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\documentclass[makeidx]{article}
\usepackage{xspace}
\usepackage{epsfig}
\usepackage{xcolor}
\usepackage{syntax}
\usepackage{amssymb}
\usepackage[fleqn]{amsmath}
\usepackage{amssymb}
\usepackage{semantic}
\usepackage{dart}
\usepackage{hyperref}
\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{makeidx}
\makeindex
\title{Dart Programming Language Specification\\
{5th edition draft}\\
{\large Version 2.2.0-dev}}
\author{}
% For information about Location Markers (and in particular the
% commands \LMHash and \LMLabel), see the long comment at the
% end of this file.
% CHANGES
% =======
% Significant changes to the specification.
% 2.2
% - Specify whether the values of literal expressions override Object.==.
% - Allow Type objects as case expressions and const map keys.
% - Introduce set literals.
% - Specify that a getter/setter and a method with the same basename is
% an error, also in the case where a class obtains both from its
% superinterfaces.
% - Specify the Dart 2.0 rule that you cannot implement, extend or mix-in
% Function.
% - Generalize specification of type aliases such that they can denote any
% type, not just function types.
%
% 2.1
% - Remove 64-bit constraint on integer literals compiled to JavaScript numbers.
% - Allow integer literals in a double context to evaluate to a double value.
% - Specify dynamic error for a failing downcast in redirecting factory
% constructor invocation.
% - Specify that type arguments passed in a redirecting factory constructor
% declaration must be taken into account during static checks.
% - Disallow any expression statement starting with `{`, not just
% those that are map literals.
% - Define a notion of lookup that is needed for super invocations, adjust
% specification of super invocations accordingly.
% - Specify that it is a dynamic error to initialize a non-static variable
% with a value that does not have the declared type (e.g., a failed cast).
% - Specify for constructor initializers that target variable must exist and
% the initializing expression must have a type which is assignable to its
% type.
% - Specify for superinitializers that the target constructor must exist and
% the argument part must have a matching shape and pass type and value
% arguments satisfying the relevant constraints.
% - Reword rules about abstract methods and inheritance to use 'class
% interface'.
% - Specify that it is an error for a concrete class with no non-trivial
% \code{noSuchMethod} to not have a concrete declaration for some member
% in its interface, or to have one which is not a correct override.
% - Use \ref{bindingActualsToFormals} in 3 locations, eliminating 2 extra
% copies of nearly the same text.
% - Add figure in \ref{bindingActualsToFormals} for improved readability.
% - Introduce a notion of lookup which is needed for superinvocations.
% - Use new lookup concept to simplify specification of getter, setter, method
% lookup.
% - Introduce several `Case<SomeTopic>` markers in order to improve
% readability.
% - Reorganize several sections to specify static analysis first and then
% dynamic semantics; clarify many details along the way. The sections are:
% \ref{variables}, \ref{new}, \ref{const}, \ref{bindingActualsToFormals},
% \ref{unqualifiedInvocation}, \ref{functionExpressionInvocation},
% \ref{superInvocations}, \ref{assignment}, \ref{compoundAssignment},
% \ref{localVariableDeclaration}, and \ref{return}.
% - Corrected error involving multiple uses of the same part in the same
% program such that it takes exports into account.
% - Eliminate all references to checked and production mode, Dart 2 does
% not have modes.
% - Integrate feature specification on noSuchMethod forwarders.
% - Specify that bound satisfaction in generic type alias type parameters
% must imply bound satisfaction everywhere in the body.
% - Specify that super-bounded generic type alias applications must trigger
% a well-boundedness check on all types occurring in the denoted type.
% - Corrected corner case of rules for generation of noSuchMethod forwarders.
% - Integrate feature specification on parameters that are
% covariant-by-declaration.
% - Integrate feature specification on parameters that are
% covariant-by-class.
% - Correct section 'Type of a function', allowing for adjustments needed
% for rules related to covariant parameters.
% - Specified the dynamic type of function objects in several contexts, such
% that the special treatment of covariant parameters can be mentioned.
% - Specified what it means for an override relation to be correct, thus
% adding the parts that are not captured by a function type subtype check.
% - Introduced the notion of member signatures, specified that they are the
% kind of entity that a class interface contains.
% - Corrected super-boundedness check to take variance into account at the
% top level.
%
% 2.0
% - Don't allow functions as assert test values.
% - Start running "async" functions synchronously.
% - It is a static warning and dynamic error to assign to a final local.
% - Specify what "is equivalent to" means.
% - Remove @proxy.
% - Don't specify the exact object used for empty positionalArguments and
% namedArguments on Invocation.
% - Remove the, now unnecessary, handling of invalid overrides of noSuchMethod.
% - Add >>> as overridable operator.
% - If initializing formal has type annotation, require subtype of field type.
% - Constant `==` operations now also allowed if just one operand is null.
% - Make flatten not be recursive.
% - Disallow implementing two instantiations of the same generic interface.
% - Update "FutureOr" specification for Dart 2.0.
% - Require that a top-level "main" declaration is a valid script-entry
% function declaration.
% - State that the return type of a setter or []= is void when not specified.
% - Clarify that "noSuchMethod" must be implemented, not just redeclared
% abstractly, to eliminate certain diagnostic messages.
% - Add generic functions and methods to the language.
% - Don't cause warning if a non-system library import shadows a system library.
% - Update mixin application forwarding constructors to correctly handle
% optional parameters and const constructors.
% - Specify `call` for Dart 2 (no function type given to enclosing class).
% - Clarify that an identifier reference denoting a top-level, static, or
% local function evaluates to the closurization of that declaration.
% - Make `mixin` and `interface` built-in identifiers.
% - Make `async` *not* a reserved word inside async functions.
% - Add 'Class Member Conflicts', simplifying and adjusting rules about
% member declaration conflicts beyond "`n` declared twice in one scope".
% - Specify that integer literals are limited to signed 64-bit values,
% and that the `int` class is intended as signed 64-bit integer, but
% that platforms may differ.
% - Specify variance and super-bounded types.
% - Introduce `subterm` and `immediate subterm`.
% - Introduce `top type`.
% - Specify configurable imports.
% - Specify the dynamic type of the Iterable/Future/Stream returned from
% invocations of functions marked sync*/async/async*.
% - Add appendix listing the major differences between 64-bit integers
% and JavaScript integers.
% - Remove appendix on naming conventions.
% - Make it explicit that "dynamic" is exported from dart:core.
% - Remove "boolean conversion". It's just an error to not be a bool.
% - Adjust cyclic subtype prevention rule for type variables.
% - Clarify that it is an error to use FutureOr<T> as a superinterface etc.
% - Eliminate the notion of static warnings, all program faults are now errors.
% - It is no longer an error for a getter to have return type `void`.
% - Specify that each redirection of a constructor is checked, statically and
% dynamically.
% - Specify that it is an error for a superinitializer to occur anywhere else
% than at the end of an initializer list.
% - Update the potentially/compile-time constant expression definitions
% so that "potentially constant" depends only on the grammar, not the types
% of sub-expressions.
% - Make `==` recognize `null` and make `&&` and `||` short-circuit in constant
% expressions.
% - Add `as` and `is` expressions as constant expressions
% - Make `^`, `|` and `&` operations on `bool` constant operations.
% - Integrate subtyping.md. This introduces the Dart 2 rules for subtyping,
% which in particular means that the notion of being a more specific type
% is eliminated, and function types are made contravariant in their
% parameter types.
% - Integrate instantiation to bound. This introduces the notions of raw
% types, the raw-depends relation, and simple bounds; and it specifies
% the algorithm which is used to expand a raw type (e.g., `C`) to a
% parameterized type (e.g., `C<int>`).
% - Integrate invalid_returns.md. This replaces the rules about when it is
% an error to have `return;` or `return e;` in a function.
% - Integrate generalized-void.md. Introduces syntactic support for using
% `void` in many new locations, including variable type annotations and
% actual type arguments; also adds errors for using values of type `void`.
% - Integrate implicit_creation.md, specifying how some constant expressions
% can be written without `const`, and all occurrences of `new` can be
% omitted.
%
% 1.15
% - Change how language specification describes control flow.
% - Object initialization now specifies initialization order correctly.
% - Specifies that leaving an await-for loop must wait for the subscription
% to be canceled.
% - An await-for loop only pauses the subscription if it does something async.
% - Assert statements allows a "message" operand and a trailing comma.
% - The Null type is now considered a subtype of all types in most cases.
% - Specify what NEWLINE means in multiline strings.
% - Specified the FutureOf type.
% - Asserts can occur in initializer lists.
%
% 1.14
% - The call "C()" where "C" is a class name, is a now compile-time error.
% - Changed description of rewrites that depended on a function literal.
% In many cases, the rewrite wasn't safe for asynchronous code.
% - Removed generalized tear-offs.
% - Allow "rethrow" to also end a switch case. Allow braces around switch cases.
% - Allow using '=' as default-value separator for named parameters.
% - Make it a compile-time error if a library includes the same part twice.
% - Now more specific about the return types of sync*/async/async* functions
% in relation to return statements.
% - Allow Unicode surrogate values in String literals.
% - Make an initializing formal's value accessible in the initializer list.
% - Allow any expression in assert statements (was only conditionalExpression).
% - Allow trailing commas in argument and parameter lists.
%
% 1.11 - ECMA 408 - 4th Edition
% - Specify that potentially constant expressions must be valid expressions
% if the parameters are non-constant.
% - Make "??" a compile-time constant operator.
% - Having multiple unnamed libraries no longer causes warnings.
% - Specify null-aware operators for static methods.
%
% 1.10
% - Allow mixins to have super-classes and super-calls.
% - Specify static type checking for the implicit for-in iterator variable.
% - Specify static types for a number of expressions where it was missing.
% - Make calls on the exact type "Function" not cause warnings.
% - Specify null-aware behavior of "e?.v++" and similar expressions.
% - Specify that `package:` URIs are treated in an implementation dependent way.
% - Require warning if for-in is used on object with no "iterator" member.
%
% 1.9 - ECMA-408 - 3rd Edition
%
\begin{document}
\maketitle
\tableofcontents
\newpage
\pagestyle{myheadings}
\markright{Dart Programming Language Specification}
% begin Ecma boilerplate
\section{Scope}
\LMLabel{ecmaScope}
\LMHash{}%
This Ecma standard specifies the syntax and semantics of the Dart programming language.
It does not specify the APIs of the Dart libraries except where those library elements are essential to the correct functioning of the language itself (e.g., the existence of class \code{Object} with methods such as \code{noSuchMethod}, \code{runtimeType}).
\section{Conformance}
\LMLabel{ecmaConformance}
\LMHash{}%
A conforming implementation of the Dart programming language must provide and support all the APIs (libraries, types, functions, getters, setters, whether top-level, static, instance or local) mandated in this specification.
\LMHash{}%
A conforming implementation is permitted to provide additional APIs, but not additional syntax, except for experimental features in support of null-aware cascades that are likely to be introduced in the next revision of this specification.
\section{Normative References}
\LMLabel{ecmaNormativeReferences}
\LMHash{}%
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any amendments) applies.
\begin{enumerate}
\item
The Unicode Standard, Version 5.0, as amended by Unicode 5.1.0, or successor.
\item
Dart API Reference, https://api.dartlang.org/
\end{enumerate}
\section{Terms and Definitions}
\LMLabel{ecmaTermsAndDefinitions}
\LMHash{}%
Terms and definitions used in this specification are given in the body of the specification proper.
Such terms are highlighted in italics when they are introduced, e.g., `we use the term \NoIndex{verbosity} to refer to the property of excess verbiage',
and add a marker in the margin.
% End Ecma Boilerplate
\section{Notation}
\LMLabel{notation}
\LMHash{}%
We distinguish between normative and non-normative text.
Normative text defines the rules of Dart.
It is given in this font.
At this time, non-normative text includes:
\begin{itemize}
\item[Rationale]
Discussion of the motivation for language design decisions appears in italics.
\rationale{
Distinguishing normative from non-normative helps clarify what part of the text is binding and what part is merely expository.
}
\item[Commentary]
Comments such as ``\commentary{The careful reader will have noticed that the name Dart has four characters}'' serve to illustrate or clarify the specification, but are redundant with the normative text.
\commentary{
The difference between commentary and rationale can be subtle.
}
\rationale{
Commentary is more general than rationale, and may include illustrative examples or clarifications.
}
\item[Open questions] (\Q{in this font}).
Open questions are points that are unsettled in the mind of the author(s) of the specification; expect them (the questions, not the authors; precision is important in a specification) to be eliminated in the final specification.
\Q{Should the text at the end of the previous bullet be rationale or commentary?}
\end{itemize}
\LMHash{}%
Reserved words and built-in identifiers (\ref{identifierReference}) appear in {\bf bold}.
\commentary{
Examples would be \SWITCH{} or \CLASS{}.
}
\LMHash{}%
Grammar productions are given in a common variant of EBNF.
The left hand side of a production ends with a colon.
On the right hand side, alternation is represented by vertical bars, and sequencing by spacing.
As in PEGs, alternation gives priority to the left.
Optional elements of a production are suffixed by a question mark like so: \code{anElephant?}.
Appending a star to an element of a production means it may be repeated zero or more times.
Appending a plus sign to a production means it occurs one or more times.
Parentheses are used for grouping.
Negation is represented by prefixing an element of a production with a tilde.
Negation is similar to the not combinator of PEGs, but it consumes input if it matches.
In the context of a lexical production it consumes a single character if there is one; otherwise, a single token if there is one.
\commentary{
An example would be:
}
\begin{grammar}\color{commentaryColor}
<aProduction> ::= <anAlternative>
\alt <anotherAlternative>
\alt <oneThing> <after> <another>
\alt <zeroOrMoreThings>*
\alt <oneOrMoreThings>+
\alt <anOptionalThing>?
\alt (<some> <grouped> <things>)
\alt \~{}<notAThing>
\alt `aTerminal'
\alt <A\_LEXICAL\_THING>
\end{grammar}
\LMHash{}%
Both syntactic and lexical productions are represented this way.
Lexical productions are distinguished by their names.
The names of lexical productions consist exclusively of upper case characters and underscores.
As always, within grammatical productions, whitespace and comments between elements of the production are implicitly ignored unless stated otherwise.
Punctuation tokens appear in quotes.
\LMHash{}%
Productions are embedded, as much as possible, in the discussion of the constructs they represent.
\LMHash{}%
A \Index{term} is a syntactic construct.
It may be considered to be a piece of text which is derivable in the grammar,
and it may be considered to be a tree created by such a derivation.
An \Index{immediate subterm} of a given term $t$ is a syntactic construct
which corresponds to an immediate subtree of $t$ considered as a derivation tree.
A \Index{subterm} of a given term $t$ is $t$,
or an immediate subterm of $t$,
or a subterm of an immediate subterm of $t$.
\LMHash{}%
A list $x_1, \ldots, x_n$ denotes any list of $n$ elements of the form $x_i, 1 \le i \le n$.
Note that $n$ may be zero, in which case the list is empty.
We use such lists extensively throughout this specification.
\LMHash{}%
For $j \in 1 .. n$,
let $y_j$ be an atomic syntactic entity (like an identifier),
$x_j$ a composite syntactic entity (like an expression or a type),
and $E$ again a composite syntactic entity.
The notation
\IndexCustom{$[x_1/y_1, \ldots, x_n/y_n]E$}{[x1/y1, ..., xn/yn]E@$[x/y\ldots]E$}
then denotes a copy of $E$
in which each occurrence of $y_i, 1 \le i \le n$ has been replaced by $x_i$.
\LMHash{}%
This operation is also known as \Index{substitution},
and it is the variant that avoids capture.
That is, when $E$ contains a construct that introduces $y_i$ into a nested scope for some $i \in 1 .. n$,
the substitution will not replace $y_i$ in that scope.
Conversely, if such a replacement would put an identifier \id{} (a subterm of $x_i$) into a scope where \id{} is declared,
the relevant declarations in $E$ are systematically renamed to fresh names.
\commentary{
In short, capture freedom ensures that the ``meaning'' of each identifier is preserved during substitution.
}
\LMHash{}%
We sometimes abuse list or map literal syntax, writing $[o_1, \ldots, o_n]$ (respectively $\{k_1: o_1, \ldots, k_n: o_n\}$) where the $o_i$ and $k_i$ may be objects rather than expressions.
The intent is to denote a list (respectively map) object whose elements are the $o_i$ (respectively, whose keys are the $k_i$ and values are the $o_i$).
\LMHash{}%
The specifications of operators often involve statements such as
\code{$x$ \metavar{op} $y$}
is equivalent to the method invocation
\IndexCustom{\rm\code{$x$.\metavar{op}($y$)}}{x.op(y)@\code{$x$.\metavar{op}($y$)}}.
Such specifications should be understood as a shorthand for:
\begin{itemize}
\item
$x$ $op$ $y$ is equivalent to the method invocation
\code{$x$.\metavar{op'}($y$)},
assuming the class of $x$ actually declared a non-operator method named $op'$
defining the same function as the operator $op$.
\end{itemize}
\rationale{
This circumlocution is required because
{\rm\code{$x$.\metavar{op}($y$)}}, where op is an operator, is not legal syntax.
However, it is painfully verbose, and we prefer to state this rule once here,
and use a concise and clear notation across the specification.
