manual.txt

=============
Nimrod Manual
=============

:Author: Andreas Rumpf
:Version: |nimrodversion|

.. contents::


  "Complexity" seems to be a lot like "energy": you can transfer it from the end
  user to one/some of the other players, but the total amount seems to remain
  pretty much constant for a given task. -- Ran

About this document
===================

**Note**: This document is a draft! Several of Nimrod's features need more
precise wording. This manual will evolve into a proper specification some
day.

This document describes the lexis, the syntax, and the semantics of Nimrod.

The language constructs are explained using an extended BNF, in
which ``(a)*`` means 0 or more ``a``'s, ``a+`` means 1 or more ``a``'s, and
``(a)?`` means an optional *a*; an alternative spelling for optional parts is
``[a]``. The ``|`` symbol is used to mark alternatives
and has the lowest precedence. Parentheses may be used to group elements.
Non-terminals start with a lowercase letter, abstract terminal symbols are in
UPPERCASE. Verbatim terminal symbols (including keywords) are quoted
with ``'``. An example::

  ifStmt ::= 'if' expr ':' stmts ('elif' expr ':' stmts)* ['else' stmts]

Other parts of Nimrod - like scoping rules or runtime semantics are only
described in an informal manner. The reason is that formal semantics are
difficult to write and understand. However, there is only one Nimrod
implementation, so one may consider it as the formal specification;
especially since the compiler's code is pretty clean (well, some parts of it).


Definitions
===========

A Nimrod program specifies a computation that acts on a memory consisting of
components called `locations`:idx:. A variable is basically a name for a
location. Each variable and location is of a certain `type`:idx:. The
variable's type is called `static type`:idx:, the location's type is called
`dynamic type`:idx:. If the static type is not the same as the dynamic type,
it is a super-type or subtype of the dynamic type.

An `identifier`:idx: is a symbol declared as a name for a variable, type,
procedure, etc. The region of the program over which a declaration applies is
called the `scope`:idx: of the declaration. Scopes can be nested. The meaning
of an identifier is determined by the smallest enclosing scope in which the
identifier is declared.

An expression specifies a computation that produces a value or location.
Expressions that produce locations are called `l-values`:idx:. An l-value
can denote either a location or the value the location contains, depending on
the context. Expressions whose values can be determined statically are called
`constant expressions`:idx:; they are never l-values.

A `static error`:idx: is an error that the implementation detects before
program execution. Unless explicitly classified, an error is a static error.

A `checked runtime error`:idx: is an error that the implementation detects
and reports at runtime. The method for reporting such errors is via *raising
exceptions*. However, the implementation provides a means to disable these
runtime checks. See the section pragmas_ for details.

An `unchecked runtime error`:idx: is an error that is not guaranteed to be
detected, and can cause the subsequent behavior of the computation to
be arbitrary. Unchecked runtime errors cannot occur if only `safe`:idx:
language features are used.


Lexical Analysis
================

Encoding
--------

All Nimrod source files are in the UTF-8 encoding (or its ASCII subset). Other
encodings are not supported. Any of the standard platform line termination
sequences can be used - the Unix form using ASCII LF (linefeed), the Windows
form using the ASCII sequence CR LF (return followed by linefeed), or the old
Macintosh form using the ASCII CR (return) character. All of these forms can be
used equally, regardless of platform.


Indentation
-----------

Nimrod's standard grammar describes an `indentation sensitive`:idx: language.
This means that all the control structures are recognized by indentation.
Indentation consists only of spaces; tabulators are not allowed.

The terminals ``IND`` (indentation), ``DED`` (dedentation) and ``SAD``
(same indentation) are generated by the scanner, denoting an indentation.

These terminals are only generated for lines that are not empty.

The parser and the scanner communicate over a stack which indentation terminal
should be generated: the stack consists of integers counting the spaces. The
stack is initialized with a zero on its top. The scanner reads from the stack:
If the current indentation token consists of more spaces than the entry at the
top of the stack, a ``IND`` token is generated, else if it consists of the same
number of spaces, a ``SAD`` token is generated. If it consists of fewer spaces,
a ``DED`` token is generated for any item on the stack that is greater than the
current. These items are later popped from the stack by the parser. At the end
of the file, a ``DED`` token is generated for each number remaining on the
stack that is larger than zero.

Because the grammar contains some optional ``IND`` tokens, the scanner cannot
push new indentation levels. This has to be done by the parser. The symbol
``indPush`` indicates that an ``IND`` token is expected; the current number of
leading spaces is pushed onto the stack by the parser. The symbol ``indPop``
denotes that the parser pops an item from the indentation stack. No token is
consumed by ``indPop``.


