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Feature: Generic Method Syntax

Author: eernst@

Status: Background material. The normative text on this topic is part of the language specification as of 673d5f0.

This document is an informal specification of the support in Dart 1.x for generic methods and functions which includes syntax and name resolution, but not reification of type arguments.

The motivation for having this feature is that it enables partial support for generic methods and functions, thus providing a bridge between not having generic methods and having full support for generic methods. In particular, code declaring and using generic methods may be type checked and compiled in strong mode, and the same code will now be acceptable in standard (non-strong) mode as well. The semantics is different in certain cases, but standard mode analysis will emit diagnostic messages (e.g., errors) for that.

In this document, the word routine will be used when referring to an entity which can be a non-operator method declaration, a top level function declaration, a local function declaration, or a function literal expression. Depending on the context, the word routine may also denote the semantic entity associated with such a declaration, e.g., a closure corresponding to a function literal.

With this feature it is possible to compile code where generic methods and functions are declared, implemented, and invoked. The runtime semantics does not include reification of type arguments. Usages of the runtime value of a routine type parameter is a runtime error or yields dynamic, depending on the context. No type checking takes place at usages of a method or function type parameter in the body, and no type checking regarding explicitly specified or omitted type arguments takes place at call sites.

In short, generic methods and functions are supported syntactically, and the runtime semantics prevents dynamic usages of the type argument values, but it allows all usages where that dynamic value is not required. For instance, a generic routine type parameter, T, cannot be used in an expression like x is T, but it can be used as a type annotation. In a context where other tools may perform type checking, this allows for a similar level of expressive power as do language designs where type arguments are erased at compile time.

The motivation for this document is that it serves as an informal specification for the implementation of support for the generic method syntax feature in all Dart tools.

Syntax

The syntactic elements which are added or modified in order to support this feature are as follows, based on grammar rules given in the Dart Language Specification (Aug 19, 2015).

formalParameterPart:
  typeParameters? formalParameterList
functionSignature:
  metadata returnType? identifier formalParameterPart
typeParameter:
  metadata identifier ('extends' type)?
functionExpression:
  formalParameterPart functionBody
fieldFormalParameter:
  metadata finalConstVarOrType? 'this' '.' identifier
  formalParameterPart?
argumentPart:
  typeArguments? arguments
selector:
  assignableSelector | argumentPart
assignableExpression:
  primary (argumentPart* assignableSelector)+ |
  'super' unconditionalAssignableSelector |
  identifier
cascadeSection:
  '..' (cascadeSelector argumentPart*)
  (assignableSelector argumentPart*)*
  (assignmentOperator expressionWithoutCascade)?

In a draft specification of generic methods from June 2015, the number of grammar changes is significantly higher, but that form can be obtained via renaming.

This extension to the grammar gives rise to an ambiguity where the same tokens may be angle brackets of a type argument list as well as relational operators. For instance, foo(a<b,c>(d))[^1] may be parsed as
a postfixExpression on the form primary arguments where the arguments are two relational expressions (a<b and c>(d)), and it may also be parsed such that there is a single argument which is an invocation of a generic function (a<b,c>(d)). The ambiguity is resolved in favor of the latter.

This is a breaking change, because existing code could include expressions like foo(a < b, c > (d)) where foo receives two arguments. That expression will now be parsed as an invocation of foo with one argument. It is unlikely that this will introduce bugs silently, because the new parsing is likely to incur diagnostic messages at compile-time.

We chose to favor the generic function invocation over the relational expression because it is considered to be a rare exception that this ambiguity arises: It requires a balanced set of angle brackets followed by a left parenthesis, which is already an unusual form. On top of that, the style guide recommendation to use named parameters for boolean arguments helps making this situation even less common.

If it does occur then there is an easy workaround: an extra set of parentheses (as in foo(a<b,(2>(d)))) will resolve the ambiguity in the direction of relational expressions; or we might simply be able to remove the parentheses around the last expression (as in foo(a<b,2>d)), which will also eliminate the ambiguity.

It should be noted that parsing techniques like recursive descent seem to conflict with this approach to disambiguation: Determining whether the remaining input starts with a balanced expression on the form < .. > seems to imply a need for unbounded lookahead. However, if some type of parsing is used where bracket tokens are matched up during lexical analysis then it takes only a simple O(1) operation in the parser to perform a check which will very frequently resolve the ambiguity.

Scope of the Mechanism

With the syntax in place, it is obvious that certain potential extensions have not been included.

For instance, constructors, setters, getters, and operators cannot be declared as generic: The syntax for passing actual type arguments at invocation sites for setters, getters, and operators is likely to be unwieldy and confusing, and for constructors there is a need to find a way to distinguish between type arguments for the new instance and type arguments for the constructor itself. However, there are plans to add support for generic constructors.