}
\LMHash{}%
When the specification refers to the order given in the program, it means the order of the program source code text, scanning left-to-right and top-to-bottom.
\LMHash{}%
When the specification refers to a
\IndexCustom{fresh variable}{variable!fresh},
it means a variable with a name that doesn't occur anywhere
in the current program.
When the specification introduces a fresh variable bound to an object,
the fresh variable is implicitly bound in a surrounding scope.
\LMHash{}%
References to otherwise unspecified names of program entities
(such as classes or functions)
are interpreted as the names of members of the Dart core library.
\commentary{%
Examples would be the classes \code{Object} and \code{Type}
representing, respectively, the root of the class hierarchy and
the reification of run-time types.
%
It would be possible to declare, e.g.,
a local variable named \code{Object},
so it is generally incorrect to assume that
the name \code{Object} will actually resolve to said core class.
However, we will generally omit mentioning this, for brevity.%
}
%% TODO(eernst): We need to get rid of the concept of `is equivalent to`,
%% cf. language issue https://github.com/dart-lang/language/issues/227.
%% In this CL the phrase `treated as` has been introduced in a few places,
%% and the above-mentioned issue 227 will give rise to a complete revision
%% of this aspect of this document. In particular, the next paragraph will
%% be deleted.
\LMHash{}%
When the specification says that one piece of syntax \Index{is equivalent to}
another piece of syntax, it means that it is equivalent in all ways,
and the former syntax should generate the same compile-time errors
and have the same run-time behavior as the latter, if any.
\commentary{%
Error messages, if any, should always refer to the original syntax.%
}
If execution or evaluation of a construct is said to be
equivalent to execution or evaluation of another construct,
then only the run-time behavior is equivalent,
and compile-time errors apply only for the original syntax.
\LMHash{}%
When the specification says that one piece of syntax $s$ is
\Index{treated as}
another piece of syntax $s'$,
it means that the static analysis of $s$ is the static analysis of $s'$
(\commentary{in particular, exactly the same compile-time errors occur}).
Moreover, if $s$ has no compile-time errors then
the behavior of $s$ at run time is exactly the behavior of $s'$.
\rationale{%
Error \emph{messages}, if any, should always refer to the original syntax $s$.%
}
\commentary{%
In short, whenever $s$ is treated as $s'$,
the reader should immediately switch to the section about $s'$
in order to get any further information about
the static analysis and dynamic semantics of $s$.%
}
\rationale{%
The notion of being `treated as' is similar to the notion of syntactic sugar:
``$s$ is treated as $s'$''
could as well have been worded
``$s$ is desugared into $s'$''.
Of course, it should then actually be called ``semantic sugar'',
because the applicability of the transformation and the construction of $s'$
may rely on information from static analysis.
The point is that we only specify the static analysis and dynamic semantics
of a core language which is a subset of Dart
(just slightly smaller than Dart),
and desugaring transforms any given Dart program to
a program in that core language.
This helps keeping the language specification consistent and comprehensible,
because it shows directly
that some language features are introducing essential semantics,
and others are better described as mere abbreviations of existing constructs.%
}
\section{Overview}
\LMLabel{overview}
\LMHash{}%
Dart is a class-based, single-inheritance, pure object-oriented programming language.
Dart is optionally typed (\ref{types}) and supports reified generics.
The run-time type of every object is represented as an instance of class \code{Type} which can be obtained by calling the getter \code{runtimeType} declared in class \code{Object}, the root of the Dart class hierarchy.
\LMHash{}%
Dart programs may be statically checked.
Programs with compile-time errors do not have a specified dynamic semantics.
This specification makes no attempt to answer additional questions
about a library or program at the point
where it is known to have a compile-time error.
\commentary{
However, tools may choose to support execution of some programs with errors.
For instance, a compiler may compile certain constructs with errors such that
a dynamic error will be raised if an attempt is made to
execute such a construct,
or an IDE integrated runtime may support opening
an editor window when such a construct is executed,
allowing developers to correct the error.
It is expected that such features would amount to a natural extension of the
dynamic semantics of Dart as specified here, but, as mentioned,
this specification makes no attempt to specify exactly what that means.
}
\LMHash{}%
As specified in this document,
dynamic checks are guaranteed to be performed in certain situations,
and certain violations of the type system throw exceptions at run time.
\commentary{
An implementation is free to omit such checks whenever they are
guaranteed to succeed, e.g., based on results from the static analysis.
}
\commentary{
The coexistence between optional typing and reification is based on the following:
\begin{enumerate}
\item
Reified type information reflects the types of objects at run time
and may always be queried by dynamic typechecking constructs
(the analogs of instanceOf, casts, typecase etc.\ in other languages).
Reified type information includes
access to instances of class \code{Type} representing types,
the run-time type (aka class) of an object,
and the actual values of type parameters
to constructors and generic function invocations.
\item
Type annotations declare the types of
variables and functions (including methods and constructors).
\item
%% TODO(eernst): Change when integrating instantiate-to-bounds.md.
Type annotations may be omitted, in which case they are generally
filled in with the type \DYNAMIC{}
(\ref{typeDynamic}).
\end{enumerate}
}
%% TODO(eernst): Update when we add inference.
\commentary{
Dart as implemented includes extensive support for inference of omitted types.
This specification makes the assumption that inference has taken place,
and hence inferred types are considered to be present in the program already.
However, in some cases no information is available
to infer an omitted type annotation,
and hence this specification still needs to specify how to deal with that.
A future version of this specification will also specify type inference.
}
\LMHash{}%
Dart programs are organized in a modular fashion into
units called \NoIndex{libraries} (\ref{librariesAndScripts}).
Libraries are units of encapsulation and may be mutually recursive.
\commentary{
However they are not first class.
To get multiple copies of a library running simultaneously, one needs to spawn an isolate.
}
\LMHash{}%
A dart program execution may occur with assertions enabled or disabled.
The method used to enable or disable assertions is implementation specific.
\subsection{Scoping}
\LMLabel{scoping}
\LMHash{}%
A \Index{namespace} is a mapping of names denoting declarations to actual declarations.
Let $NS$ be a namespace.
We say that a name $n$ \Index{is in} $NS$ if $n$ is a key of $NS$.
We say a declaration $d$ \NoIndex{is in} $NS$ if a key of $NS$ maps to $d$.
\LMHash{}%
A scope $S_0$ induces a namespace $NS_0$ that maps the simple name of each variable, type or function declaration $d$ declared in $S_0$ to $d$.
Labels are not included in the induced namespace of a scope; instead they have their own dedicated namespace.
\commentary{
It is therefore impossible, e.g., to define a class that declares a method and a getter with the same name in Dart.
Similarly one cannot declare a top-level function with the same name as a library variable or a class.
}
\LMHash{}%
It is a compile-time error if there is more than one entity with the same name declared in the same scope.
\commentary{
In some cases, the name of the declaration differs from the identifier used to declare it.
Setters have names that are distinct from the corresponding getters because they always have an = automatically added at the end, and unary minus has the special name unary-.
}
\LMHash{}%
Dart is lexically scoped.
Scopes may nest.
A name or declaration $d$ is \Index{available in scope} $S$ if $d$ is in the namespace induced by $S$ or if $d$ is available in the lexically enclosing scope of $S$.
We say that a name or declaration $d$ is \Index{in scope} if $d$ is available in the current scope.
\LMHash{}%
If a declaration $d$ named $n$ is in the namespace induced by a scope $S$, then $d$ \Index{hides} any declaration named $n$ that is available in the lexically enclosing scope of $S$.
\commentary{
A consequence of these rules is that it is possible to hide a type with a method or variable.
Naming conventions usually prevent such abuses.
Nevertheless, the following program is legal:
}
\begin{dartCode}
\CLASS{} HighlyStrung \{
String() => "?";
\}
\end{dartCode}
\LMHash{}%
Names may be introduced into a scope by declarations within the scope or by other mechanisms such as imports or inheritance.
\rationale{
The interaction of lexical scoping and inheritance is a subtle one.
Ultimately, the question is whether lexical scoping takes precedence over inheritance or vice versa.
Dart chooses the former.
Allowing inherited names to take precedence over locally declared names could create unexpected situations as code evolves.
Specifically, the behavior of code in a subclass could silently change if a new name is introduced in a superclass.
Consider:
}
\begin{dartCode}
\LIBRARY{} L1;
\CLASS{} S \{\}
\LIBRARY{} L2;
\IMPORT{} `L1.dart';
foo() => 42;
\CLASS{} C \EXTENDS{} S\{ bar() => foo();\}
\end{dartCode}
\rationale{
Now assume a method \code{foo()} is added to \code{S}.
}
\begin{dartCode}
\LIBRARY{} L1;
\CLASS{} S \{foo() => 91;\}
\end{dartCode}
\rationale{
If inheritance took precedence over the lexical scope, the behavior of \code{C} would change in an unexpected way.
Neither the author of \code{S} nor the author of \code{C} are necessarily aware of this.
In Dart, if there is a lexically visible method \code{foo()}, it will always be called.
Now consider the opposite scenario.
We start with a version of \code{S} that contains \code{foo()}, but do not declare \code{foo()} in library \code{L2}.
Again, there is a change in behavior - but the author of \code{L2} is the one who introduced the discrepancy that effects their code, and the new code is lexically visible.
Both these factors make it more likely that the problem will be detected.
These considerations become even more important if one introduces constructs such as nested classes, which might be considered in future versions of the language.
Good tooling should of course endeavor to inform programmers of such situations (discreetly).
For example, an identifier that is both inherited and lexically visible could be highlighted (via underlining or colorization).
Better yet, tight integration of source control with language aware tools would detect such changes when they occur.
}
\subsection{Privacy}
\LMLabel{privacy}
\LMHash{}%
Dart supports two levels of \Index{privacy}: public and private.
A declaration is \IndexCustom{private}{private!declaration}
if{}f its name is private,
otherwise it is \IndexCustom{public}{public!declaration}.
A name $q$ is \IndexCustom{private}{private!name}
if{}f any one of the identifiers that comprise $q$ is private,
otherwise it is \IndexCustom{public}{public!name}.
An identifier is \IndexCustom{private}{private!identifier}
if{}f it begins with an underscore (the \_ character)
otherwise it is \IndexCustom{public}{public!identifier}.
\LMHash{}%
A declaration $m$ is \Index{accessible to a library} $L$
if $m$ is declared in $L$ or if $m$ is public.
\commentary{
This means private declarations may only be accessed within the library in which they are declared.
}
\LMHash{}%
Privacy applies only to declarations within a library, not to library declarations themselves.
\rationale{
Libraries do not reference each other by name and so the idea of a private library is meaningless.
Thus, if the name of a library begins with an underscore, it has no effect on the accessibility of the library or its members.
}
\rationale{
Privacy is, at this point, a static notion tied to a particular piece of code (a library).
It is designed to support software engineering concerns rather than security concerns.
Untrusted code should always run in an another isolate.
It is possible that libraries will become first class objects and privacy will be a dynamic notion tied to a library instance.
Privacy is indicated by the name of a declaration - hence privacy and naming are not orthogonal.
This has the advantage that both humans and machines can recognize access to private declarations at the point of use without knowledge of the context from which the declaration is derived.
}
\subsection{Concurrency}
\LMLabel{concurrency}
\LMHash{}%
Dart code is always single threaded.
There is no shared-state concurrency in Dart.
Concurrency is supported via actor-like entities called \Index{isolates}.
\LMHash{}%
An isolate is a unit of concurrency.
It has its own memory and its own thread of control.
Isolates communicate by message passing (\ref{sendingMessages}).
No state is ever shared between isolates.
Isolates are created by spawning (\ref{spawningAnIsolate}).
\section{Errors and Warnings}
\LMLabel{errorsAndWarnings}
\LMHash{}%
This specification distinguishes between several kinds of errors.
\LMHash{}%
\IndexCustom{Compile-time errors}{compile-time error}
are errors that preclude execution.
A compile-time error must be reported by a Dart compiler before the erroneous code is executed.
\rationale{
A Dart implementation has considerable freedom as to when compilation takes place.
Modern programming language implementations often interleave compilation and execution, so that compilation of a method may be delayed, e.g., until it is first invoked.
Consequently, compile-time errors in a method $m$ may be reported as late as the time of $m$'s first invocation.
Dart is often loaded directly from source, with no intermediate binary representation.
In the interests of rapid loading, Dart implementations may choose to avoid full parsing of method bodies, for example.
This can be done by tokenizing the input and checking for balanced curly braces on method body entry.
In such an implementation, even syntax errors will be detected only when the method needs to be executed, at which time it will be compiled (JITed).
In a development environment a compiler should of course report compilation errors eagerly so as to best serve the programmer.
A Dart development environment might choose to support error eliminating program transformations, e.g.,
replacing an erroneous expression by the invocation of a debugger.
It is outside the scope of this document to specify how such transformations work, and where they may be applied.
}
\LMHash{}%
If an uncaught compile-time error occurs within the code of a running isolate $A$, $A$ is immediately suspended.
The only circumstance where a compile-time error could be caught would be via code run reflectively, where the mirror system can catch it.
\rationale{
Typically, once a compile-time error is thrown and $A$ is suspended, $A$ will then be terminated.
However, this depends on the overall environment.
A Dart engine runs in the context of an \Index{embedder},
a program that interfaces between the engine and the surrounding computing environment.
The embedder will often be a web browser, but need not be; it may be a C++ program on the server for example.
When an isolate fails with a compile-time error as described above, control returns to the embedder, along with an exception describing the problem.
This is necessary so that the embedder can clean up resources etc.
It is then the embedder's decision whether to terminate the isolate or not.
}
\LMHash{}%
\IndexCustom{Static warnings}{static warning}
are situations that do not preclude execution,
but which are unlikely to be intended,
and likely to cause bugs or inconveniences.
A static warning must be reported by a Dart compiler before the associated code is executed.
\LMHash{}%
When this specification says that a \Index{dynamic error} occurs,
it means that a corresponding error object is thrown.
When it says that a \Index{dynamic type error} occurs,
it represents a failed type check at run time,
and the object which is thrown implements \code{TypeError}.
\LMHash{}%
Whenever we say that an exception $ex$ is
\IndexCustom{thrown}{throwing an exception},
it acts like an expression had thrown (\ref{statementCompletion})
with $ex$ as exception object and with a stack trace
corresponding to the current system state.
When we say that a $C$ \IndexCustom{is thrown}{throwing a class},
where $C$ is a class, we mean that an instance of class $C$ is thrown.
\LMHash{}%
If an uncaught exception is thrown by a running isolate $A$, $A$ is immediately suspended.
\section{Variables}
\LMLabel{variables}
\LMHash{}%
Variables are storage locations in memory.
\begin{grammar}
<variableDeclaration> ::= <declaredIdentifier> (`,' <identifier>)*
<declaredIdentifier> ::= <metadata> \COVARIANT{}? <finalConstVarOrType> <identifier>
<finalConstVarOrType> ::= \FINAL{} <type>?
\alt \CONST{} <type>?
\alt <varOrType>
<varOrType> ::= \VAR{}
\alt <type>
<initializedVariableDeclaration> ::= \gnewline{}
<declaredIdentifier> (`=' <expression>)? (`,' <initializedIdentifier>)*
<initializedIdentifier> ::= <identifier> (`=' <expression>)?
<initializedIdentifierList> ::= <initializedIdentifier> (`,' <initializedIdentifier>)*
\end{grammar}
\LMHash{}%
A \synt{variableDeclaration} that declares two or more variables
is equivalent to multiple variable declarations declaring
the same set of variable names in the same order,
with the same type and modifiers.
\LMHash{}%
An \synt{initializedVariableDeclaration}
that declares two or more variables
is equivalent to multiple variable declarations declaring
the same set of variable names, in the same order,
with the same initialization, type, and modifiers.
\commentary{
For example,
\code{\VAR{} x, y;}
is equivalent to
\code{\VAR{} x; \VAR{} y;}
and
\code{\STATIC{} \FINAL{} String s1, s2 = "foo";}
is equivalent to
\code{\STATIC{} \FINAL{} String s1; \STATIC{} \FINAL{} String s2 = "foo";}.
}
\LMHash{}%
It is possible for a variable declaration to include the modifier \COVARIANT{}.
The effect of doing this with an instance variable is described elsewhere
(\ref{instanceVariables}).
It is a compile-time error for the declaration of
a variable which is not an instance variable
to include the modifier \COVARIANT{}.
\LMHash{}%
In a variable declaration of one of the forms
\code{$N$ $v$;}
\code{$N$ $v$ = $e$;}
where $N$ is derived from
\syntax{<metadata> <finalConstVarOrType>},
we say that $v$ is the \Index{declaring occurrence} of the identifier.
For every identifier which is not a declaring occurrence,
we say that it is an \Index{referencing occurrence}.
We also abbreviate that to say that an identifier is
a \Index{declaring identifier} respectively an \Index{referencing identifier}.
\commentary{
In an expression of the form \code{$e$.\id} it is possible that
$e$ has static type \DYNAMIC{} and \id{} cannot be associated with
any specific declaration named \id{} at compile-time,
but in this situation \id{} is still a referencing identifier.
}
\LMHash{}%
An \Index{initializing variable declaration}
is a variable declaration whose declaring identifier is
immediately followed by `\code{=}' and an \Index{initializing expression}.