Comments
--------

`Comments`:idx: start anywhere outside a string or character literal with the
hash character ``#``.
Comments consist of a concatenation of `comment pieces`:idx:. A comment piece
starts with ``#`` and runs until the end of the line. The end of line characters
belong to the piece. If the next line only consists of a comment piece which is
aligned to the preceding one, it does not start a new comment:

.. code-block:: nimrod

  i = 0     # This is a single comment over multiple lines belonging to the
            # assignment statement. The scanner merges these two pieces.
  # This is a new comment belonging to the current block, but to no particular
  # statement.
  i = i + 1 # This a new comment that is NOT
  echo(i)   # continued here, because this comment refers to the echo statement

Comments are tokens; they are only allowed at certain places in the input file
as they belong to the syntax tree! This feature enables perfect source-to-source
transformations (such as pretty-printing) and superior documentation generators.
A nice side-effect is that the human reader of the code always knows exactly
which code snippet the comment refers to.


Identifiers & Keywords
----------------------

`Identifiers`:idx: in Nimrod can be any string of letters, digits
and underscores, beginning with a letter. Two immediate following
underscores ``__`` are not allowed::

  letter ::= 'A'..'Z' | 'a'..'z' | '\x80'..'\xff'
  digit ::= '0'..'9'
  IDENTIFIER ::= letter ( ['_'] (letter | digit) )*

The following `keywords`:idx: are reserved and cannot be used as identifiers:

.. code-block:: nimrod
   :file: keywords.txt

Some keywords are unused; they are reserved for future developments of the
language.

Nimrod is a `style-insensitive`:idx: language. This means that it is not
case-sensitive and even underscores are ignored:
**type** is a reserved word, and so is **TYPE** or **T_Y_P_E**. The idea behind
this is that this allows programmers to use their own preferred spelling style
and libraries written by different programmers cannot use incompatible
conventions. A Nimrod-aware editor or IDE can show the identifiers as
preferred. Another advantage is that it frees the programmer from remembering
the exact spelling of an identifier.


String literals
---------------

`String literals`:idx: can be delimited by matching double quotes, and can
contain the following `escape sequences`:idx:\ :

==================         ===================================================
  Escape sequence          Meaning
==================         ===================================================
  ``\n``                   `newline`:idx:
  ``\r``, ``\c``           `carriage return`:idx:
  ``\l``                   `line feed`:idx:
  ``\f``                   `form feed`:idx:
  ``\t``                   `tabulator`:idx:
  ``\v``                   `vertical tabulator`:idx:
  ``\\``                   `backslash`:idx:
  ``\"``                   `quotation mark`:idx:
  ``\'``                   `apostrophe`:idx:
  ``\d+``                  `character with decimal value d`:idx:;
                           all decimal digits directly
                           following are used for the character
  ``\a``                   `alert`:idx:
  ``\b``                   `backspace`:idx:
  ``\e``                   `escape`:idx: `[ESC]`:idx:
  ``\xHH``                 `character with hex value HH`:idx:;
                           exactly two hex digits are allowed
==================         ===================================================


Strings in Nimrod may contain any 8-bit value, except embedded zeros.


Triple quoted string literals
-----------------------------

String literals can also be delimited by three double quotes
``"""`` ... ``"""``.
Literals in this form may run for several lines, may contain ``"`` and do not
interpret any escape sequences.
For convenience, when the opening ``"""`` is immediately followed by a newline,
the newline is not included in the string. The ending of the string literal is
defined by the pattern ``"""[^"]``, so this:
  
.. code-block:: nimrod 
  """"long string within quotes""""
  
Produces::
  
  "long string within quotes"


Raw string literals
-------------------

There are also `raw string literals` that are preceded with the letter ``r``
(or ``R``) and are delimited by matching double quotes (just like ordinary
string literals) and do not interpret the escape sequences. This is especially
convenient for regular expressions or Windows paths:

.. code-block:: nimrod

  var f = openFile(r"C:\texts\text.txt") # a raw string, so ``\t`` is no tab

To produce a single ``"`` within a raw string literal, it has to be doubled:

.. code-block:: nimrod

  r"a""b"
  
Produces::
  
  a"b

``r""""`` is not possible with this notation, because the three leading 
quotes introduce a triple quoted string literal. 


Generalized raw string literals
-------------------------------

The construct ``identifier"string literal"`` (without whitespace between the
identifier and the opening quotation mark) is a
`generalized raw string literal`:idx:. It is a shortcut for the construct
``identifier(r"string literal")``, so it denotes a procedure call with a
raw string literal as its only argument. Generalized raw string literals
are especially convenient for embedding mini languages directly into Nimrod
(for example regular expressions).

The construct ``identifier"""string literal"""`` exists too. It is a shortcut
for ``identifier("""string literal""")``.