This informal specification specifies a dynamic semantics where the values of actual type arguments are not reified at run time. A future extension of this mechanism may add this reification, such that dynamic type tests and type casts involving routine type variables will be supported.

Resolution and Type Checking

In order to be useful, the support for generic methods and functions must be sufficiently complete and consistent to avoid spurious diagnostic messages. In particular, even though no regular type checks take place at usages of routine type parameters in the body where they are in scope, those type parameters should be resolved. If they had been ignored then any usage of a routine type parameter X would give rise to a Cannot resolve type X error message, or the usage might resolve to other declarations of X in enclosing scopes such as a class type parameter, both of which is unacceptable.

In dart2js resolution, the desired behavior has been achieved by adding a new type parameter scope and putting the type parameters into that scope, giving each of them the bound dynamic. The type parameter scope is the current scope during resolution of the routine signature and the type parameter bounds, it encloses the formal parameter scope of the routine, and the formal parameter scope in turn encloses the body scope.

This implies that every usage of a routine type parameter is treated during type checking as if it had been an alias for the type dynamic.

Static checks for invocations of methods or functions where type arguments are passed are omitted entirely: The type arguments are parsed, but no checks are applied to certify that the given routine accepts type arguments, and no checks are applied for bound violations. Similarly, no checks are performed for invocations where no type arguments are passed, whether or not the given routine is statically known to accept type arguments.

Certain usages of a routine type parameter X give rise to errors: It is a compile-time error if X is used as a type literal expression (e.g., foo(X)), or in an expression on the form e is X or e is! X, or in a try/catch statement like .. on T catch ...

It could be argued that it should be a warning or an error if a routine type parameter X is used in an expression on the form e as X. The blind success of this test at runtime may introduce bugs into correct programs in situations where the type constraint is violated; in particular, this could cause "wrong" objects to propagate through local variables and parameters and even into data structures (say, when a List<T> is actually a List<dynamic>, because T is not present at runtime when the list is created). However, considering that these type constraint violations are expected to be rare, and considering that it is common to require that programs compile without warnings, we have chosen to omit this warning. A tool is still free to emit a hint, or in some other way indicate that there is an issue.

Dynamic semantics

If a routine invocation specifies actual type arguments, e.g., int in the invocation f<int>(42), those type arguments will not be evaluated at runtime, and they will not be passed to the routine in the invocation. Similarly, no type arguments are ever passed to a generic routine due to call-site inference. This corresponds to the fact that the type arguments have no runtime representation.

When the body of a generic routine is executed, usages of the formal type parameters will either result in a run-time error, or they will yield the type dynamic, following the treatment of malformed types in Dart. There are the following cases:

When X is a routine type parameter, the evaluation of e is X, e is! X, and X used as an expression proceeds as if X had been a malformed type, producing a dynamic error; the evaluation of e as X has the same outcome as the evaluation of e.

Note that the forms containing is are compile-time errors, which means that compilers may reject the program or offer ways to compile the program with a different runtime semantics for these expressions. The rationale for dart2js allowing the construct and compiling it to a run time error is that (1) this allows more programs using generic methods to be compiled, and (2) an is expression that blindly returns true every time (or false every time) may silently introduce a bug into an otherwise correct program, so the expression must fail if it is ever evaluated.

When X is a routine type parameter which is passed as a type argument to a generic class instantiation G, it is again treated like a malformed type, i.e., it is considered to denote the type dynamic.

This may be surprising, so let us consider a couple of examples: When X is a routine type parameter, 42 is X raises a dynamic error, <int>[42] is List<X> yields the value true, and 42 as X yields 42, no matter whether the syntax for the invocation of the routine included an actual type argument, and, if so, no matter which value the actual type argument would have had at the invocation.

Object construction is similar: When X is a routine type parameter which is a passed as a type argument in a constructor invocation, the actual value of the type type argument will be the type dynamic, as it would have been with a malformed type.

In checked mode, when X is a routine type parameter, no checked mode checks will ever fail for initialization or assignment to a local variable or parameter whose type annotation is X, and if the type annotation is a generic type G that contains X, checked mode checks will succeed or fail as if X had been the type dynamic. Note that this differs from the treatment of malformed types.

Changes

2017-Jan-04: Changed 'static error' to 'compile-time error', which is the phrase that the language specification uses.

Notes

[^1]: These expressions violate the common style in Dart with respect to spacing and capitalization. That is because the ambiguity implies conflicting requirements, and we do not want to bias the appearance in one of the two directions.

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