\LMHash{}%
A variable declared at the top-level of a library is referred to as either a
\IndexCustom{library variable}{variable!library} or a
\IndexCustom{top-level variable}{variable!top-level}.
\LMHash{}%
A \IndexCustom{static variable}{variable!static}
is a variable that is not associated with a particular instance,
but rather with an entire library or class.
Static variables include library variables and class variables.
Class variables are variables whose declaration is immediately nested inside a class declaration and includes the modifier \STATIC{}.
A library variable is implicitly static.
It is a compile-time error to preface a top-level variable declaration with the built-in identifier (\ref{identifierReference}) \STATIC{}.
\LMHash{}%
A \IndexCustom{constant variable}{variable!constant}
is a variable whose declaration includes the modifier \CONST{}.
A constant variable must be initialized to a constant expression (\ref{constants}) or a compile-time error occurs.
\commentary{%
An initializing expression of a constant variable occurs in a constant context
(\ref{constantContexts}),
which means that \CONST{} modifiers need not be specified explicitly.%
}
\LMHash{}%
A \IndexCustom{final variable}{variable!final}
is a variable whose binding is fixed upon initialization;
a final variable $v$ will always refer to the same object after $v$ has been initialized.
A variable is final if{}f its declaration includes the modifier \FINAL{} or the modifier \CONST{}.
\LMHash{}%
A \IndexCustom{mutable variable}{variable!mutable}
is a variable which is not final.
%% Note that the following relies on the assumption that inference has
%% already taken place, including member signature inference. For instance,
%% if `var x;` is an instance variable declaration that overrides `T get x;`
%% then we treat `var x;` as if it had been `T x;`.
\LMHash{}%
The following rules on implicitly induced getters and setters
apply to all static and instance variables.
\LMHash{}%
A variable declaration of one of the forms
\code{$T$ $v$;}
\code{$T$ $v$ = $e$;}
\code{\CONST{} $T$ $v$ = $e$;}
\code{\FINAL{} $T$ $v$;}
or \code{\FINAL{} $T$ $v$ = $e$;}
induces an implicit getter function (\ref{getters}) with signature
\code{$T$ \GET{} $v$}
whose invocation evaluates as described below
(\ref{evaluationOfImplicitVariableGetters}).
In these cases the static type of $v$ is $T$.
\LMHash{}%
A variable declaration of one of the forms
\code{\VAR{} $v$;}
\code{\VAR{} $v$ = $e$;}
\code{\CONST{} $v$ = $e$;}
\code{\FINAL{} $v$;}
or \code{\FINAL{} $v$ = $e$;}
induces an implicit getter function with signature
\code{\DYNAMIC{} \GET{} $v$}
whose invocation evaluates as described below
(\ref{evaluationOfImplicitVariableGetters}).
%% TODO[inference]: We assume inference has taken place, i.e., inferred types
%% are written explicitly. Does this mean that the initialized variants
%% cannot exist (not even for `$e$` of type `dynamic`?). We probably don't
%% want to start talking about a grammar before inference and another one
%% after inference.
In these cases, the static type of $v$ is \DYNAMIC{}
(\ref{typeDynamic}).
\LMHash{}%
A mutable variable declaration of the form
\code{{} $T$ $v$;}
or \code{$T$ $v$ = $e$;}
induces an implicit setter function (\ref{setters}) with signature
\code{\VOID{} \SET{} $v$=($T$ $x$)}
whose execution sets the value of $v$ to the incoming argument $x$.
\LMHash{}%
A mutable variable declaration of the form
\code{\VAR{} $v$;}
or \code{\VAR{} $v$ = $e$;}
induces an implicit setter function with signature
\code{\VOID{} \SET{} $v$=(\DYNAMIC{} $x$)}
whose execution sets the value of $v$ to the incoming argument $x$.
\LMHash{}%
The scope into which the implicit getters and setters are introduced depends on the kind of variable declaration involved.
\LMHash{}%
A library variable introduces a getter into the top level scope of the enclosing library.
A static class variable introduces a static getter into the immediately enclosing class.
An instance variable introduces an instance getter into the immediately enclosing class.
\LMHash{}%
A mutable library variable introduces a setter into the top level scope of the enclosing library.
A mutable static class variable introduces a static setter into the immediately enclosing class.
A mutable instance variable introduces an instance setter into the immediately enclosing class.
\LMHash{}%
Let $v$ be variable declared in an initializing variable declaration,
and let $e$ be the associated initializing expression.
It is a compile-time error if the static type of $e$ is not assignable to the declared type of $v$.
It is a compile-time error if a final instance variable whose declaration has an initializer expression
is also initialized by a constructor, either by an initializing formal or an initializer list entry.
\commentary{
It is a compile-time error if a final instance variable
that has been initialized by means of an initializing formal of a constructor $k$
is also initialized in the initializer list of $k$ (\ref{initializerLists}).
%% TODO(eernst): Not quite true, because of special lookup for assignment!
A static final variable $v$ does not induce a setter,
so unless a setter named \code{$v$=} is in scope
it is a compile-time error to assign to $v$.
Similarly, assignment to a final instance variable $v$
is a compile-time error,
unless a setter named \code{$v$=} is in scope,
or the receiver has type \DYNAMIC{}.
$v$ can be initialized in its declaration or in initializer lists,
but initialization and assignment is not the same thing.
When the receiver has type \DYNAMIC{}
such an assignment is not a compile-time error,
% This error can occur because the receiver is dynamic.
but if there is no setter it will cause a dynamic error.
}
\LMHash{}%
A variable that has no initializing expression has the null object (\ref{null}) as its initial value.
Otherwise, variable initialization proceeds as follows:
\LMHash{}%
Static variable declarations with an initializing expression are initialized lazily
(\ref{evaluationOfImplicitVariableGetters}).
\rationale{
The lazy semantics are given because we do not want a language where one tends to define expensive initialization computations, causing long application startup times.
This is especially crucial for Dart, which must support the coding of client applications.
}
\commentary{
Initialization of an instance variable with no initializing expression
takes place during constructor execution
(\ref{initializerLists}).
}
\LMHash{}%
Initialization of an instance variable $v$
with an initializing expression $e$
proceeds as follows:
$e$ is evaluated to an object $o$
and the variable $v$ is bound to $o$.
\commentary{
It is specified elsewhere when this initialization occurs,
and in which environment
(p.\,\pageref{executionOfGenerativeConstructors},
\ref{localVariableDeclaration},
\ref{bindingActualsToFormals}).
}
\commentary{
If the initializing expression throws then
access to the uninitialized variable is prevented,
because the instance creation
that caused this initialization to take place
will throw.
}
\LMHash{}%
% This error can occur due to implicit casts, and
% for instance variables also when a setter is called dynamically.
It is a dynamic type error if the dynamic type of $o$ is not
a subtype of the actual type of the variable $v$
(\ref{actualTypeOfADeclaration}).
\subsection{Evaluation of Implicit Variable Getters}
\LMLabel{evaluationOfImplicitVariableGetters}
\LMHash{}%
Let $d$ be the declaration of a static or instance variable $v$.
If $d$ is an instance variable,
then the invocation of the implicit getter of $v$ evaluates to
the value stored in $v$.
If $d$ is a static variable
(\commentary{which can be a library variable})
then the implicit getter method of $v$ executes as follows:
\begin{itemize}
\item {\bf Non-constant variable declaration with initializer}.
If $d$ is of one of the forms
\code{\VAR{} $v$ = $e$;},
\code{$T$ $v$ = $e$;},
\code{\FINAL{} $v$ = $e$;},
\code{\FINAL{} $T$ $v$ = $e$;},
\code{\STATIC{} $v$ = $e$;},
\code{\STATIC{} $T$ $v$ = $e$; },
\code{\STATIC{} \FINAL{} $v$ = $e$; } or
\code{\STATIC{} \FINAL{} $T$ $v$ = $e$;}
and no value has yet been stored into $v$
then the initializing expression $e$ is evaluated.
If, during the evaluation of $e$, the getter for $v$ is invoked,
a \code{CyclicInitializationError} is thrown.
If the evaluation of $e$ throws an exception $e$ and stack trace $s$,
the null object (\ref{null}) is stored into $v$;
the execution of the getter then throws $e$ and stack trace $s$.
Otherwise, the evaluation of $e$ succeeded yielding an object $o$;
then $o$ is stored into $v$ and
the execution of the getter completes by returning $o$.
Otherwise,
(\commentary{when a value $o$ has been stored in $v$})
execution of the getter completes by returning $o$.
\item {\bf Constant variable declaration}.
If $d$ is of one of the forms
\code{\CONST{} $v$ = $e$;},
\code{\CONST{} $T$ $v$ = $e$;},
\code{\STATIC{} \CONST{} $v$ = $e$;} or
\code{\STATIC{} \CONST{} $T$ $v$ = $e$;}
the result of the getter is the value of the constant expression $e$.
\commentary{
Note that a constant expression cannot depend on itself,
so no cyclic references can occur.
}
\item {\bf Variable declaration without initializer}.
The result of executing the getter method is the value stored in $v$.
\commentary{This may be the initial value, that is, the null object.}
\end{itemize}
\section{Functions}
\LMLabel{functions}
\LMHash{}%
Functions abstract over executable actions.
\begin{grammar}
<functionSignature> ::= \gnewline{}
<metadata> <type>? <identifier> <formalParameterPart>
<formalParameterPart> ::= <typeParameters>? <formalParameterList>
<functionBody> ::= \ASYNC{}? `=>' <expression> `;'
\alt (\ASYNC{} | \ASYNC `*' | \SYNC `*')? <block>
<block> ::= `{' <statements> `}'
\end{grammar}
\LMHash{}%
Functions can be introduced by function declarations (\ref{functionDeclarations}),
method declarations (\ref{instanceMethods}, \ref{staticMethods}),
getter declarations (\ref{getters}),
setter declarations (\ref{setters}),
and constructor declarations (\ref{constructors});
and they can be introduced by function literals (\ref{functionExpressions}).
\LMHash{}%
A function is \IndexCustom{asynchronous}{function!asynchronous}
if its body is marked with the \ASYNC{} or \code{\ASYNC*} modifier.
Otherwise the function is \IndexCustom{synchronous}{function!synchronous}.
A function is a \IndexCustom{generator}{function!generator}
if its body is marked with the \code{\SYNC*} or \code{\ASYNC*} modifier.
Further details about these concepts are given below.
\commentary{%
Whether a function is synchronous or asynchronous is orthogonal to
whether it is a generator or not.
Generator functions are a sugar for functions
that produce collections in a systematic way,
by lazily applying a function that \emph{generates}
individual elements of a collection.
Dart provides such a sugar in both the synchronous case,
where one returns an iterable,
and in the asynchronous case, where one returns a stream.
Dart also allows both synchronous and asynchronous functions
that produce a single value.%
}
\LMHash{}%
Each declaration that introduces a function has a signature that specifies
its return type, name, and formal parameter part,
except that the return type may be omitted,
and getters never have a formal parameter part.
Function literals have a formal parameter part, but no return type and no name.
The formal parameter part optionally specifies
the formal type parameter list of the function,
and it always specifies its formal parameter list.
A function body is either:
\begin{itemize}
\item
a block statement (\ref{blocks}) containing
the statements (\ref{statements}) executed by the function,
optionally marked with one of the modifiers:
\ASYNC, \code{\ASYNC*} or \code{\SYNC*}.
%
Unless it is statically known that the body of the function
cannot complete normally
(\commentary{that is, it cannot reach the end and ``fall through''},
cf.~\ref{statementCompletion}),
it is a compile-time error if
the addition of \code{\RETURN;} at the end of the body
would be a compile-time error.
\commentary{%
For instance, it is an error if
the return type of a synchronous function is \code{int},
and the body may complete normally.
The precise rules are given in section~\ref{return}.%
}
\commentary{%
Because Dart supports dynamic function invocations,
we cannot guarantee that a function that does not return a value
will not be used in the context of an expression.
Therefore, every function must return a value.
A function body that ends without doing a throw or return
will cause the function to return the null object (\ref{null}),
as will a \RETURN{} without an expression.
For generator functions, the situation is more subtle.
See further discussion in section~\ref{return}.%
}
OR
\item
of the form \code{=> $e$} or the form \code{\ASYNC{} => $e$},
which both return the value of the expression $e$ as if by a
\code{return $e$}.
\commentary{%
The other modifiers do not apply here,
because they apply only to generators, discussed below.
Generators are not allowed to return a value,
values are added to the generated stream or iterable using
\YIELD{} or \YIELD*.%
}
Let $T$ be the declared return type of the function that has this body.
It is a compile-time error if one of the following conditions hold:
\begin{itemize}
\item The function is synchronous, $T$ is not \VOID{},
and it would have been a compile-time error to declare the function with the body
\code{\{ \RETURN{} $e$; \}}
rather than \code{=> $e$}.
\commentary{%
In particular, $e$ can have \emph{any} type when the return type is \VOID.%
}
\rationale{%
This enables concise declarations of \VOID{} functions.
It is reasonably easy to understand such a function,
because the return type is textually near to the returned expression $e$.
In contrast, \code{\RETURN{} $e$;} in a block body is only allowed
for an $e$ with one of a few specific static types,
because it is less likely that the developer understands
that the returned value will not be used
(\ref{return}).%
}
\item The function is asynchronous, \flatten{T} is not \VOID{},
and it would have been a compile-time error to declare the function with the body
\code{\ASYNC{} \{ \RETURN{} $e$; \}}
rather than \code{\ASYNC{} => $e$}.
\commentary{%
In particular, $e$ can have \emph{any} type
when the flattened return type is \VOID,%
}
\rationale{%
and the rationale is similar to the synchronous case.%
}
\end{itemize}
\end{itemize}
\LMHash{}%
It is a compile-time error if an \ASYNC, \code{\ASYNC*} or \code{\SYNC*} modifier is attached to the body of a setter or constructor.
\rationale{
An asynchronous setter would be of little use, since setters can only be used in the context of an assignment (\ref{assignment}),
and an assignment expression always evaluates to the value of the assignment's right hand side.
If the setter actually did its work asynchronously,
one might imagine that one would return a future that resolved to the assignment's right hand side after the setter did its work.
An asynchronous constructor would, by definition, never return an instance of the class it purports to construct, but instead return a future.
Calling such a beast via \NEW{} would be very confusing.
If you need to produce an object asynchronously, use a method.
One could allow modifiers for factories.
A factory for \code{Future} could be modified by \ASYNC{}, a factory for \code{Stream} could be modified by \code{\ASYNC*} and a factory for \code{Iterable} could be modified by \code{\SYNC*}.
No other scenario makes sense because the object returned by the factory would be of the wrong type.
This situation is very unusual so it is not worth making an exception to the general rule for constructors in order to allow it.
}
\LMHash{}%
It is a compile-time error if the declared return type of a function marked \ASYNC{} is not a supertype of \code{Future<$T$>} for some type $T$.
It is a compile-time error if the declared return type of a function marked \code{\SYNC*} is not a supertype of \code{Iterable<$T$>} for some type $T$.
It is a compile-time error if the declared return type of a function marked \code{\ASYNC*} is not a supertype of \code{Stream<$T$>} for some type $T$.
\subsection{Function Declarations}
\LMLabel{functionDeclarations}
\LMHash{}%
A \Index{function declaration} is a function that is neither a member of a class nor a function literal.
Function declarations include exactly the following:
\IndexCustom{library functions}{function!library},
which are function declarations
%(including getters and setters)
at the top level of a library, and
\IndexCustom{local functions}{function!local},
which are function declarations declared inside other functions.
Library functions are often referred to simply as top-level functions.
\LMHash{}%
A function declaration consists of an identifier indicating the function's name, possibly prefaced by a return type.
The function name is followed by a signature and body.
For getters, the signature is empty.
The body is empty for functions that are external.
\LMHash{}%
The scope of a library function is the scope of the enclosing library.
The scope of a local function is described in section \ref{localFunctionDeclaration}.
In both cases, the name of the function is in scope in its formal parameter scope (\ref{formalParameters}).
\LMHash{}%
It is a compile-time error to preface a function declaration with the built-in identifier \STATIC{}.
\LMHash{}%
When we say that a function $f_1$ \Index{forwards} to another function $f_2$, we mean that invoking $f_1$ causes $f_2$ to be executed with the same arguments and/or receiver as $f_1$, and returns the result of executing $f_2$ to the caller of $f_1$, unless $f_2$ throws an exception, in which case $f_1$ throws the same exception.
Furthermore, we only use the term for synthetic functions introduced by the specification.
\subsection{Formal Parameters}
\LMLabel{formalParameters}
\LMHash{}%
Every non-getter function declaration includes a \Index{formal parameter list},
which consists of a list of required positional parameters (\ref{requiredFormals}),
followed by any optional parameters (\ref{optionalFormals}).
The optional parameters may be specified either as a set of named parameters or as a list of positional parameters, but not both.
\LMHash{}%
Some function declarations include a
\Index{formal type parameter list} (\ref{functions}),
in which case we say that it is a
\IndexCustom{generic function}{function!generic}.
A \IndexCustom{non-generic function}{function!non-generic}
is a function which is not generic.
\LMHash{}%
The \Index{formal parameter part} of a function declaration consists of the formal type parameter list, if any, and the formal parameter list.