Character literals
------------------

Character literals are enclosed in single quotes ``''`` and can contain the
same escape sequences as strings - with one exception: ``\n`` is not allowed
as it may be wider than one character (often it is the pair CR/LF for example).
A character is not an Unicode character but a single byte. The reason for this
is efficiency: for the overwhelming majority of use-cases, the resulting
programs will still handle UTF-8 properly as UTF-8 was specially designed for
this.
Another reason is that Nimrod can thus support ``array[char, int]`` or
``set[char]`` efficiently as many algorithms rely on this feature.


Numerical constants
-------------------

`Numerical constants`:idx: are of a single type and have the form::

  hexdigit ::= digit | 'A'..'F' | 'a'..'f'
  octdigit ::= '0'..'7'
  bindigit ::= '0'..'1'
  INT_LIT ::= digit ( ['_'] digit )*
            | '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )*
            | '0o' octdigit ( ['_'] octdigit )*
            | '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )*

  INT8_LIT ::= INT_LIT '\'' ('i' | 'I' ) '8'
  INT16_LIT ::= INT_LIT '\'' ('i' | 'I' ) '16'
  INT32_LIT ::= INT_LIT '\'' ('i' | 'I' ) '32'
  INT64_LIT ::= INT_LIT '\'' ('i' | 'I' ) '64'

  exponent ::= ('e' | 'E' ) ['+' | '-'] digit ( ['_'] digit )*
  FLOAT_LIT ::= digit (['_'] digit)*  ('.' (['_'] digit)* [exponent] |exponent)
  FLOAT32_LIT ::= ( FLOAT_LIT | INT_LIT ) '\'' ('f' | 'F') '32'
  FLOAT64_LIT ::= ( FLOAT_LIT | INT_LIT ) '\'' ('f' | 'F') '64'


As can be seen in the productions, numerical constants can contain underscores
for readability. Integer and floating point literals may be given in decimal (no
prefix), binary (prefix ``0b``), octal (prefix ``0o``) and hexadecimal
(prefix ``0x``) notation.

There exists a literal for each numerical type that is
defined. The suffix starting with an apostrophe ('\'') is called a
`type suffix`:idx:. Literals without a type suffix are of the type ``int``,
unless the literal contains a dot or ``E|e`` in which case it is of
type ``float``.

The type suffixes are:

=================    =========================
  Type Suffix        Resulting type of literal
=================    =========================
  ``'i8``            int8
  ``'i16``           int16
  ``'i32``           int32
  ``'i64``           int64
  ``'f32``           float32
  ``'f64``           float64
=================    =========================

Floating point literals may also be in binary, octal or hexadecimal
notation:
``0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64``
is approximately 1.72826e35 according to the IEEE floating point standard.



Other tokens
------------

The following strings denote other tokens::

       (     )     {     }     [     ]     ,  ;   [.    .]  {.   .}  (.  .)
       :     =     ^     ..    `

`..`:tok: takes precedence over other tokens that contain a dot: `{..}`:tok: are
the three tokens `{`:tok:, `..`:tok:, `}`:tok: and not the two tokens
`{.`:tok:, `.}`:tok:.

In Nimrod one can define his own operators. An `operator`:idx: is any
combination of the following characters that is not listed above::

       +     -     *     /     <     >
       =     @     $     ~     &     %
       !     ?     ^     .     |     \

These keywords are also operators:
``and or not xor shl shr div mod in notin is isnot``.


Syntax
======

This section lists Nimrod's standard syntax in ENBF. How the parser receives
indentation tokens is already described in the `Lexical Analysis`_ section.

Nimrod allows user-definable operators.
Binary operators have 8 different levels of precedence. For user-defined
operators, the precedence depends on the first character the operator consists
of. All binary operators are left-associative.

================  ==============================================  ==================  ===============
Precedence level    Operators                                     First characters    Terminal symbol
================  ==============================================  ==================  ===============
  7 (highest)                                                     ``$``               OP7
  6               ``*    /    div   mod   shl  shr  %``           ``* % \  /``        OP6
  5               ``+    -``                                      ``+  ~  |``         OP5
  4               ``&``                                           ``&``               OP4
  3               ``==  <= < >= > !=  in  not_in  is  isnot``     ``= <  > !``        OP3
  2               ``and``                                                             OP2
  1               ``or xor``                                                          OP1
  0 (lowest)                                                      ``? @ ^ ` : .``     OP0
================  ==============================================  ==================  ===============


The grammar's start symbol is ``module``.

.. include:: grammar.txt
   :literal:



Semantics
=========

Constants
---------

`Constants`:idx: are symbols which are bound to a value. The constant's value
cannot change. The compiler must be able to evaluate the expression in a
constant declaration at compile time.

Nimrod contains a sophisticated compile-time evaluator, so procedures which
have no side-effect can be used in constant expressions too:

.. code-block:: nimrod
  import strutils
  const
    constEval = contains("abc", 'b') # computed at compile time!


Types
-----

All expressions have a `type`:idx: which is known at compile time. Nimrod
is statically typed. One can declare new types, which is in essence defining
an identifier that can be used to denote this custom type.

These are the major type classes:

* ordinal types (consist of integer, bool, character, enumeration
  (and subranges thereof) types)
* floating point types
* string type
* structured types
* reference (pointer) type
* procedural type
* generic type


Ordinal types
~~~~~~~~~~~~~
`Ordinal types`:idx: have the following characteristics:

- Ordinal types are countable and ordered. This property allows
  the operation of functions as ``Inc``, ``Ord``, ``Dec`` on ordinal types to
  be defined.
- Ordinal values have a smallest possible value. Trying to count further
  down than the smallest value gives a checked runtime or static error.
- Ordinal values have a largest possible value. Trying to count further
  than the largest value gives a checked runtime or static error.

Integers, bool, characters and enumeration types (and subranges of these
types) belong to ordinal types.


Pre-defined integer types
~~~~~~~~~~~~~~~~~~~~~~~~~
These integer types are pre-defined:

``int``
  the generic signed integer type; its size is platform dependent
  (the compiler chooses the processor's fastest integer type).
  This type should be used in general. An integer literal that has no type
  suffix is of this type.

intXX
  additional signed integer types of XX bits use this naming scheme
  (example: int16 is a 16 bit wide integer).
  The current implementation supports ``int8``, ``int16``, ``int32``, ``int64``.
  Literals of these types have the suffix 'iXX.


There are no `unsigned integer`:idx: types, only `unsigned operations`:idx:
that treat their arguments as unsigned. Unsigned operations all wrap around;
they cannot lead to over- or underflow errors. Unsigned operations use the
``%`` suffix as convention:

======================   ======================================================
operation                meaning
======================   ======================================================
``a +% b``               unsigned integer addition
``a -% b``               unsigned integer subtraction
``a *% b``               unsigned integer multiplication
``a /% b``               unsigned integer division
``a %% b``               unsigned integer modulo operation
``a <% b``               treat ``a`` and ``b`` as unsigned and compare
``a <=% b``              treat ``a`` and ``b`` as unsigned and compare
``ze(a)``                extends the bits of ``a`` with zeros until it has the
                         width of the ``int`` type
``toU8(a)``              treats ``a`` as unsigned and converts it to an
                         unsigned integer of 8 bits (but still the
                         ``int8`` type)
``toU16(a)``             treats ``a`` as unsigned and converts it to an
                         unsigned integer of 16 bits (but still the
                         ``int16`` type)
``toU32(a)``             treats ``a`` as unsigned and converts it to an
                         unsigned integer of 32 bits (but still the
                         ``int32`` type)
======================   ======================================================

`Automatic type conversion`:idx: is performed in expressions where different
kinds of integer types are used: the smaller type is converted to the larger.
For further details, see `Convertible relation`_.


Pre-defined floating point types
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The following floating point types are pre-defined:

``float``
  the generic floating point type; its size is platform dependent
  (the compiler chooses the processor's fastest floating point type).
  This type should be used in general.

floatXX
  an implementation may define additional floating point types of XX bits using
  this naming scheme (example: float64 is a 64 bit wide float). The current
  implementation supports ``float32`` and ``float64``. Literals of these types
  have the suffix 'fXX.


Automatic type conversion in expressions with different kinds
of floating point types is performed: See `Convertible relation`_ for further
details. Arithmetic performed on floating point types follows the IEEE
standard. Integer types are not converted to floating point types automatically
and vice versa.

The IEEE standard defines five types of floating-point exceptions:

* Invalid: operations with mathematically invalid operands,
  for example 0.0/0.0, sqrt(-1.0), and log(-37.8).
* Division by zero: divisor is zero and dividend is a finite nonzero number,
  for example 1.0/0.0.
* Overflow: operation produces a result that exceeds the range of the exponent, 
  for example MAXDOUBLE+0.0000000000001e308.
* Underflow: operation produces a result that is too small to be represented 
  as a normal number, for example, MINDOUBLE * MINDOUBLE.
* Inexact: operation produces a result that cannot be represented with infinite 
  precision, for example, 2.0 / 3.0, log(1.1) and 0.1 in input.

The IEEE exceptions are either ignored at runtime or mapped to the 
Nimrod exceptions: `EFloatInvalidOp`:idx, `EFloatDivByZero`:idx:,
`EFloatOverflow`:idx:, `EFloatUnderflow`:idx:, and `EFloatInexact`:idx:\. 
These exceptions inherit from the `EFloatingPoint`:idx: base class.