\commentary{
The following kinds of functions cannot be generic:
Getters, setters, operators, and constructors.
}
\LMHash{}%
The formal type parameter list of a function declaration introduces
a new scope known as the function's
\IndexCustom{type parameter scope}{scope!type parameter}.
The type parameter scope of a generic function $f$ is enclosed in the scope where $f$ is declared.
Every formal type parameter introduces a type into the type parameter scope.
\LMHash{}%
If it exists, the type parameter scope of a function $f$ is the current scope for the signature of $f$, and for the formal type parameter list itself;
otherwise the scope where $f$ is declared is the current scope for the signature of $f$.
\commentary{
This means that formal type parameters are in scope in the bounds of parameter declarations,
allowing for so-called F-bounded type parameters like
\code{class C<X \EXTENDS{} Comparable<X>{}> \{ \ldots{} \}},
\noindent
and the formal type parameters are in scope for each other, allowing dependencies like
\code{class D<X \EXTENDS{} Y, Y> \{ \ldots{} \}}.
}
\LMHash{}%
The formal parameter list of a function declaration introduces a new scope known as the function's
\IndexCustom{formal parameter scope}{scope!formal parameter}.
The formal parameter scope of a non-generic function $f$ is enclosed in the scope where $f$ is declared.
The formal parameter scope of a generic function $f$ is enclosed in the type parameter scope of $f$.
Every formal parameter introduces a local variable into the formal parameter scope.
The current scope for the function's signature is the scope that encloses the formal parameter scope.
\commentary{
This means that in a generic function declaration,
the return type and parameter type annotations can use the formal type parameters,
but the formal parameters are not in scope in the signature.
}
\LMHash{}%
The body of a function declaration introduces a new scope known as the function's
\IndexCustom{body scope}{scope!function body}.
The body scope of a function $f$ is enclosed in the scope introduced by the formal parameter scope of $f$.
%The formal parameter scope of a function maps the name of each formal parameter $p$ to the value $p$ is bound to.
% The formal parameters of a function are processed in the enclosing scope of the function.
% \commentary{this means that the parameters themselves may not be referenced within the formal parameter list.}
\LMHash{}%
It is a compile-time error if a formal parameter is declared as a constant variable (\ref{variables}).
\begin{grammar}
<formalParameterList> ::= `(' `)'
\alt `(' <normalFormalParameters> `,'? `)'
\alt `(' <normalFormalParameters> `,' <optionalFormalParameters> `)'
\alt `(' <optionalFormalParameters> `)'
<normalFormalParameters> ::= \gnewline{}
<normalFormalParameter> (`,' <normalFormalParameter>)*
<optionalFormalParameters> ::= <optionalPositionalFormalParameters>
\alt <namedFormalParameters>
<optionalPositionalFormalParameters> ::= \gnewline{}
`[' <defaultFormalParameter> (`,' <defaultFormalParameter>)* `,'? `]'
<namedFormalParameters> ::= \gnewline{}
`{' <defaultNamedParameter> (`,' <defaultNamedParameter>)* `,'? `}'
\end{grammar}
Formal parameter lists allow an optional trailing comma after the last parameter (\syntax{`,'?}).
A parameter list with such a trailing comma is equivalent in all ways to the same parameter list without the trailing comma.
All parameter lists in this specification are shown without a trailing comma, but the rules and semantics apply equally to the corresponding parameter list with a trailing comma.
\subsubsection{Required Formals}
\LMLabel{requiredFormals}
\LMHash{}%
A \Index{required formal parameter} may be specified in one of three ways:
\begin{itemize}
\item By means of a function signature that names the parameter and describes its type as a function type (\ref{functionTypes}).
It is a compile-time error if any default values are specified in the signature of such a function type.% explain what the type is in this case? Where is this described in general?
\item As an initializing formal, which is only valid as a parameter to a generative constructor (\ref{generativeConstructors}). % do we need to say this, or anything more?
\item Via an ordinary variable declaration (\ref{variables}).
\end{itemize}
\begin{grammar}
<normalFormalParameter> ::= <functionFormalParameter>
\alt <fieldFormalParameter>
\alt <simpleFormalParameter>
<functionFormalParameter> ::= \gnewline{}
<metadata> \COVARIANT{}? <type>? <identifier> <formalParameterPart>
<simpleFormalParameter> ::= <declaredIdentifier>
\alt <metadata> \COVARIANT{}? <identifier>
<fieldFormalParameter> ::= \gnewline{}
<metadata> <finalConstVarOrType>? \THIS{} `.' <identifier> \gnewline{}
<formalParameterPart>?
\end{grammar}
\LMHash{}%
It is possible to include the modifier \COVARIANT{}
in some forms of parameter declarations.
The effect of doing this is described in a separate section
(\ref{covariantParameters}).
\commentary{
Note that the non-terminal \synt{normalFormalParameter} is also used
in the grammar rules for optional parameters,
which means that such parameters can also be covariant.
}
\LMHash{}%
It is a compile-time error if the modifier \COVARIANT{} occurs on a parameter of a function which is not an instance method, instance setter, or instance operator.
\subsubsection{Optional Formals}
\LMLabel{optionalFormals}
\LMHash{}%
Optional parameters may be specified and provided with default values.
\begin{grammar}
<defaultFormalParameter> ::= <normalFormalParameter> (`=' <expression>)?
<defaultNamedParameter> ::= <normalFormalParameter> (`=' <expression>)?
\alt <normalFormalParameter> ( `:' <expression>)?
\end{grammar}
The form \syntax{<normalFormalParameter> `:' <expression>}
is equivalent to the form
\syntax{<normalFormalParameter> `=' <expression>}.
The colon-syntax is included only for backwards compatibility.
It is deprecated and will be removed in a later version of the language specification.
\LMHash{}%
It is a compile-time error if the default value of an optional parameter is not a constant expression (\ref{constants}).
If no default is explicitly specified for an optional parameter an implicit default of \NULL{} is provided.
\LMHash{}%
It is a compile-time error if the name of a named optional parameter begins with an `_' character.
\rationale{
The need for this restriction is a direct consequence of the fact that naming and privacy are not orthogonal.
If we allowed named parameters to begin with an underscore, they would be considered private and inaccessible to callers from outside the library where it was defined.
If a method outside the library overrode a method with a private optional name, it would not be a subtype of the original method.
The static checker would of course flag such situations, but the consequence would be that adding a private named formal would break clients outside the library in a way they could not easily correct.
}
\subsubsection{Covariant Parameters}
\LMLabel{covariantParameters}
\LMHash{}%
Dart allows formal parameters of instance methods,
including setters and operators,
to be declared \COVARIANT{}.
\commentary{
The syntax for doing this is specified in an earlier section (\ref{requiredFormals}).
}
\LMHash{}%
It is a compile-time error if the modifier \COVARIANT{} occurs
in the declaration of a formal parameter of a function
which is not an instance method, an instance setter, or an operator.
\commentary{
As specified below, a parameter can also be covariant for other reasons.
The overall effect of having a covariant parameter $p$
in the signature of a given method $m$
is to allow the type of $p$ to be overridden covariantly,
which means that the type required at run time for a given actual argument
may be a proper subtype of the type which is known at compile time
at the call site.
}
\rationale{
This mechanism allows developers to explicitly request that
a compile-time guarantee which is otherwise supported
(namely: that an actual argument whose static type satisfies the requirement
will also do so at run time)
is replaced by dynamic type checks.
In return for accepting these dynamic type checks,
developers can use covariant parameters to express software designs
where the dynamic type checks are known (or at least trusted) to succeed,
based on reasoning that the static type analysis does not capture.
}
\LMHash{}%
Let $m$ be a method signature with formal type parameters
\List{X}{1}{s},
positional formal parameters \List{p}{1}{n},
and named formal parameters \List{q}{1}{k}.
Let $m'$ be a method signature with formal type parameters
\List{X'\!}{1}{s},
positional formal parameters \List{p'\!}{1}{n'},
and named formal parameters \List{q'\!}{1}{k'}.
%
Assume that $j \in 1 .. n'$, and $j \leq n$;
we say that $p'_j$ is the parameter in $m'$ that
\IndexCustom{corresponds}{parameter corresponds to parameter}
to the formal parameter $p_j$ in $m$.
Assume that $j \in 1 .. k'$ and $l \in 1 .. k$;
we say that $q'_j$ is the parameter in $m'$ that
\NoIndex{corresponds} to the formal parameter
$q_l$ in $m$ if $q'_j = q_l$.
%
Similarly, we say that the formal type parameter
$X'_j$ from $m'$
\NoIndex{corresponds} to the formal type parameter
$X_j$ from $m$, for all $j \in 1 .. s$.
\commentary{
This includes the case where $m$ respectively $m'$ has
optional positional parameters,
in which case $k = 0$ respectively $k' = 0$ must hold,
but we can have $n \not= n'$.
The case where the numbers of formal type parameters differ is not relevant.
}
% Being covariant is a property of a parameter of the interface of a class;
% this means that we only talk about the originating keyword \COVARIANT{}
% and the class that contains the relevant declaration when we detect for
% the first time that a given parameter is covariant. From that point and on
% it is "carried" along the subtype links associated with class interfaces,
% such that we can get it inductively from an indirect superinterface just
% by checking whether the direct superinterfaces "have" a method signature
% with the relevant name and a corresponding parameter, and then checking
% that parameter. The same approach is applicable for covariant-by-class.
\LMHash{}%
Let $C$ be a class that declares a method $m$ which has
a parameter $p$ whose declaration has the modifier \COVARIANT{};
in this case we say that the parameter $p$ is
\IndexCustom{covariant-by-declaration}{parameter!covariant-by-declaration}.
%
In this case the interface of $C$ has the method signature $m$,
and that signature has the parameter $p$;
we also say that the parameter $p$ in this method signature is
\NoIndex{covariant-by-declaration}.
%
Finally, the parameter $p$ of the method signature $m$
of the interface of a class $C$ is
\NoIndex{covariant-by-declaration}
if a direct superinterface of $C$
has an accessible method signature $m'$ with the same name as $m$,
which has a parameter $p'$ that corresponds to $p$,
such that $p'$ is covariant-by-declaration.
\LMHash{}%
Assume that $C$ is a generic class with formal type parameter declarations
\code{$X_1\ \EXTENDS\ B_1 \ldots,\ X_s\ \EXTENDS\ B_s$},
let $m$ be a declaration of an instance method in $C$
(which can be a method, a setter, or an operator),
let $p$ be a parameter declared by $m$, and
let $T$ be the declared type of $p$.
%
The parameter $p$ is
\IndexCustom{covariant-by-class}{parameter!covariant-by-class}
if, for any $j \in 1 .. s$,
$X_j$ occurs in a covariant or an invariant position in $T$.
%
In this case the interface of $C$ also has the method signature $m$,
and that signature has the parameter $p$;
we also say that the parameter $p$ in this method signature is
\NoIndex{covariant-by-class}.
Finally, the parameter $p$ of the method signature $m$
of the interface of the class $C$ is
\NoIndex{covariant-by-class}
if a direct superinterface of $C$
has an accessible method signature $m'$ with the same name as $m$,
which has a parameter $p'$ that corresponds to $p$,
such that $p'$ is covariant-by-class.
\LMHash{}%
A formal parameter $p$ is
\IndexCustom{covariant}{parameter!covariant}
if $p$ is covariant-by-declaration or $p$ is covariant-by-class.
\commentary{
It is possible for a parameter to be simultaneously
covariant-by-declaration and covariant-by-class.
Note that a parameter may be
covariant-by-declaration or covariant-by-class
based on a declaration in any direct or indirect superinterface,
including any superclass:
The definitions above propagate these properties
to an interface from each of its direct superinterfaces,
but they will in turn receive the property from their direct superinterfaces,
and so on.
}
\subsection{Type of a Function}
\LMLabel{typeOfAFunction}
\LMHash{}%
This section specifies the static type which is ascribed to
the function denoted by a function declaration,
and the dynamic type of the corresponding function object.
\LMHash{}%
In this specification,
the notation used to denote the type of a function,
that is, a \Index{function type},
follows the syntax of the language,
except that \EXTENDS{} is abbreviated to
\FunctionTypeExtends.
This means that every function type is of one of the forms
\FunctionTypePositionalStd{T_0}
\FunctionTypeNamedStd{T_0}
\noindent
where $T_0$ is the return type,
$X_j$ are the formal type parameters with bounds $B_j$, $j \in 1 .. s$,
$T_j$ are the formal parameter types for $j \in 1 .. n + k$,
and $x_{n+j}$ are the names of named parameters for $j \in 1 .. k$.
Non-generic function types are covered by the case $s = 0$,
where the type parameter declaration list
\code{<\ldots{}>}
as a whole is omitted.
%
Similarly, the optional brackets \code{[]} and \code{\{\}} are omitted
when there are no optional parameters.
% We promise that the two forms always get the same treatment for k=0.
\commentary{
Both forms with optionals cover function types with no optionals when $k = 0$,
and every rule in this specification is such that
any of the two forms may be used without ambiguity
to determine the treatment of function types with no optionals.
}
\LMHash{}%
If a function declaration does not declare a return type explicitly,
its return type is \DYNAMIC{} (\ref{typeDynamic}),
unless it is a constructor,
in which case it is not considered to have a return type,
or it is a setter or operator \code{[]=},
in which case its return type is \VOID{}.
\LMHash{}%
A function declaration may declare formal type parameters.
The type of the function includes the names of the type parameters
and for each type parameter the upper bound,
which is considered to be the built-in class \code{Object} if no bound is specified.
When consistent renaming of type parameters can make two function types identical,
they are considered to be the same type.
\commentary{
It is convenient to include the formal type parameter names in function types
because they are needed in order to express such things as relations among
different type parameters, F-bounds, and the types of formal parameters.
However, we do not wish to distinguish between two function types if they have
the same structure and only differ in the choice of names.
This treatment of names is also known as alpha-equivalence.
}
\LMHash{}%
In the following three paragraphs,
if the number $m$ of formal type parameters is zero then
the type parameter list in the function type is omitted.
\LMHash{}%
Let $F$ be a function with
type parameters \TypeParametersStd,
required formal parameter types \List{T}{1}{n},
return type $T_0$,
and no optional parameters.
Then the static type of $F$ is
\FunctionTypeAllRequiredStd{T_0}.
\LMHash{}%
Let $F$ be a function with
type parameters \TypeParametersStd,
required formal parameter types \List{T}{1}{n},
return type $T_0$
and positional optional parameter types \List{T}{n+1}{n+k}.
Then the static type of $F$ is
\FunctionTypePositionalStd{T_0}.
\LMHash{}%
Let $F$ be a function with
type parameters \TypeParametersStd,
required formal parameter types \List{T}{1}{n},
return type $T_0$,
and named parameters \PairList{T}{x}{n+1}{n+k},
where $x_{n+j}$, $j \in 1 .. k$ may or may not have a default value.
Then the static type of $F$ is
\FunctionTypeNamedStd{T_0}.
\LMHash{}%
Let $T$ be the static type of a function declaration $F$.
Let $u$ be the run-time type of a function object $o$ obtained by
function closurization
(\ref{functionClosurization})
or instance method closurization
(\ref{ordinaryMemberClosurization})
applied to $F$,
and let $t$ be the actual type corresponding to $T$
at the occasion where $o$ was created
(\ref{actualTypeOfADeclaration}).
\commentary{$T$ may contain free type variables, but $t$ contains their actual values.}
The following must then hold:
$u$ is a class that implements the built-in class \FUNCTION{};
$u$ is a subtype of $t$;
and $u$ is not a subtype of any function type which is a proper subtype of $t$.
\commentary{%
If we had omitted the last requirement then
\code{f \IS{} int\,\FUNCTION([int])}
could evaluate to \TRUE{} with the declaration
\code{\VOID{} f()\,\{\}},
which is obviously not the intention.%
}
\rationale{
It is up to the implementation to choose
an appropriate representation for function objects.
For example, consider that
a function object produced via property extraction
treats equality differently from other function objects,
and is therefore likely a different class.
Implementations may also use different classes for function objects
based on arity and or type.
Arity may be implicitly affected by whether a function is
an instance method (with an implicit receiver parameter) or not.
The variations are manifold and, e.g.,
one cannot assume that any two distinct function objects
will necessarily have the same run-time type.
}
\subsection{External Functions}
\LMLabel{externalFunctions}
\LMHash{}%
An \IndexCustom{external function}{function!external}
is a function whose body is provided separately from its declaration.
An external function may be a top-level function (\ref{librariesAndScripts}), a method (\ref{instanceMethods}, \ref{staticMethods}), a getter (\ref{getters}), a setter (\ref{setters}) or a non-redirecting constructor (\ref{generativeConstructors}, \ref{factories}).
External functions are introduced via the built-in identifier \EXTERNAL{} (\ref{identifierReference}) followed by the function signature.
\rationale{
External functions allow us to introduce type information for code that is not statically known to the Dart compiler.
}
\commentary{
Examples of external functions might be foreign functions (defined in C, or Javascript etc.), primitives of the implementation (as defined by the Dart run-time system), or code that was dynamically generated but whose interface is statically known.
However, an abstract method is different from an external function,
as it has \emph{no} body.
}
\LMHash{}%
An external function is connected to its body by an implementation specific mechanism.