Nimrod provides the pragmas `NaNChecks`:idx and `InfChecks`:idx:\ to control
whether the IEEE exceptions are ignored or trap a Nimrod exception:

.. code-block:: nimrod
  {.NanChecks: on, InfChecks: on.}
  var a = 1.0
  var b = 0.0
  echo b / b # raises EFloatInvalidOp
  echo a / b # raises EFloatOverflow

In the current implementation ``EFloatDivByZero`` and ``EFloatInexact`` are 
never raised. ``EFloatOverflow`` is raised instead of ``EFloatDivByZero``.
There is also a `floatChecks`:idx: pragma that is a short-cut for the 
combination of ``NaNChecks`` and ``InfChecks`` pragmas. ``floatChecks`` are
turned off as default.

The only operations that are affected by the ``floatChecks`` pragma are
the ``+``, ``-``, ``*``, ``/`` operators for floating point types.


Boolean type
~~~~~~~~~~~~
The `boolean`:idx: type is named ``bool`` in Nimrod and can be one of the two
pre-defined values ``true`` and ``false``. Conditions in while,
if, elif, when statements need to be of type bool.

This condition holds::

  ord(false) == 0 and ord(true) == 1

The operators ``not, and, or, xor, <, <=, >, >=, !=, ==`` are defined
for the bool type. The ``and`` and ``or`` operators perform short-cut
evaluation. Example:

.. code-block:: nimrod

  while p != nil and p.name != "xyz":
    # p.name is not evaluated if p == nil
    p = p.next


The size of the bool type is one byte.


Character type
~~~~~~~~~~~~~~
The `character type`:idx: is named ``char`` in Nimrod. Its size is one byte.
Thus it cannot represent an UTF-8 character, but a part of it.
The reason for this is efficiency: for the overwhelming majority of use-cases,
the resulting programs will still handle UTF-8 properly as UTF-8 was specially
designed for this.
Another reason is that Nimrod can support ``array[char, int]`` or
``set[char]`` efficiently as many algorithms rely on this feature. The
`TRune` type is used for Unicode characters, it can represent any Unicode
character. ``TRune`` is declared in the ``unicode`` module.



Enumeration types
~~~~~~~~~~~~~~~~~
`Enumeration`:idx: types define a new type whose values consist of the ones
specified. The values are ordered. Example:

.. code-block:: nimrod

  type
    TDirection = enum
      north, east, south, west


Now the following holds::

  ord(north) == 0
  ord(east) == 1
  ord(south) == 2
  ord(west) == 3

Thus, north < east < south < west. The comparison operators can be used
with enumeration types.

For better interfacing to other programming languages, the fields of enum
types can be assigned an explicit ordinal value. However, the ordinal values
have to be in ascending order. A field whose ordinal value is not
explicitly given is assigned the value of the previous field + 1.

An explicit ordered enum can have *holes*:

.. code-block:: nimrod
  type
    TTokenType = enum
      a = 2, b = 4, c = 89 # holes are valid

However, it is then not an ordinal anymore, so it is not possible to use these
enums as an index type for arrays. The procedures ``inc``, ``dec``, ``succ``
and ``pred`` are not available for them either.


Subrange types
~~~~~~~~~~~~~~
A `subrange`:idx: type is a range of values from an ordinal type (the base
type). To define a subrange type, one must specify it's limiting values: the
lowest and highest value of the type:

.. code-block:: nimrod
  type
    TSubrange = range[0..5]


``TSubrange`` is a subrange of an integer which can only hold the values 0
to 5. Assigning any other value to a variable of type ``TSubrange`` is a
checked runtime error (or static error if it can be statically
determined). Assignments from the base type to one of its subrange types
(and vice versa) are allowed.

A subrange type has the same size as its base type (``int`` in the example).


String type
~~~~~~~~~~~
All string literals are of the type `string`:idx:. A string in Nimrod is very
similar to a sequence of characters. However, strings in Nimrod are both
zero-terminated and have a length field. One can retrieve the length with the
builtin ``len`` procedure; the length never counts the terminating zero.
The assignment operator for strings always copies the string.
The ``&`` operator concatenates strings.

Strings are compared by their lexicographical order. All comparison operators
are available. Strings can be indexed like arrays (lower bound is 0). Unlike
arrays, they can be used in case statements:

.. code-block:: nimrod

  case paramStr(i)
  of "-v": incl(options, optVerbose)
  of "-h", "-?": incl(options, optHelp)
  else: write(stdout, "invalid command line option!\n")

Per convention, all strings are UTF-8 strings, but this is not enforced. For
example, when reading strings from binary files, they are merely a sequence of
bytes. The index operation ``s[i]`` means the i-th *char* of ``s``, not the
i-th *unichar*. The iterator ``runes`` from the ``unicode``
module can be used for iteration over all Unicode characters.


Structured types
~~~~~~~~~~~~~~~~
A variable of a `structured type`:idx: can hold multiple values at the same
time. Structured types can be nested to unlimited levels. Arrays, sequences,
tuples, objects and sets belong to the structured types.

Array and sequence types
~~~~~~~~~~~~~~~~~~~~~~~~
`Arrays`:idx: are a homogeneous type, meaning that each element in the array
has the same type. Arrays always have a fixed length which is specified at
compile time (except for open arrays). They can be indexed by any ordinal type.
A parameter ``A`` may be an *open array*, in which case it is indexed by
integers from 0 to ``len(A)-1``. An array expression may be constructed by the
array constructor ``[]``.

`Sequences`:idx: are similar to arrays but of dynamic length which may change
during runtime (like strings). A sequence ``S`` is always indexed by integers
from 0 to ``len(S)-1`` and its bounds are checked. Sequences can be
constructed by the array constructor ``[]`` in conjunction with the array to
sequence operator ``@``. Another way to allocate space for a sequence is to
call the built-in ``newSeq`` procedure.

A sequence may be passed to a parameter that is of type *open array*.

Example:

.. code-block:: nimrod

  type
    TIntArray = array[0..5, int] # an array that is indexed with 0..5
    TIntSeq = seq[int] # a sequence of integers
  var
    x: TIntArray
    y: TIntSeq
  x = [1, 2, 3, 4, 5, 6]  # [] is the array constructor
  y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence

The lower bound of an array or sequence may be received by the built-in proc
``low()``, the higher bound by ``high()``. The length may be
received by ``len()``. ``low()`` for a sequence or an open array always returns
0, as this is the first valid index.
One can append elements to a sequence with the ``add()`` proc or the ``&``
operator, and remove (and get) the last element of a sequence with the
``pop()`` proc.

The notation ``x[i]`` can be used to access the i-th element of ``x``.

Arrays are always bounds checked (at compile-time or at runtime). These
checks can be disabled via pragmas or invoking the compiler with the
``--boundChecks:off`` command line switch.

An open array is  also a means to implement passing a variable number of
arguments to a procedure. The compiler converts the list of arguments
to an array automatically:

.. code-block:: nimrod
  proc myWriteln(f: TFile, a: openarray[string]) =
    for s in items(a):
      write(f, s)
    write(f, "\n")

  myWriteln(stdout, "abc", "def", "xyz")
  # is transformed by the compiler to:
  myWriteln(stdout, ["abc", "def", "xyz"])

This transformation is only done if the openarray parameter is the
last parameter in the procedure header. The current implementation does not
support nested open arrays.


Tuples and object types
~~~~~~~~~~~~~~~~~~~~~~~
A variable of a `tuple`:idx: or `object`:idx: type is a heterogeneous storage
container.
A tuple or object defines various named *fields* of a type. A tuple also
defines an *order* of the fields. Tuples are meant for heterogeneous storage
types with no overhead and few abstraction possibilities. The constructor ``()``
can be used to construct tuples. The order of the fields in the constructor
must match the order of the tuple's definition. Different tuple-types are
*equivalent* if they specify the same fields of the same type in the same
order.

The assignment operator for tuples copies each component.
The default assignment operator for objects copies each component. Overloading
of the assignment operator for objects is not possible, but this may change in
future versions of the compiler.

.. code-block:: nimrod

  type
    TPerson = tuple[name: string, age: int] # type representing a person
                                            # a person consists of a name
                                            # and an age
  var
    person: TPerson
  person = (name: "Peter", age: 30)
  # the same, but less readable:
  person = ("Peter", 30)

The implementation aligns the fields for best access performance. The alignment
is compatible with the way the C compiler does it.

Objects provide many features that tuples do not. Object provide inheritance
and information hiding. Objects have access to their type at runtime, so that
the ``is`` operator can be used to determine the object's type.

.. code-block:: nimrod

  type
    TPerson = object
      name*: string   # the * means that `name` is accessible from other modules
      age: int        # no * means that the field is hidden

    TStudent = object of TPerson # a student is a person
      id: int                    # with an id field

  var
    student: TStudent
    person: TPerson
  assert(student is TStudent) # is true

Object fields that should be visible from outside the defining module, have to
be marked by ``*``. In contrast to tuples, different object types are
never *equivalent*.