Attempting to invoke an external function that has not been connected to its body will throw a \code{NoSuchMethodError} or some subclass thereof.
\LMHash{}%
The actual syntax is given in sections \ref{classes} and \ref{librariesAndScripts} below.
\section{Classes}
\LMLabel{classes}
\LMHash{}%
A \Index{class} defines the form and behavior of a set of objects which are its
\IndexCustom{instances}{instance}.
Classes may be defined by class declarations as described below, or via mixin applications (\ref{mixinApplication}).
\begin{grammar}
<classDefinition> ::= <metadata> \ABSTRACT{}? \CLASS{} <identifier> <typeParameters>?
\gnewline{} <superclass>? <interfaces>?
\gnewline{} `{' (<metadata> <classMemberDefinition>)* `}'
\alt <metadata> \ABSTRACT{}? \CLASS{} <mixinApplicationClass>
<typeNotVoidList> ::= <typeNotVoid> (`,' <typeNotVoid>)*
<classMemberDefinition> ::= <declaration> `;'
\alt <methodSignature> <functionBody>
<methodSignature> ::= <constructorSignature> <initializers>?
\alt <factoryConstructorSignature>
\alt \STATIC{}? <functionSignature>
\alt \STATIC{}? <getterSignature>
\alt \STATIC{}? <setterSignature>
\alt <operatorSignature>
<declaration> ::= <constantConstructorSignature> (<redirection> | <initializers>)?
\alt <constructorSignature> (<redirection> | <initializers>)?
\alt \EXTERNAL{} <constantConstructorSignature>
\alt \EXTERNAL{} <constructorSignature>
\alt (\EXTERNAL{} \STATIC{}?)? <getterSignature>
\alt (\EXTERNAL{} \STATIC{}?)? <setterSignature>
\alt \EXTERNAL{}? <operatorSignature>
\alt (\EXTERNAL{} \STATIC{}?)? <functionSignature>
\alt \STATIC{} (\FINAL{} | \CONST{}) <type>? <staticFinalDeclarationList>
\alt \FINAL{} <type>? <initializedIdentifierList>
\alt (\STATIC{} | \COVARIANT{})? (\VAR{} | <type>) <initializedIdentifierList>
<staticFinalDeclarationList> ::= \gnewline{}
<staticFinalDeclaration> (`,' <staticFinalDeclaration>)*
<staticFinalDeclaration> ::= <identifier> `=' <expression>
\end{grammar}
\LMHash{}%
It is possible to include the modifier \COVARIANT{} in some forms of declarations.
The effect of doing this is described elsewhere
(\ref{covariantParameters}).
\LMHash{}%
A class has constructors, instance members and static members.
The \IndexCustom{instance members}{members!instance} of a class
are its instance methods, getters, setters and instance variables.
The \IndexCustom{static members}{members!static} of a class
are its static methods, getters, setters and class variables.
The \IndexCustom{members}{members} of a class
are its static and instance members.
\LMHash{}%
A class has several scopes:
\begin{itemize}
\item A \IndexCustom{type-parameter scope}{scope!type parameter},
which is empty if the class is not generic (\ref{generics}).
The enclosing scope of the type-parameter scope of a class is the enclosing scope of the class declaration.
\item A \IndexCustom{static scope}{scope!static}.
The enclosing scope of the static scope of a class is the type parameter scope (\ref{generics}) of the class.
\item An \IndexCustom{instance scope}{scope!instance}.
The enclosing scope of a class' instance scope is the class' static scope.
\end{itemize}
\LMHash{}%
The enclosing scope of an instance member declaration is the instance scope of the class in which it is declared.
\LMHash{}%
The enclosing scope of a static member declaration is the static scope of the class in which it is declared.
\LMHash{}%
Every class has a single superclass except class \code{Object} which has no superclass.
A class may implement a number of interfaces by declaring them in its implements clause (\ref{superinterfaces}).
\LMHash{}%
An \IndexCustom{abstract class declaration}{class declaration!abstract}
is a class declaration that is explicitly declared
with the \ABSTRACT{} modifier.
A \IndexCustom{concrete class declaration}{class declaration!concrete}
is a class declaration that is not abstract.
An \IndexCustom{abstract class}{class!abstract} is a class
whose declaration is abstract, and
a \IndexCustom{concrete class}{class!concrete} is a class
whose declaration is concrete.
\rationale{
We want different behavior for concrete classes and abstract classes.
If $A$ is intended to be abstract,
we want the static checker to warn about any attempt to instantiate $A$,
and we do not want the checker to complain about unimplemented methods in $A$.
In contrast, if $A$ is intended to be concrete,
the checker should warn about all unimplemented methods,
but allow clients to instantiate it freely.
}
\commentary{
The interface of a class $C$ is
an implicit interface that declares instance member signatures
that correspond to the instance members declared by $C$,
and whose direct superinterfaces are
the direct superinterfaces of $C$
(\ref{interfaces}, \ref{superinterfaces}).
}
\LMHash{}%
When a class name appears as a type,
that name denotes the interface of the class.
\LMHash{}%
% The use of 'concrete member' below may seem redundant, because a class
% does not inherit abstract members from its superclass, but this
% underscores the fact that even when an abstract declaration of $m$ is
% declared in $C$, $C$ does not "have" $m$.
A concrete class must fully implement its interface:
Let $C$ be a concrete class with interface $I$.
Assume that $I$ has an accessible member signature $m$.
It is a compile-time error if $C$ does not have
a concrete accessible member with the same name as $m$,
unless $C$ has a non-trivial \code{noSuchMethod}
(\ref{theMethodNoSuchMethod}).
It is a compile-time error if $C$ has
a concrete accessible member with the same name as $m$,
with a method signature $m'$ which is not a correct override of $m$
(\ref{correctMemberOverrides}),
unless that concrete member is a \code{noSuchMethod} forwarder
(\ref{theMethodNoSuchMethod}).
\commentary{
In particular, it is an error for a class to be concrete even if it inherits
a member implementation for every member signature in its interface,
unless each of them has parameters and types such that they satisfy
the corresponding member signature.
But when there is a non-trivial \code{noSuchMethod} it is allowed
to leave some members unimplemented,
and it is allowed to to have a \code{noSuchMethod} forwarder which does not
satisfy the class interface
(in which case it will be overridden by another \code{noSuchMethod} forwarder).
}
\commentary{
It is a compile-time error if a class declares two members of the same name,
either because it declares the same name twice in the same scope
(\ref{scoping}),
or because it declares a static member and an instance member
with the same name
(\ref{classMemberConflicts}).
}
\commentary{
Here are simple examples, that illustrate the difference between ``has a member'' and ``declares a member''.
For example, \code{B} \IndexCustom{declares}{declares member}
one member named \code{f},
but \IndexCustom{has}{has member} two such members.
The rules of inheritance determine what members a class has.
}
\begin{dartCode}
\CLASS{} A \{
\VAR{} i = 0;
\VAR{} j;
f(x) => 3;
\}
\\
\CLASS{} B \EXTENDS{} A \{
int i = 1; // \comment{getter i and setter i= override versions from A}
\STATIC{} j; // \comment{compile-time error: static getter \& setter conflict with}
// \comment{instance getter \& setter}
\\
// \comment{compile-time error: static method conflicts with instance method}
\STATIC{} f(x) => 3;
\}
\end{dartCode}
\LMHash{}%
It is a compile-time error if a class named $C$ declares
a member with basename (\ref{classMemberConflicts}) $C$.
If a generic class named $G$ declares a type variable named $X$,
it is a compile-time error
if $X$ is equal to $G$,
if $G$ has a member whose basename is $X$,
and if $G$ has a constructor named \code{$G$.$X$}.
\subsection{Instance Methods}
\LMLabel{instanceMethods}
\LMHash{}%
\IndexCustom{Instance methods}{method!instance}
are functions (\ref{functions})
whose declarations are immediately contained within a class declaration
and that are not declared \STATIC{}.
The \Index{instance methods of a class} $C$ are the instance methods declared by $C$
and the instance methods inherited by $C$ from its superclass
(\ref{inheritanceAndOverriding}).
\LMHash{}%
Consider a class $C$.
It is a compile-time error if an instance method declaration in $C$ has
a member signature $m$
(\ref{interfaces})
% Note that $m'$ is accessible, due to the definition of 'overrides'.
which overrides a member signature $m'$
from a direct superinterface of $C$
(\ref{interfaceInheritanceAndOverriding}),
unless this is a correct member override
(\ref{correctMemberOverrides}).
\commentary{
This is not the only kind of conflict that may exist:
An instance member declaration $D$ may conflict with another declaration $D'$,
even in the case where they do not have the same name
or they are not the same kind of declaration.
E.g., $D$ could be an instance getter and $D'$ a static setter
(\ref{classMemberConflicts}).
}
\LMHash{}%
For each parameter $p$ of $m$ where \COVARIANT{} is present,
it is a compile-time error if there exists
a direct or indirect superinterface $J$ of $C$ which has
an accessible method signature $m''$ with the same name as $m$,
such that $m''$ has a parameter $p''$ that corresponds to $p$
(\ref{covariantParameters}),
unless the type of $p$ is assignable to the type of $p''$.
\commentary{
This means that
a parameter which is covariant-by-declaration can have a type
which is a supertype or a subtype of the type of
a corresponding parameter in a superinterface,
but the two types cannot be unrelated.
Note that this requirement must be satisfied
for each direct or indirect superinterface separately,
because assignability is not transitive.
}
\rationale{
The superinterface may be the statically known type of the receiver,
so this means that we relax the potential typing relationship
between the statically known type of a parameter and the
type which is actually required at run time
to the assignability relationship,
rather than the strict supertype relationship
which applies to a parameter which is not covariant.
It should be noted that it is not statically known
at the call site whether any given parameter is covariant,
because the covariance could be introduced in
a proper subtype of the statically known type of the receiver.
We chose to give priority to flexibility rather than safety here,
because the whole point covariant parameters is that developers
can make the choice to increase the flexibility
in a trade-off where some static type safety is lost.
}
\subsubsection{Operators}
\LMLabel{operators}
\LMHash{}%
\IndexCustom{Operators}{operators} are instance methods with special names.
\begin{grammar}
<operatorSignature> ::= \gnewline{}
<type>? \OPERATOR{} <operator> <formalParameterList>
<operator> ::= `~'
\alt <binaryOperator>
\alt `[]'
\alt `[]='
<binaryOperator> ::= <multiplicativeOperator>
\alt <additiveOperator>
\alt <shiftOperator>
\alt <relationalOperator>
\alt `=='
\alt <bitwiseOperator>
\end{grammar}
\LMHash{}%
An operator declaration is identified using the built-in identifier (\ref{identifierReference}) \OPERATOR{}.
\LMHash{}%
The following names are allowed for user-defined operators:
\lit{<},
\lit{>},
\lit{<=},
\lit{>=},
\lit{==},
\lit{-},
\lit{+},
\lit{/},
\lit{\~{}/},
\lit{*},
\lit{\%},
\lit{|},
\lit{\^},
\lit{\&},
\lit{\ltlt},
\lit{\gtgt},
\lit{\gtgtgt},
\lit{[]=},
\lit{[]},
\lit{\~{}}.
\LMHash{}%
It is a compile-time error if the arity of the user-declared operator
\lit{[]=} is not 2.
It is a compile-time error if the arity of a user-declared operator with one of the names:
\lit{<},
\lit{>},
\lit{<=},
\lit{>=},
\lit{==},
\lit{-},
\lit{+},
\lit{\~{}/},
\lit{/},
\lit{*},
\lit{\%},
\lit{|},
\lit{\^},
\lit{\&},
\lit{\ltlt},
\lit{\gtgt},
\lit{\gtgtgt},
\lit{[]}
is not 1.
It is a compile-time error if the arity of the user-declared operator
\lit{-}
is not 0 or 1.
\commentary{
The \lit{-} operator is unique
in that two overloaded versions are permitted.
If the operator has no arguments, it denotes unary minus.
If it has an argument, it denotes binary subtraction.
}
\LMHash{}%
The name of the unary operator \lit{-} is \code{unary-}.
\rationale{
This device allows the two methods to be distinguished
for purposes of method lookup, override and reflection.
}
\LMHash{}%
It is a compile-time error if the arity of the user-declared operator
\lit{\~{}}
is not 0.
\LMHash{}%
It is a compile-time error to declare an optional parameter in an operator.
\LMHash{}%
It is a static warning if the return type of a user-declared operator
\lit{[]=}
is explicitly declared and not \VOID{}.
\commentary{
If no return type is specified for a user-declared operator
\lit{[]=},
its return type is \VOID{} (\ref{typeOfAFunction}).
}
\rationale{
The return type is \VOID{} because
a return statement in an implementation of operator
\lit{[]=}
does not return a value.
Consider a non-throwing evaluation of an expression $e$ of the form
\code{$e_1$[$e_2$] = $e_3$},
and assume that the evaluation of $e_3$ yields an instance $o$.
$e$ will then evaluate to $o$,
and even if the executed body of operator
\lit{[]=}
completes with a value $o'$,
that is, if $o'$ is returned, that value is simply ignored.
The rationale for this behavior is that assignments should be guaranteed to evaluate to the assigned value.
}
\subsubsection{The Method \code{noSuchMethod}}
\LMLabel{theMethodNoSuchMethod}
\LMHash{}%
The method \code{noSuchMethod} is invoked implicitly during execution
in situations where one or more member lookups fail
(\ref{ordinaryInvocation},
\ref{getterAccessAndMethodExtraction},
\ref{assignment}).
\commentary{
We may think of \code{noSuchMethod} as a backup
which kicks in when an invocation of a member $m$ is attempted,
but there is no member named $m$,
or it exists,
but the given invocation has an argument list shape
that does not fit the declaration of $m$
(passing fewer positional arguments than required or more than supported,
or passing named arguments with names not declared by $m$).
% The next sentence covers both function objects and instances of
% a class with a method named \code{call}, because we would have a
% compile-time error invoking \code{call} with a wrongly shaped argument
% list unless the type is \DYNAMIC{} or \FUNCTION.
This can only occur for an ordinary method invocation
when the receiver has static type \DYNAMIC,
or for a function invocation when
the invoked function has static type \FUNCTION{} or \DYNAMIC.
%
The method \code{noSuchMethod} can also be invoked in other ways, e.g.,
it can be called explicitly like any other method,
and it can be invoked from a \code{noSuchMethod} forwarder,
as explained below.
}
\LMHash{}%
We say that a class $C$ \Index{has a non-trivial \code{noSuchMethod}}
if $C$ has a concrete member named \code{noSuchMethod}
which is distinct from the one declared in the built-in class \code{Object}.
\commentary{
Note that it must be a method that accepts one positional argument,
in order to correctly override \code{noSuchMethod} in \code{Object}.
For instance, it can have signature
\code{noSuchMethod(Invocation i)} or
\code{noSuchMethod(Object i, [String s])},
but not
\code{noSuchMethod(Invocation i, String s)}.
This implies that the situation where \code{noSuchMethod} is invoked
(explicitly or implicitly)
with one actual argument cannot fail for the reason that
``there is no such method'',
such that we would enter an infinite loop trying to invoke \code{noSuchMethod}.
It \emph{is} possible, however, to encounter a dynamic error
during an invocation of \code{noSuchMethod}
because the actual argument fails to satisfy a type check,
but that situation will give rise to a dynamic type error
rather than a repeated attempt to invoke \code{noSuchMethod}
(\ref{bindingActualsToFormals}).
Here is an example where a dynamic type error occurs because
an attempt is made to pass an \code{Invocation}
where only the null object is accepted:
}
\begin{dartCode}
\CLASS{} A \{
noSuchMethod(\COVARIANT{} Null n) => n;
\}
\\
\VOID{} main() \{
\DYNAMIC{} d = A();
d.foo(42); // Dynamic type error when invoking noSuchMethod.
\}
\end{dartCode}
\LMHash{}%
Let $C$ be a concrete class and
let $L$ be the library that contains the declaration of $C$.
The member $m$ is \Index{noSuchMethod forwarded} in $C$ if{}f
one of the following is true:
\begin{itemize}
\item $C$ has a non-trivial \code{noSuchMethod},
the interface of $C$ contains $m$,
and $C$ has no concrete declaration of $m$
(\commentary{that is, no member $m$ is declared or inherited by $C$}).
\item
% Inaccessible private methods are not present in the interface of a class,
% so we need to find a class that can access $m$.
There exists a direct or indirect superinterface
$D$ of $C$ which is declared in the library $L_2$,
the interface of $D$ contains $m$
(\commentary{which implies that $m$ is accessible to $L_2$}),
$m$ is inaccessible to $L$,
and no superclass of $C$ has
a concrete declaration of $m$ accessible to $L_2$.
\end{itemize}
\LMHash{}%
For a concrete class $C$, a
\IndexCustom{\code{noSuchMethod} forwarder}{noSuchMethod forwarder}
is implicitly induced for each member $m$
which is noSuchMethod forwarded.
This is a concrete member of $C$
with the signature taken from the interface of $C$ respectively $D$ above,
and with the same default value for each optional parameter.
It can be invoked in an ordinary invocation and in a superinvocation,
and when $m$ is a method it can be closurized
(\ref{ordinaryMemberClosurization})
using a property extraction
(\ref{propertyExtraction}).