Object variants
~~~~~~~~~~~~~~~
Often an object hierarchy is overkill in certain situations where simple
`variant`:idx: types are needed.

An example:

.. code-block:: nimrod

  # This is an example how an abstract syntax tree could be modelled in Nimrod
  type
    TNodeKind = enum  # the different node types
      nkInt,          # a leaf with an integer value
      nkFloat,        # a leaf with a float value
      nkString,       # a leaf with a string value
      nkAdd,          # an addition
      nkSub,          # a subtraction
      nkIf            # an if statement
    PNode = ref TNode
    TNode = object
      case kind: TNodeKind  # the ``kind`` field is the discriminator
      of nkInt: intVal: int
      of nkFloat: floatVal: float
      of nkString: strVal: string
      of nkAdd, nkSub:
        leftOp, rightOp: PNode
      of nkIf:
        condition, thenPart, elsePart: PNode

  var
    n: PNode
  new(n)  # creates a new node
  n.kind = nkFloat
  n.floatVal = 0.0 # valid, because ``n.kind==nkFloat``, so that it fits

  # the following statement raises an `EInvalidField` exception, because
  # n.kind's value does not fit:
  n.strVal = ""

As can been seen from the example, an advantage to an object hierarchy is that
no casting between different object types is needed. Yet, access to invalid
object fields raises an exception.


Set type
~~~~~~~~
The `set type`:idx: models the mathematical notion of a set. The set's
basetype can only be an ordinal type. The reason is that sets are implemented
as high performance bit vectors.

Sets can be constructed via the set constructor: ``{}`` is the empty set. The
empty set is type compatible with any special set type. The constructor
can also be used to include elements (and ranges of elements) in the set:

.. code-block:: nimrod

  {'a'..'z', '0'..'9'} # This constructs a set that contains the
                       # letters from 'a' to 'z' and the digits
                       # from '0' to '9'

These operations are supported by sets:

==================    ========================================================
operation             meaning
==================    ========================================================
``A + B``             union of two sets
``A * B``             intersection of two sets
``A - B``             difference of two sets (A without B's elements)
``A == B``            set equality
``A <= B``            subset relation (A is subset of B or equal to B)
``A < B``             strong subset relation (A is a real subset of B)
``e in A``            set membership (A contains element e)
``A -+- B``           symmetric set difference (= (A - B) + (B - A))
``card(A)``           the cardinality of A (number of elements in A)
``incl(A, elem)``     same as A = A + {elem}
``excl(A, elem)``     same as A = A - {elem}
==================    ========================================================


Reference and pointer types
~~~~~~~~~~~~~~~~~~~~~~~~~~~
References (similar to `pointers`:idx: in other programming languages) are a
way to introduce many-to-one relationships. This means different references can
point to and modify the same location in memory.

Nimrod distinguishes between `traced`:idx: and `untraced`:idx: references.
Untraced references are also called *pointers*. Traced references point to
objects of a garbage collected heap, untraced references point to
manually allocated objects or to objects somewhere else in memory. Thus
untraced references are *unsafe*. However for certain low-level operations
(accessing the hardware) untraced references are unavoidable.

Traced references are declared with the **ref** keyword, untraced references
are declared with the **ptr** keyword.

The ``^`` operator can be used to derefer a reference, the ``addr`` procedure
returns the address of an item. An address is always an untraced reference.
Thus the usage of ``addr`` is an *unsafe* feature.

The ``.`` (access a tuple/object field operator)
and ``[]`` (array/string/sequence index operator) operators perform implicit
dereferencing operations for reference types:

.. code-block:: nimrod

  type
    PNode = ref TNode
    TNode = object
      le, ri: PNode
      data: int

  var
    n: PNode
  new(n)
  n.data = 9 # no need to write n^ .data

To allocate a new traced object, the built-in procedure ``new`` has to be used.
To deal with untraced memory, the procedures ``alloc``, ``dealloc`` and
``realloc`` can be used. The documentation of the system module contains
further information.

If a reference points to *nothing*, it has the value ``nil``.

Special care has to be taken if an untraced object contains traced objects like
traced references, strings or sequences: in order to free everything properly,
the built-in procedure ``GCunref`` has to be called before freeing the
untraced memory manually!

.. XXX finalizers for traced objects

Procedural type
~~~~~~~~~~~~~~~
A `procedural type`:idx: is internally a pointer to a procedure. ``nil`` is
an allowed value for variables of a procedural type. Nimrod uses procedural
types to achieve `functional`:idx: programming techniques.

Example:

.. code-block:: nimrod

  type
    TCallback = proc (x: int) {.cdecl.}

  proc printItem(x: Int) = ...

  proc forEach(c: TCallback) =
    ...

  forEach(printItem)  # this will NOT work because calling conventions differ

A subtle issue with procedural types is that the calling convention of the
procedure influences the type compatibility: procedural types are only
compatible if they have the same calling convention.

Nimrod supports these `calling conventions`:idx:, which are all incompatible to
each other:

`stdcall`:idx:
    This the stdcall convention as specified by Microsoft. The generated C
    procedure is declared with the ``__stdcall`` keyword.

`cdecl`:idx:
    The cdecl convention means that a procedure shall use the same convention
    as the C compiler. Under windows the generated C procedure is declared with
    the ``__cdecl`` keyword.

`safecall`:idx:
    This is the safecall convention as specified by Microsoft. The generated C
    procedure is declared with the ``__safecall`` keyword. The word *safe*
    refers to the fact that all hardware registers shall be pushed to the
    hardware stack.