\commentary{
This implies that a \code{noSuchMethod} forwarder has the same
properties as an explicitly declared concrete member,
except of course that a \code{noSuchMethod} forwarder
does not prevent itself from being induced.
We do not specify the body of a \code{noSuchMethod} forwarder,
but it will invoke \code{noSuchMethod},
and we specify the dynamic semantics of executing it below.
}
\commentary{
At the beginning of this section we mentioned that implicit invocations
of \code{noSuchMethod} can only occur
with a receiver of static type \DYNAMIC{}
or a function of static type \DYNAMIC{} or \FUNCTION{}.
With a \code{noSuchMethod} forwarder,
\code{noSuchMethod} can also be invoked
on a receiver whose static type is not \DYNAMIC{}.
No similar situation exists for functions,
because it is impossible to induce a \code{noSuchMethod} forwarder
into the class of a function object.
}
\commentary{
For a concrete class $C$,
we may think of a non-trivial \code{noSuchMethod}
(declared in or inherited by $C$)
as a request for ``automatic implementation'' of all unimplemented members
in the interface of $C$ as \code{noSuchMethod} forwarders.
Similarly, there is an implicit request for
automatic implementation of all unimplemented
inaccessible members of any concrete class,
whether or not there is a non-trivial \code{noSuchMethod}.
Note that the latter cannot be written explicitly in Dart,
because their names are inaccessible;
but the language can still specify that they are induced implicitly,
because compilers control the treatment of private names.
}
\LMHash{}%
It is a compile-time error if a concrete class $C$ has
a \code{noSuchMethod} forwarded method signature $S$
for a method named $m$,
and a superclass of $C$ has an accessible concrete declaration of $m$
which is not a \code{noSuchMethod} forwarder.
\commentary{
This can only happen if that concrete declaration does not
correctly override $S$. Consider the following example:
}
\begin{dartCode}
\CLASS{} A \{
foo(int i) => \NULL;
\}
\ABSTRACT{} \CLASS{} B \{
foo([int i]);
\}
\CLASS{} C \EXTENDS{} A \IMPLEMENTS{} B \{
noSuchMethod(Invocation i) => ...;
// Error: Forwarder would override `A.foo`.
\}
\end{dartCode}
\commentary{
In this example,
an implementation with signature \code{foo(int i)} is inherited by \code{C},
and the superinterface \code{B} declares
the signature \code{foo([int i])}.
This is a compile-time error because \code{C} does not have
a method implementation with signature \code{foo([int])}.
We do not wish to implicitly induce
a \code{noSuchMethod} forwarder with signature \code{foo([int])}
because it would override \code{A.foo},
and that is likely to be highly confusing for developers.
%
In particular, it would cause an invocation like \code{C().foo(42)}
to invoke \code{noSuchMethod},
even though that is an invocation which is correct for
the declaration of \code{foo} in \code{A}.
%
Hence, we require developers to explicitly resolve the conflict
whenever an implicitly induced \code{noSuchMethod} forwarder
would override an explicitly declared inherited implementation.
%
It is no problem, however,
to let a \code{noSuchMethod} forwarder override
another \code{noSuchMethod} forwarder,
and hence there is no error in that situation.
}
\LMHash{}%
For the dynamic semantics,
assume that a class $C$ has an implicitly induced
\code{noSuchMethod} forwarder named $m$,
with formal type parameters
\code{$X_1,\ \ldots,\ X_r$},
positional formal parameters
\code{$a1,\ \ldots,\ a_k$}
(\commentary{some of which may be optional when $m = 0$}),
and named formal parameters with names
\code{$x_1,\ \ldots,\ x_m$}
(\commentary{with default values as mentioned above}).
\commentary{
For this purpose we need not distinguish between
a signature that has optional positional parameters and
a signature that has named parameters,
because the former is covered by $m = 0$.
}
\LMHash{}%
The execution of the body of $m$ creates
an instance $im$ of the predefined class \code{Invocation}
such that:
\begin{itemize}
\item \code{$im$.isMethod} evaluates to \code{\TRUE{}} if{}f $m$ is a method.
\item \code{$im$.isGetter} evaluates to \code{\TRUE{}} if{}f $m$ is a getter.
\item \code{$im$.isSetter} evaluates to \code{\TRUE{}} if{}f $m$ is a setter.
\item \code{$im$.memberName} evaluates to the symbol \code{m}.
\item \code{$im$.positionalArguments} evaluates to an unmodifiable list
with the same values as the list resulting from evaluation of
\code{<Object>[$a_1, \ldots,\ a_k$]}.
\item \code{$im$.namedArguments} evaluates to an unmodifiable map
with the same keys and values as the map resulting from evaluation of
\code{<Symbol, Object>\{$\#x_1$: $x_1, \ldots,\ \#x_m$: $x_m$\}}.
\item \code{$im$.typeArguments} evaluates to an unmodifiable list
with the same values as the list resulting from evaluation of
\code{<Type>[$X_1, \ldots,\ X_r$]}.
\end{itemize}
\LMHash{}%
Next, \code{noSuchMethod} is invoked with $i$ as the actual argument,
and the result obtained from there is returned by the execution of $m$.
\commentary{
This is an ordinary method invocation of \code{noSuchMethod}
(\ref{ordinaryInvocation}).
That is, a \code{noSuchMethod} forwarder in a class $C$ can invoke
an implementation of \code{noSuchMethod} that is declared in
a subclass of $C$.
Dynamic type checks on the actual arguments passed to $m$
are performed in the same way as for an invocation of an
explicitly declared method.
In particular, an actual argument passed to a covariant parameter
will be checked dynamically.
Also, like other ordinary method invocations,
it is a dynamic type error if the result returned by
a \code{noSuchMethod} forwarder has a type which is not a subtype
of the return type of the forwarder.
One special case to be aware of is where a forwarder is torn off
and then invoked with an actual argument list which does not match
the formal parameter list.
In that situation we will get an invocation of
\code{Object.noSuchMethod}
rather than the \code{noSuchMethod} in the original receiver,
because this is an invocation of a function object
(and they do not override \code{noSuchMethod}):
}
\begin{dartCode}
\CLASS{} A \{
noSuchMethod(Invocation i) => \NULL;
\VOID{} foo();
\}
\\
\VOID{} main() \{
A a = A();
\FUNCTION{} f = a.foo;
// Invokes `Object.noSuchMethod`, which throws.
f(42);
\}
\end{dartCode}
\subsubsection{The Operator `=='}
\LMLabel{theOperatorEqualsEquals}
\LMHash{}%
The operator \lit{==} is used implicitly in certain situations,
and in particular constant expressions
(\ref{constants})
give rise to constraints on that operator.
In order to specify these constraints just once we introduce the notion of a
% Neither \syntax nor \lit works, so we fall back to `\code{==}'.
\IndexCustom{primitive operator `\code{==}'}{%
operator `\code{==}'!primitive}:
\begin{itemize}
\item Every instance of type \code{int} and \code{String}
has a primitive operator \lit{==}.
\item Every instance of type \code{Symbol}
which was originally obtained by evaluation of a literal symbol or
a constant invocation of a constructor of the \code{Symbol} class
has a primitive operator \lit{==}.
\item Every instance of type \code{Type}
which was originally obtained by evaluating a constant type literal
(\ref{dynamicTypeSystem})
has a primitive operator \lit{==}.
\item An instance $o$ has a primitive operator \lit{==}
if the dynamic type of $o$ is a class $C$,
and $C$ has a primitive operator \lit{==}.
\item The class \code{Object} has a primitive operator \lit{==}.
\item A class $C$ has a primitive operator \lit{==}
if it does not have an implementation of the operator \lit{==}
that overrides the one inherited from \code{Object}.
\commentary{%
In particular, the following have a primitive operator \lit{==}:
The null object (\ref{null}),
function objects obtained by function closurization of
a static method or a top-level function
(\ref{functionClosurization}),
instances of type \code{bool}
(\ref{booleans}),
and instances obtained by evaluation of a list literal
(\ref{lists}),
a map literal
(\ref{maps}), or
a set literal
(\ref{sets}).
}
\end{itemize}
\LMHash{}%
When we say that the operator \lit{==} of a given instance or class
\IndexCustom{is not primitive}{operator `\code{==}'!is not primitive},
it means that it is not true that said instance or class
has a primitive operator \lit{==}.
\subsection{Getters}
\LMLabel{getters}
\LMHash{}%
Getters are functions (\ref{functions}) that are used to retrieve the values of object properties.
\begin{grammar}
<getterSignature> ::= <type>? \GET{} <identifier>
\end{grammar}
\LMHash{}%
If no return type is specified, the return type of the getter is \DYNAMIC{}.
\LMHash{}%
A getter definition that is prefixed with the \STATIC{} modifier defines a static getter.
Otherwise, it defines an instance getter.
The name of the getter is given by the identifier in the definition.
\LMHash{}%
The \Index{instance getters of a class} $C$ are
those instance getters declared by $C$,
either implicitly or explicitly,
and the instance getters inherited by $C$ from its superclass.
The \Index{static getters of a class} $C$ are
those static getters declared by $C$.
\commentary{
A getter declaration may conflict with other declarations
(\ref{classMemberConflicts}).
In particular, a getter can never override a method,
and a method can never override a getter or an instance variable.
The rules for when a getter correctly overrides another member
are given elsewhere
(\ref{correctMemberOverrides}).
}
\subsection{Setters}
\LMLabel{setters}
\LMHash{}%
Setters are functions (\ref{functions}) that are used to set the values of object properties.
\begin{grammar}
<setterSignature> ::= <type>? \SET{} <identifier> <formalParameterList>
\end{grammar}
\commentary{
If no return type is specified, the return type of the setter is \VOID{} (\ref{typeOfAFunction}).
}
\LMHash{}%
A setter definition that is prefixed with the \STATIC{} modifier defines a static setter.
Otherwise, it defines an instance setter.
The name of a setter is obtained by appending the string `=' to the identifier given in its signature.
\commentary{
Hence, a setter name can never conflict with, override or be overridden by a getter or method.
}
\LMHash{}%
The \Index{instance setters of a class} $C$ are
those instance setters declared by $C$
either implicitly or explicitly,
and the instance setters inherited by $C$ from its superclass.
The \Index{static setters of a class} $C$ are
those static setters declared by $C$,
either implicitly or explicitly.
\LMHash{}%
It is a compile-time error if a setter's formal parameter list
does not consist of exactly one required formal parameter $p$.
\rationale{
We could enforce this via the grammar, but we'd have to specify the evaluation rules in that case.
}
\LMHash{}%
It is a static warning if a setter declares a return type other than \VOID{}.
It is a static warning if a class has
a setter named \code{$v$=} with argument type $T$ and
a getter named $v$ with return type $S$,
and $S$ may not be assigned to $T$.
\commentary{
The rules for when a setter correctly overrides another member
are given elsewhere
(\ref{correctMemberOverrides}).
A setter declaration may conflict with other declarations as well
(\ref{classMemberConflicts}).
}
\subsection{Abstract Instance Members}
\LMLabel{abstractInstanceMembers}
\LMHash{}%
An \IndexCustom{abstract method}{method!abstract}
(respectively,
\IndexCustom{abstract getter}{getter!abstract} or
\IndexCustom{abstract setter}{setter!abstract})
is an instance method, getter or setter that is not declared \EXTERNAL{} and does not provide an implementation.
A \IndexCustom{concrete method}{method!concrete}
(respectively,
\IndexCustom{concrete getter}{getter!concrete} or
\IndexCustom{concrete setter}{setter!concrete})
is an instance method, getter or setter that is not abstract.
\rationale{
Abstract instance members are useful because of their interplay with classes.
Every Dart class induces an implicit interface,
and Dart does not support specifying interfaces explicitly.
Using an abstract class instead of a traditional interface
has important advantages.
An abstract class can provide default implementations.
It can also provide static methods,
obviating the need for service classes such as \code{Collections} or \code{Lists},
whose entire purpose is to group utilities related to a given type.
}
\commentary{
Invocation of an abstract method, getter, or setter cannot occur,
because lookup (\ref{lookup}) will never yield an abstract member as its result.
One way to think about this is that
an abstract member declaration in a subclass
does not override or shadow an inherited member implementation.
It only serves to specify the signature of the given member that
every concrete subtype must have an implementation of;
that is, it contributes to the interface of the class,
not to the class itself.
}
\rationale{
The purpose of an abstract method is to provide a declaration
for purposes such as type checking and reflection.
In mixins, it is often useful to introduce such declarations for methods that
the mixin expects will be provided by the superclass the mixin is applied to.
}
\rationale{
We wish to detect if one declares a concrete class with abstract members.
However, code like the following should work:
}
\begin{dartCode}
class Base \{
int get one => 1;
\}
\\
\ABSTRACT{} \CLASS{} Mix \{
int get one;
int get two => one + one;
\}
\\
\CLASS{} C extends Base with Mix \{ \}
\end{dartCode}
\rationale{
At run time, the concrete method \code{one} declared in \code{Base} will be executed, and no problem should arise.
Therefore no error should be raised if a corresponding concrete member exists in the hierarchy.
}
\subsection{Instance Variables}
\LMLabel{instanceVariables}
\LMHash{}%
\IndexCustom{Instance variables}{variables!instance}
are variables whose declarations
are immediately contained within a class declaration
and that are not declared \STATIC{}.
The \Index{instance variables of a class} $C$ are
the instance variables declared by $C$
and the instance variables inherited by $C$ from its superclass.
\LMHash{}%
It is a compile-time error if an instance variable is declared to be constant.
\rationale{
The notion of a constant instance variable is subtle and confusing to programmers.
An instance variable is intended to vary per instance.
A constant instance variable would have the same value for all instances, and as such is already a dubious idea.
The language could interpret const instance variable declarations as instance getters that return a constant.
However, a constant instance variable could not be treated as a true compile-time constant, as its getter would be subject to overriding.
Given that the value does not depend on the instance, it is better to use a static class variable.
An instance getter for it can always be defined manually if desired.
}
\LMHash{}%
It is possible for the declaration of an instance variable
to include the modifier \COVARIANT{}
(\ref{variables}).
The effect of this is that the formal parameter of
the corresponding implicitly induced setter
is considered to be covariant-by-declaration
(\ref{covariantParameters}).
\commentary{
The modifier \COVARIANT{} on an instance variable has no other effects.
In particular, the return type of the implicitly induced getter
can already be overridden covariantly without \COVARIANT{},
and it can never be overridden to a supertype or an unrelated type,
regardless of whether the modifier \COVARIANT{} is present.
}
\subsection{Constructors}
\LMLabel{constructors}
\LMHash{}%
A \Index{constructor} is a special function that is used in instance creation expressions (\ref{instanceCreation}) to obtain objects, typically by creating or initializing them.
Constructors may be generative (\ref{generativeConstructors}) or they may be factories (\ref{factories}).
\LMHash{}%
A \Index{constructor name} always begins with the name of its immediately enclosing class, and may optionally be followed by a dot and an identifier \id.
It is a compile-time error if the name of a constructor is not a constructor name.
\LMHash{}%
The
\IndexCustom{function type of a constructor}{function type!of a constructor}
$k$ is the function type
whose return type is the class that contains the declaration of $k$,
and whose formal parameter types, optionality, and names of named parameters
correspond to the declaration of $k$.
\commentary{
Note that the function type $F$ of a constructor $k$ may contain
type variables declared by the enclosing class $C$.
In that case we can apply a substitution to $F$, as in
$[T_1/X_1, \ldots, T_m/X_m]F$,
where $X_j, j \in 1 .. m$ are the formal type parameters of $C$
and $T_j, j \in 1 .. m$ are specified in the given context.
We may also omit such a substitution when the given context is
the instance scope of $C$, where $X_1, \ldots, X_m$ are in scope.
}
\commentary{
A constructor declaration may conflict with static member declarations
(\ref{classMemberConflicts}).
}
% In what scope do constructors go? The simple names of named constructors go in the static scope of the class. Unnamed ones go nowhere, but we use the class name to refer to them; the class name could also in the static scope of the class as well to prevent weird errors, or we could ban it explicitly and avoiding duplication. Similarly, the instance scope could contain the constructor names and class name, or we could have special rules to prevent collisions between instance members and constructors or the class.
% The enclosing scope of a generative constructor is the instance scope of the class in which it is declared (but what about redirecting?)
\LMHash{}%
If{}f no constructor is specified for a class $C$, it implicitly has a default constructor \code{C() : \SUPER{}() \{\}}, unless $C$ is class \code{Object}.
\subsubsection{Generative Constructors}
\LMLabel{generativeConstructors}
\LMHash{}%
A \IndexCustom{generative constructor}{constructor!generative}
declaration consists of a constructor name, a constructor parameter list,
and either a redirect clause or an initializer list and an optional body.
\begin{grammar}
<constructorSignature> ::= \gnewline{}
<identifier> (`.' <identifier>)? <formalParameterList>
\end{grammar}
\LMHash{}%
A \Index{constructor parameter list} is a parenthesized, comma-separated list of formal constructor parameters.
A \Index{formal constructor parameter} is either a formal parameter (\ref{formalParameters}) or an initializing formal.
An \Index{initializing formal} has the form \code{\THIS{}.\id}, where \id{} is the name of an instance variable of the immediately enclosing class.
It is a compile-time error if \id{} is not an instance variable of the immediately enclosing class.
It is a compile-time error if an initializing formal is used by a function other than a non-redirecting generative constructor.
\LMHash{}%
If an explicit type is attached to the initializing formal, that is its static type.
Otherwise, the type of an initializing formal named \id{} is $T_{id}$, where $T_{id}$ is the type of the instance variable named \id{} in the immediately enclosing class.
It is a compile-time error if the static type of \id{} is not a subtype of $T_{id}$.
\LMHash{}%
Initializing formals constitute an exception to the rule that
every formal parameter introduces a local variable into
the formal parameter scope (\ref{formalParameters}).
When the formal parameter list of a non-redirecting generative constructor
contains any initializing formals, a new scope is introduced, the
\IndexCustom{formal parameter initializer scope}{scope!formal parameter initializer},
which is the current scope of the initializer list of the constructor,
and which is enclosed in the scope where the constructor is declared.
Each initializing formal in the formal parameter list introduces a final local variable into the formal parameter initializer scope, but not into the formal parameter scope; every other formal parameter introduces a local variable into both the formal parameter scope and the formal parameter initializer scope.
\commentary{
This means that formal parameters, including initializing formals, must have distinct names, and that initializing formals are in scope for the initializer list, but they are not in scope for the body of the constructor.
When a formal parameter introduces a local variable into two scopes, it is still one variable and hence one storage location.
The type of the constructor is defined in terms of its formal parameters, including the initializing formals.
}
\LMHash{}%
Initializing formals are executed during
the execution of generative constructors detailed below.
Executing an initializing formal \code{\THIS{}.\id}
causes the instance variable \id{} of the immediately surrounding class
to be assigned the value of the corresponding actual parameter,
% This can occur due to a failing implicit cast.
unless the assigned value has a dynamic type
which is not a subtype of the declared type of the instance variable \id{},
in which case a dynamic error occurs.
\commentary{
The above rule allows initializing formals to be used as optional parameters:
}
\begin{dartCode}
class A \{
int x;
A([this.x]);
\}
\end{dartCode}
\commentary{
is legal, and has the same effect as
}
\begin{dartCode}
class A \{
int x;
A([int x]): this.x = x;
\}
\end{dartCode}
\LMHash{}%
A \Index{fresh instance} is an instance whose identity is distinct from any previously allocated instance of its class.
A generative constructor always operates on a fresh instance of its immediately enclosing class.
\commentary{
The above holds if the constructor is actually run, as it is by \NEW{}.
If a constructor $c$ is referenced by \CONST{}, $c$ may not be run; instead, a canonical object may be looked up.
See the section on instance creation (\ref{instanceCreation}).
}
\LMHash{}%
If a generative constructor $c$ is not a redirecting constructor and no body is provided, then $c$ implicitly has an empty body \code{\{\}}.
\paragraph{Redirecting Generative Constructors}
\LMLabel{redirectingGenerativeConstructors}
\LMHash{}%
A generative constructor may be
\IndexCustom{redirecting}{constructor!redirecting},
in which case its only action is to invoke another generative constructor.
A redirecting constructor has no body;
instead, it has a redirect clause that specifies which constructor the invocation is redirected to, and with which arguments.
\begin{grammar}
<redirection> ::= `:' \THIS{} (`.' <identifier>)? <arguments>
\end{grammar}
\def\ConstMetavar{\mbox{\CONST{}?}}
\LMHash{}%
Assume that
\code{$C$<$X_1\ \EXTENDS\ B_1 \ldots,\ X_m\ \EXTENDS\ B_m$>}
is the name and formal type parameters of the enclosing class,
$\ConstMetavar$ stands for either \CONST{} or nothing,
$N$ is $C$ or $C.\id_0$ for some identifier $\id_0$,
and \id{} is an identifier.
Consider a declaration of a redirecting generative constructor $k$ of one of the forms
\code{$\ConstMetavar$ $N$($T_1\ x_1 \ldots,\ T_n\ x_n,\ $[$T_{n+1}\ x_{n+1} = d_1 \ldots,\ T_{n+k}\ x_{n+k} = d_k$]): $R$;}
\code{$\ConstMetavar$ $N$($T_1\ x_1 \ldots,\ T_n\ x_n,\ $\{$T_{n+1}\ x_{n+1} = d_1 \ldots,\ T_{n+k}\ x_{n+k} = d_k$\}): $R$;}
\noindent
where $R$ is of one of the forms
\code{$\THIS{}$($e_1 \ldots,\ e_p,\ x_1$: $e_{p+1}, \ldots,\ x_q$: $e_{p+q}$)}
\code{$\THIS{}.\id$($e_1 \ldots,\ e_p,\ x_1$: $e_{p+1}, \ldots,\ x_q$: $e_{p+q}$)}
\LMHash{}%
The
\IndexCustom{redirectee constructor}{constructor!redirectee}
for this declaration is then the constructor denoted by
\code{$C$<$X_1 \ldots,\ X_m$>} respectively \code{$C$<$X_1 \ldots,\ X_m$>.\id}.
It is a compile-time error if the static argument list type (\ref{actualArgumentLists}) of
\code{($e_1 \ldots,\ e_p,\ x_1$: $e_{p+1}, \ldots,\ x_q$: $e_{p+q}$)}
is not an assignable match for the formal parameter list of the redirectee.
\commentary{
Note that the case where no named parameters are passed is covered by letting $q$ be zero,
and the case where $C$ is a non-generic class is covered by letting $m$ be zero,
in which case the formal type parameter list and actual type argument lists are omitted (\ref{generics}).
}
\rationale{
We require an assignable match rather than the stricter subtype match
because a generative redirecting constructor $k$ invokes its redirectee $k'$
in a manner which resembles function invocation in general.
For instance, $k$ could accept an argument \code{x}
and pass on an expression $e_j$ using \code{x} such as \code{x.f(42)} to $k'$,
and it would be surprising
if $e_j$ were subject to more strict constraints than the ones applied to
actual arguments to function invocations in general.
}
\LMHash{}%
When $\ConstMetavar$ is \CONST{},
it is a compile-time error if the redirectee is not a constant constructor.
Moreover, when $\ConstMetavar$ is \CONST{}, each
$e_i,\ i \in 1 .. p+q$,
must be a potentially constant expression (\ref{constantConstructors}).
\LMHash{}%
% This error can occur due to a failed implicit cast.
It is a dynamic type error if an actual argument passed
in an invocation of a redirecting generative constructor $k$
is not a subtype of the actual type (\ref{actualTypeOfADeclaration})
of the corresponding formal parameter in the declaration of $k$.
% This error can occur due to a failed implicit cast.
It is a dynamic type error if an actual argument passed
to the redirectee $k'$ of a redirecting generative constructor
is not a subtype of the actual type
(\ref{actualTypeOfADeclaration})
of the corresponding formal parameter in the declaration of the redirectee.
\paragraph{Initializer Lists}
\LMLabel{initializerLists}
\LMHash{}%
An initializer list begins with a colon, and consists of a comma-separated list of individual \Index{initializers}.
\commentary{
There are three kinds of initializers.
\begin{itemize}
\item[$\bullet$] A \emph{superinitializer} identifies a
\emph{superconstructor}\,---\,that is,
a specific constructor of the superclass.
Execution of the superinitializer causes
the initializer list of the superconstructor to be executed.
\item[$\bullet$] An \emph{instance variable initializer}
assigns a value to an individual instance variable.
\item[$\bullet$] An assertion.
\end{itemize}
}
\begin{grammar}
<initializers> ::= `:' <initializerListEntry> (`,' <initializerListEntry>)*
<initializerListEntry> ::= \SUPER{} <arguments>
\alt \SUPER{} `.' <identifier> <arguments>
\alt <fieldInitializer>
\alt <assertion>
<fieldInitializer> ::= \gnewline{}
(\THIS{} `.')? <identifier> `=' <conditionalExpression> <cascadeSection>*
\end{grammar}
\LMHash{}%
An initializer of the form \code{$v$ = $e$} is equivalent to
an initializer of the form \code{\THIS{}.$v$ = $e$},
both forms are called \Index{instance variable initializers}.
It is a compile-time error if the enclosing class does not declare an instance variable named $v$.
Otherwise, let $T$ be the static type of $v$.
It is a compile-time error unless the static type of $e$ is assignable to $T$.
\LMHash{}%
Consider a \Index{superinitializer} $s$ of the form
\code{\SUPER{}($a_1, \ldots,\ a_n,\ x_{n+1}: a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}
respectively
\code{\SUPER{}.\id($a_1, \ldots,\ a_n,\ x_{n+1}: a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}.
\noindent{}%
Let $S$ be the superclass of the enclosing class of $s$.
It is a compile-time error if class $S$ does not declare a generative constructor named $S$ (respectively \code{$S$.\id}).
Otherwise, the static analysis of $s$ is performed as specified in Section~\ref{bindingActualsToFormals},
as if \code{\SUPER{}} respectively \code{\SUPER{}.\id}
had had the function type of the denoted constructor,
%% TODO(eernst): The following is very imprecise, it just serves to remember
%% that we must specify how to deal with the type variables in that parameter
%% part. One thing that we weasel over is that the superclass may be a mixin
%% application.
and substituting the formal type variables of the superclass
for the corresponding actual type arguments passed to the superclass
in the header of the current class.
\LMHash{}%
Let $k$ be a generative constructor.
Then $k$ may include at most one superinitializer in its initializer list or a compile-time error occurs.
If no superinitializer is provided, an implicit superinitializer of the form \SUPER{}() is added at the end of $k$'s initializer list,
unless the enclosing class is class \code{Object}.
It is a compile-time error if a superinitializer appears in $k$'s initializer list at any other position than at the end.
It is a compile-time error if more than one initializer corresponding to a given instance variable appears in $k$'s initializer list.
It is a compile-time error if $k$'s initializer list contains an initializer for a variable that is initialized by means of an initializing formal of $k$.
It is a compile-time error if $k$'s initializer list contains an initializer for a final variable $f$ whose declaration includes an initialization expression.
It is a compile-time error if $k$ includes an initializing formal for a final variable $f$ whose declaration includes an initialization expression.
\LMHash{}%
Let $f$ be a final instance variable declared in
the immediately enclosing class.
A compile-time error occurs unless $f$ is initialized
by one of the following means:
\begin{itemize}
\item $f$ is declared by an initializing variable declaration.
\item $f$ is initialized by means of an initializing formal of $k$.
\item $f$ has an initializer in $k$'s initializer list.
\end{itemize}
\LMHash{}%
It is a compile-time error if $k$'s initializer list contains an initializer for a variable that is not an instance variable declared in the immediately surrounding class.
\commentary{
The initializer list may of course contain an initializer for any instance variable declared by the immediately surrounding class, even if it is not final.
}
\LMHash{}%
It is a compile-time error if a generative constructor of class \code{Object} includes a superinitializer.
\paragraph{Execution of Generative Constructors}
\LMLabel{executionOfGenerativeConstructors}
\LMHash{}%
Execution of a generative constructor $k$ of type $T$ to initialize a fresh instance $i$
is always done with respect to a set of bindings for its formal parameters
and the type parameters of the immediately enclosing class bound to
a set of actual type arguments of $T$, $t_1, \ldots, t_m$.
\commentary{
These bindings are usually determined by the instance creation expression that invoked the constructor (directly or indirectly).
However, they may also be determined by a reflective call.
}
\LMHash{}%
If $k$ is redirecting then its redirect clause has the form
\code{\THIS{}.$g$($a_1, \ldots,\ a_n,\ x_{n+1}$: $a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}
where $g$ identifies another generative constructor of the immediately surrounding class.
Then execution of $k$ to initialize $i$ proceeds by evaluating the argument list
\code{($a_1, \ldots,\ a_n,\ x_{n+1}$: $a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}
to an actual argument list $a$ of the form
\code{($o_1, \ldots,\ o_n,\ x_{n+1}$: $o_{n+1}, \ldots,\ x_{n+k}$: $o_{n+k}$)}
in an environment where the type parameters of the enclosing class are bound to
$t_1, \ldots, t_m$.
\LMHash{}%
Next, the body of $g$ is executed to initialize $i$ with respect to the bindings that map
the formal parameters of $g$ to the corresponding objects in the actual argument list $a$,
with \THIS{} bound to $i$,
and the type parameters of the immediately enclosing class bound to $t_1, \ldots, t_m$.
\LMHash{}%
Otherwise, $k$ is not redirecting.
Execution then proceeds as follows:
\LMHash{}%
The instance variable declarations of the immediately enclosing class
are visited in the order they appear in the program text.
For each such declaration $d$, if $d$ has the form
\code{\synt{finalConstVarOrType} $v$ = $e$; }
then $e$ is evaluated to an object $o$
and the instance variable $v$ of $i$ is bound to $o$.
\LMHash{}%
Any initializing formals declared in $k$'s parameter list are executed in the order they appear in the program text.
% In fact, this order is unobservable; this could be done any time prior to running the body, since
% these only effect \THIS{}.
Then, the initializers of $k$'s initializer list are executed to initialize $i$
in the order they appear in the program, as described below
(p.\,\pageref{executionOfInitializerLists}).
\rationale{
We could observe the order by side effecting external routines called.
So we need to specify the order.
}
\LMHash{}%
Then if any instance variable of $i$ declared by the immediately enclosing class
is not yet bound to an object,
all such variables are initialized with the null object (\ref{null}).
\LMHash{}%
Then, unless the enclosing class is \code{Object}, the explicitly specified or
implicitly added superinitializer (\ref{initializerLists}) is executed to
further initialize $i$.
\LMHash{}%
After the superinitializer has completed, the body of $k$ is executed in a scope where \THIS{} is bound to $i$.
\rationale{
This process ensures that no uninitialized final instance variable is ever seen by code.
Note that \THIS{} is not in scope on the right hand side of an initializer (see \ref{this}) so no instance method can execute during initialization:
an instance method cannot be directly invoked,
nor can \THIS{} be passed into any other code being invoked in the initializer.
}
\paragraph{Execution of Initializer Lists}
\LMLabel{executionOfInitializerLists}
\LMHash{}%
During the execution of a generative constructor to initialize an instance $i$,
execution of an initializer of the form \code{\THIS{}.$v$ = $e$}
proceeds as follows:
\LMHash{}%
First, the expression $e$ is evaluated to an object $o$.
Then, the instance variable $v$ of $i$ is bound to $o$.
% This error can occur due to an implicit cast.
It is a dynamic type error if the dynamic type of $o$ is not
a subtype of the actual type
(\ref{actualTypeOfADeclaration})
of the instance variable $v$.
\LMHash{}%
Execution of an initializer that is an assertion proceeds by executing the assertion (\ref{assert}).
\LMHash{}%
Consider a superinitializer $s$ of the form
\code{\SUPER{}($a_1, \ldots,\ a_n,\ x_{n+1}: a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}
respectively
\code{\SUPER{}.\id($a_1, \ldots,\ a_n,\ x_{n+1}$: $a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}.
\LMHash{}%
Let $C$ be the class in which $s$ appears and let $S$ be the superclass of $C$.
If $S$ is generic (\ref{generics}),
let $u_1, \ldots, u_p$ be the actual type arguments passed to $S$,
obtained by substituting $t_1, \ldots, t_m$
for the formal type parameters of $C$
in the superclass as specified in the header of $C$, and
$t_1, \ldots, t_m$
are the actual bindings of the type variables of $C$.
Let $k$ be the constructor declared in $S$ and named
$S$ respectively \code{$S$.\id}.
\LMHash{}%
Execution of $s$ proceeds as follows:
The argument list
\code{($a_1, \ldots,\ a_n,\ x_{n+1}$: $a_{n+1}, \ldots,\ x_{n+k}$: $a_{n+k}$)}
is evaluated to an actual argument list $a$ of the form
\code{($o_1, \ldots,\ o_n,\ x_{n+1}$: $o_{n+1}, \ldots,\ x_{n+k}$: $o_{n+k}$)}.
Then the body of the superconstructor $k$ is executed
in an environment where the formal parameters of $k$ are bound to
the corresponding actual arguments from $a$,
and the formal type parameters of $S$ are bound to $u_1, \ldots, u_p$.
\subsubsection{Factories}
\LMLabel{factories}
\LMHash{}%
A \IndexCustom{factory}{constructor!factory}
is a constructor prefaced by the built-in identifier
(\ref{identifierReference})
\FACTORY{}.
\begin{grammar}
<factoryConstructorSignature> ::= \gnewline{}
\FACTORY{} <identifier> (`.' <identifier>)? <formalParameterList>
\end{grammar}
%The enclosing scope of a factory constructor is the static scope \ref{} of the class in which it is declared.
\LMHash{}%
The return type of a factory whose signature is of the form \FACTORY{} $M$ or the form \FACTORY{} \code{$M$.\id} is $M$ if $M$ is not a generic type;
otherwise the return type is \code{$M$<$T_1, \ldots,\ T_n$>} where $T_1, \ldots, T_n$ are the type parameters of the enclosing class.
\LMHash{}%
It is a compile-time error if $M$ is not the name of the immediately enclosing class.
\LMHash{}%
% This error can occur due to an implicit cast.
It is a dynamic type error if a factory returns a non-null object
whose type is not a subtype of its actual
(\ref{actualTypeOfADeclaration})
return type.
\rationale{
It seems useless to allow a factory to return the null object (\ref{null}).
But it is more uniform to allow it, as the rules currently do.
}
\rationale{
Factories address classic weaknesses associated with constructors in other languages.
Factories can produce instances that are not freshly allocated: they can come from a cache.
Likewise, factories can return instances of different classes.
}
\paragraph{Redirecting Factory Constructors}
\LMLabel{redirectingFactoryConstructors}
\LMHash{}%
A \IndexCustom{redirecting factory constructor}{constructor!redirecting factory}
specifies a call to a constructor of another class that is to be used
whenever the redirecting constructor is called.
\begin{grammar}
<redirectingFactoryConstructorSignature> ::= \gnewline{}
\CONST{}? \FACTORY{} <identifier> (`.' <identifier>)? <formalParameterList> `='
\gnewline{} <typeNotVoid> (`.' <identifier>)?
\end{grammar}
Assume that
\code{$C$<$X_1\ \EXTENDS\ B_1 \ldots,\ X_m\ \EXTENDS\ B_m$>}
is the name and formal type parameters of the enclosing class,
$\ConstMetavar$ is \CONST{} or empty,
$N$ is $C$ or $C.\id_0$ for some identifier $\id_0$,
$T$ is a type name, and \id{} is an identifier,
then consider a declaration of a redirecting factory constructor $k$ of one of the forms
\begin{normativeDartCode}
$\ConstMetavar$ \FACTORY{}
$N$($T_1\ x_1 \ldots,\ T_n\ x_n,\ $[$T_{n+1}\ x_{n+1}$=$d_1, \ldots,\ T_{n+k}\ x_{n+k}$=$d_k$]) = $R$;
\\
$\ConstMetavar$ \FACTORY{}
$N$($T_1\ x_1 \ldots,\ T_n\ x_n,\ $\{$T_{n+1}\ x_{n+1}$=$d_1, \ldots,\ T_{n+k}\ x_{n+k}$=$d_k$\}) = $R$;
\end{normativeDartCode}
\noindent
where $R$ is of one of the forms
\code{$T$<$S_1 \ldots,\ S_p$>} or
\code{$T$<$S_1 \ldots,\ S_p$>.\id}.
\LMHash{}%
It is a compile-time error if $T$ does not denote
a class accessible in the current scope.
If $T$ does denote such a class $D$,
it is a compile-time error if $R$ does not denote a constructor.
% It is by induction sufficient to check for abstractness one level down,
% because it is an error on the redirectee if this occurs after multiple
% redirections:
Otherwise, it is a compile-time error
if $R$ denotes a generative constructor and $D$ is abstract.
Otherwise, the
\IndexCustom{redirectee constructor}{constructor!redirectee}
for this declaration is the constructor denoted by $R$.
\LMHash{}%
A redirecting factory constructor $q'$ is \Index{redirection-reachable}
from a redirecting factory constructor $q$ if{}f
$q'$ is the redirectee constructor of $q$,
or $q''$ is the redirectee constructor of $q$
and $q'$ is redirection-reachable from $q''$.
It is a compile-time error if a redirecting factory constructor
is redirection-reachable from itself.
\LMHash{}%
Let $\argumentList{T}$ be the static argument list type (\ref{actualArgumentLists})
\code{($T_1 \ldots,\ T_{n+k}$)}
when $k$ takes no named arguments, and
\code{($T_1 \ldots,\ T_n,\ T_{n+1}\ x_{n+1},\ \ldots,\ T_{n+k}\ x_{n+k}$)}
when $k$ takes some named arguments.
It is a compile-time error if $\argumentList{T}$
is not a subtype match for the formal parameter list of the redirectee.
\rationale{
We require a subtype match
(rather than the more forgiving assignable match
which is used with a generative redirecting constructor),
because a factory redirecting constructor $k$ always invokes
its redirectee $k'$ with
exactly the same actual arguments that $k$ received.
This means that a downcast on an actual argument
``between'' $k$ and $k'$
would either be unused because the actual argument has
the type required by $k'$,
or it would amount to a dynamic error which is simply delayed a single step.
}
\commentary{
Note that the non-generic case is covered by letting $m$ or $p$ or both be zero,
in which case the formal type parameter list of the class $C$
and/or the actual type argument list of the redirectee constructor is omitted (\ref{generics}).
}
\LMHash{}%
It is a compile-time error if $k$ explicitly specifies a default value for an optional parameter.
\rationale{%
Default values specified in $k$ would be ignored,
since it is the \emph{actual} parameters that are passed to $k'$.
Hence, default values are disallowed.%
}
\LMHash{}%
It is a compile-time error if a formal parameter of $k'$ has a default value
whose type is not a subtype of the type annotation
on the corresponding formal parameter in $k$.
\commentary{
Note that it is not possible to modify the arguments being passed to $k'$.
}
\rationale{
At first glance, one might think that ordinary factory constructors could simply create instances of other classes and return them, and that redirecting factories are unnecessary.
However, redirecting factories have several advantages:
\begin{itemize}
\item An abstract class may provide a constant constructor that utilizes the constant constructor of another class.
\item A redirecting factory constructor avoids the need for forwarders to repeat the formal parameters and their default values.
\end{itemize}
}
\LMHash{}%
It is a compile-time error if $k$ is prefixed with the \CONST{} modifier but $k'$ is not a constant constructor (\ref{constantConstructors}).
\LMHash{}%
Let $T_1, \ldots, T_m$ be the actual type arguments passed to $k'$
in the declaration of $k$.
Let $X_1, \ldots, X_m$ be the formal type arguments declared by
the class that contains the declaration of $k'$.
Let $F'$ be the function type of $k'$ (\ref{constructors}).
It is a compile-time error if $[T_1/X_1, \ldots, T_m/X_m]F'$
is not a subtype of the function type of $k$.
\commentary{
In the case where the two classes are non-generic
this is just a subtype check on the function types of the two constructors.
In general, this implies that the resulting object conforms to
the interface of the immediately enclosing class of $k$.
}
\LMHash{}%
For the dynamic semantics,
assume that $k$ is a redirecting factory constructor
and $k'$ is the redirectee of $k$.
\LMHash{}%
% This error can occur due to an implicit cast.
It is a dynamic type error if an actual argument passed in an invocation of $k$
is not a subtype of the actual type (\ref{actualTypeOfADeclaration})
of the corresponding formal parameter in the declaration of $k$.
\LMHash{}%
When the redirectee $k'$ is a factory constructor,
execution of $k$ amounts to execution of $k'$ with the actual arguments passed to $k$.
The result of the execution of $k'$ is the result of $k$.
\LMHash{}%
When the redirectee $k'$ is a generative constructor,
let $o$ be a fresh instance (\ref{generativeConstructors})
of the class that contains $k'$.
Execution of $k$ then amounts to execution of $k'$ to initialize $o$,
governed by the same rules as an instance creation expression (\ref{instanceCreation}).
If $k$ completed normally then the execution of $k'$ completes normally returning $o$,
otherwise $k'$ completes by throwing the exception and stack trace thrown by $k$.
\subsubsection{Constant Constructors}
\LMLabel{constantConstructors}
\LMHash{}%
A \IndexCustom{constant constructor}{constructor!constant}
may be used to create compile-time constant (\ref{constants}) objects.
A constant constructor is prefixed by the reserved word \CONST{}.
\begin{grammar}
<constantConstructorSignature> ::= \CONST{} <qualified> <formalParameterList>
\end{grammar}
%\commentary{Spell out subtleties: a constant constructor call within the initializer of a constant constructor is treated as a ordinary constructor call (a new), because the arguments cannot be assumed constant anymore. In practice, this means two versions are compiled and analyzed. One for new and one for const.}
\commentary{
All the work of a constant constructor must be handled via its initializers.
}
\LMHash{}%
It is a compile-time error if a constant constructor is declared by a class that has a mutable instance variable.
\commentary{
The above refers to both locally declared and inherited instance variables.
}
\LMHash{}%
It is a compile-time error if a constant constructor is declared by a class $C$ if any instance variable declared in $C$ is initialized with an expression that is not a constant expression.
\commentary{
A superclass of $C$ cannot declare such an initializer either, because it must necessarily declare constant constructor as well (unless it is \code{Object}, which declares no instance variables).
}
\LMHash{}%
The superinitializer that appears, explicitly or implicitly, in the initializer list of a constant constructor must specify a constant constructor of the superclass of the immediately enclosing class or a compile-time error occurs.
\LMHash{}%
Any expression that appears within the initializer list of a constant constructor must be a potentially constant expression, or a compile-time error occurs.
\LMHash{}%
A \Index{potentially constant expression} is an expression $e$ that could be a valid constant expression if all formal parameters of $e$'s immediately enclosing constant constructor were treated as compile-time constants of appropriate types, and where $e$ is also a valid expression if all the formal parameters are treated as non-constant variables.
\commentary{
The difference between a potentially constant expression and a constant expression (\ref{const}) deserves some explanation.
The key issue is how one treats the formal parameters of a constructor.
If a constant constructor is invoked from a constant object expression, the actual arguments will be required to be constant expressions.
Therefore, if we were assured that constant constructors were always invoked from constant object expressions, we could assume that the formal parameters of a constructor were compile-time constants.
However, constant constructors can also be invoked from ordinary instance creation expressions (\ref{new}), and so the above assumption is not generally valid.
Nevertheless, the use of the formal parameters of a constant constructor within the constructor is of considerable utility.
The concept of potentially constant expressions is introduced to facilitate limited use of such formal parameters.
Specifically, we allow the usage of the formal parameters of a constant constructor for expressions that involve built-in operators, but not for constant objects, lists and maps.
This allows for constructors such as:
}
\begin{dartCode}
\CLASS{} C \{
\FINAL{} x; \FINAL{} y; \FINAL{} z;
\CONST{} C(p, q): x = q, y = p + 100, z = p + q;
\}
\end{dartCode}
\commentary{
The assignment to \code{x} is allowed under the assumption that \code{q} is constant (even though \code{q} is not, in general a compile-time constant).
The assignment to \code{y} is similar, but raises additional questions.
In this case, the superexpression of \code{p} is \code{p + 100}, and it requires that \code{p} be a numeric constant expression for the entire expression to be considered constant.
The wording of the specification allows us to assume that \code{p} evaluates to an integer.
A similar argument holds for \code{p} and \code{q} in the assignment to \code{z}.
However, the following constructors are disallowed:
}
\begin{dartCode}
\CLASS{} D \{
\FINAL{} w;
\CONST{} D.makeList(p): w = \CONST{} [p]; // \comment{compile-time error}
\CONST{} D.makeMap(p): w = \CONST{} \{"help": q\}; // \comment{compile-time error}
\CONST{} D.makeC(p): w = \CONST{} C(p, 12); // \comment{compile-time error}
\}
\end{dartCode}
\commentary{
The problem is not that the assignments to \code{w} are not potentially constant; they are.
However, all these run afoul of the rules for constant lists (\ref{lists}), maps (\ref{maps}) and objects (\ref{const}), all of which independently require their subexpressions to be constant expressions.
}
\rationale{
All of the illegal constructors of \code{D} above could not be sensibly invoked via \NEW{}, because an expression that must be constant cannot depend on a formal parameter, which may or may not be constant.
In contrast, the legal examples make sense regardless of whether the constructor is invoked via \CONST{} or via \NEW{}.
Careful readers will of course worry about cases where the actual arguments to \code{C()} are constants, but are not numeric.
This is precluded by the following rule, combined with the rules for evaluating constant objects (\ref{const}).
}
\LMHash{}%
When a constant constructor $k$ is invoked from a constant object expression,
it is a compile-time error if
the invocation of $k$ at run time would throw an exception,
and it is a compile-time error if
substitution of the actual arguments for the formal parameters
yields an initializing expression $e$ in the initializer list of $k$
which is not a constant expression.
\commentary{
For instance, if $e$ is \code{a.length}
where \code{a} is a formal argument of $k$ with type \DYNAMIC{},
$e$ is potentially constant and can be used in the initializer list of $k$.
It is an error to invoke $k$ with an argument of type \code{C}
if \code{C} is a class different from \code{String},
even if \code{C} has a \code{length} getter,
and that same expression would evaluate without errors at run time.
}
%Discuss External Constructors in ne subsubsection here
\subsection{Static Methods}
\LMLabel{staticMethods}
\LMHash{}%
\IndexCustom{Static methods}{method!static}
are functions, other than getters or setters, whose declarations are immediately contained within a class declaration and that are declared \STATIC{}.
The static methods of a class $C$ are those static methods declared by $C$.
\rationale{
Inheritance of static methods has little utility in Dart.
Static methods cannot be overridden.
Any required static function can be obtained from its declaring library,
and there is no need to bring it into scope via inheritance.
Experience shows that developers are confused by
the idea of inherited methods that are not instance methods.
Of course, the entire notion of static methods is debatable,
but it is retained here because so many programmers are familiar with it.
Dart static methods may be seen as functions of the enclosing library.
}
\commentary{
Static method declarations may conflict with other declarations
(\ref{classMemberConflicts}).
}
\subsection{Superclasses}
\LMLabel{superclasses}
%% TODO(eernst): We need to say that the superclass which is obtained
%% by mixin application is generic when $C$ is generic, or at least
%% when one or more of $C$'s type variables are used by the classes
%% in the \EXTENDS{} or \WITH{} clause of $C$. It says below that
%% these clauses are in the type parameter scope of $C$, but that does
%% not allow us to talk about the superclass as an actual, stand-alone
%% class (unless we start defining nested classes, such that the
%% superclass can be declared in that scope).
\LMHash{}%
The superclass $S'$ of a class $C$ whose declaration has a with clause
\code{\WITH{} $M_1, \ldots,\ M_k$}
and an extends clause
\code{\EXTENDS{} $S$}
is the abstract class obtained by application of
mixin composition (\ref{mixins}) $M_k* \cdots * M_1$ to $S$.
The name $S'$ is a fresh identifier.
If no \WITH{} clause is specified then the \EXTENDS{} clause of
a class $C$ specifies its superclass.
If no \EXTENDS{} clause is specified, then either:
\begin{itemize}
\item $C$ is \code{Object}, which has no superclass. OR
\item Class $C$ is deemed to have an \EXTENDS{} clause of the form
\code{\EXTENDS{} Object}, and the rules above apply.
\end{itemize}
\LMHash{}%
It is a compile-time error to specify an \EXTENDS{} clause
for class \code{Object}.
\begin{grammar}
<superclass> ::= \EXTENDS{} <typeNotVoid> <mixins>?
\alt <mixins>
<mixins> ::= \WITH{} <typeNotVoidList>
\end{grammar}
%The superclass clause of a class C is processed within the enclosing scope of the static scope of C.
%\commentary{
%This means that in a generic class, the type parameters of the generic are available in the superclass clause.
%}
\LMHash{}%
The scope of the \EXTENDS{} and \WITH{} clauses of a class $C$ is the type-parameter scope of $C$.
\LMHash{}%
It is a compile-time error if the type in the \EXTENDS{} clause of a class $C$ is
a type variable (\ref{generics}), a type alias that does not denote a class (\ref{typedef}),
an enumerated type (\ref{enums}),
a deferred type (\ref{staticTypes}), type \DYNAMIC{} (\ref{typeDynamic}),
or type \code{FutureOr<$T$>} for any $T$ (\ref{typeFutureOr}).
\commentary{%
Note that \VOID{} is a reserved word,
which implies that the same restrictions apply for the type \VOID,
and similar restrictions are specified for other types like
\code{Null} (\ref{null}) and
\code{String} (\ref{strings}).%
}
\commentary{%
The type parameters of a generic class are available in the lexical scope of the superclass clause, potentially shadowing classes in the surrounding scope.
The following code is therefore illegal and should cause a compile-time error:
}
\begin{dartCode}
class T \{\}
\\
/* Compilation error: Attempt to subclass a type parameter */
class G<T> extends T \{\}
\end{dartCode}
\LMHash{}%
%% TODO(eernst): Consider replacing all occurrences of `a superclass`
%% by `a direct or indirect superclass`, because it's too confusing.
A class $S$ is a \Index{superclass} of a class $C$ if{}f either:
\begin{itemize}
\item $S$ is the superclass of $C$, or
\item $S$ is a superclass of a class $S'$,
and $S'$ is the superclass of $C$.
\end{itemize}
\LMHash{}%
It is a compile-time error if a class $C$ is a superclass of itself.
\subsubsection{Inheritance and Overriding}
\LMLabel{inheritanceAndOverriding}
\LMHash{}%
Let $C$ be a class, let $A$ be a superclass of $C$, and
let $S_1, \ldots, S_k$ be superclasses of $C$ that are also subclasses of $A$.
$C$ \Index{inherits} all concrete, accessible instance members of $A$
that have not been overridden by a concrete declaration in $C$ or in at least one of $S_1, \ldots, S_k$.
\rationale{
It would be more attractive to give a purely local definition of inheritance, that depended only on the members of the direct superclass $S$.
However, a class $C$ can inherit a member $m$ that is not a member of its superclass $S$.
This can occur when the member $m$ is private to the library $L_1$ of $C$,
whereas $S$ comes from a different library $L_2$,
but the superclass chain of $S$ includes a class declared in