`inline`:idx:
    The inline convention means the the caller should not call the procedure,
    but inline its code directly. Note that Nimrod does not inline, but leaves
    this to the C compiler. Thus it generates ``__inline`` procedures. This is
    only a hint for the compiler: it may completely ignore it and
    it may inline procedures that are not marked as ``inline``.

`fastcall`:idx:
    Fastcall means different things to different C compilers. One gets whatever
    the C ``__fastcall`` means.

`nimcall`:idx:
    Nimcall is the default convention used for Nimrod procedures. It is the
    same as ``fastcall``, but only for C compilers that support ``fastcall``.

`closure`:idx:
    indicates that the procedure expects a context, a closure that needs
    to be passed to the procedure. The calling convention ``nimcall`` is
    compatible to ``closure``.

`syscall`:idx:
    The syscall convention is the same as ``__syscall`` in C. It is used for
    interrupts.

`noconv`:idx:
    The generated C code will not have any explicit calling convention and thus
    use the C compiler's default calling convention. This is needed because
    Nimrod's default calling convention for procedures is ``fastcall`` to
    improve speed.

Most calling conventions exist only for the Windows 32-bit platform.

Assigning/passing a procedure to a procedural variable is only allowed if one
of the following conditions hold:
1) The procedure that is accessed resists in the current module.
2) The procedure is marked with the ``procvar`` pragma (see `procvar pragma`_).
3) The procedure has a calling convention that differs from ``nimcall``.
4) The procedure is anonymous.

The rules' purpose is to prevent the case that extending a non-``procvar`` 
procedure with default parameters breaks client code.


Distinct type
~~~~~~~~~~~~~

A distinct type is new type derived from a `base type`:idx: that is
incompatible with its base type. In particular, it is an essential property
of a distinct type that it **does not** imply a subtype relation between it
and its base type. Explicit type conversions from a distinct type to its
base type and vice versa are allowed.

A distinct type can be used to model different physical `units`:idx: with a
numerical base type, for example. The following example models currencies.

Different currencies should not be mixed in monetary calculations. Distinct
types are a perfect tool to model different currencies:

.. code-block:: nimrod
  type
    TDollar = distinct int
    TEuro = distinct int
  
  var
    d: TDollar
    e: TEuro

  echo d + 12
  # Error: cannot add a number with no unit and a ``TDollar``

Unfortunately, ``d + 12.TDollar`` is not allowed either,
because ``+`` is defined for ``int`` (among others), not for ``TDollar``. So
a ``+`` for dollars needs to be defined:

.. code-block::
  proc `+` (x, y: TDollar): TDollar =
    result = TDollar(int(x) + int(y))

It does not make sense to multiply a dollar with a dollar, but with a
number without unit; and the same holds for division:

.. code-block::
  proc `*` (x: TDollar, y: int): TDollar =
    result = TDollar(int(x) * y)

  proc `*` (x: int, y: TDollar): TDollar =
    result = TDollar(x * int(y))
    
  proc `div` ...

This quickly gets tedious. The implementations are trivial and the compiler
should not generate all this code only to optimize it away later - after all
``+`` for dollars should produce the same binary code as ``+`` for ints.
The pragma ``borrow`` has been designed to solve this problem; in principle
it generates the above trivial implementations:

.. code-block:: nimrod
  proc `*` (x: TDollar, y: int): TDollar {.borrow.}
  proc `*` (x: int, y: TDollar): TDollar {.borrow.}
  proc `div` (x: TDollar, y: int): TDollar {.borrow.}

The ``borrow`` pragma makes the compiler use the same implementation as
the proc that deals with the distinct type's base type, so no code is
generated.

But it seems all this boilerplate code needs to be repeated for the ``TEuro``
currency. This can be solved with templates_.

.. code-block:: nimrod
  template Additive(typ: typeDesc): stmt =
    proc `+` *(x, y: typ): typ {.borrow.}
    proc `-` *(x, y: typ): typ {.borrow.}
    
    # unary operators:
    proc `+` *(x: typ): typ {.borrow.}
    proc `-` *(x: typ): typ {.borrow.}

  template Multiplicative(typ, base: typeDesc): stmt =
    proc `*` *(x: typ, y: base): typ {.borrow.}
    proc `*` *(x: base, y: typ): typ {.borrow.}
    proc `div` *(x: typ, y: base): typ {.borrow.}
    proc `mod` *(x: typ, y: base): typ {.borrow.}

  template Comparable(typ: typeDesc): stmt =
    proc `<` * (x, y: typ): bool {.borrow.}
    proc `<=` * (x, y: typ): bool {.borrow.}
    proc `==` * (x, y: typ): bool {.borrow.}

  template DefineCurrency(typ, base: expr): stmt =
    type
      typ* = distinct base
    Additive(typ)
    Multiplicative(typ, base)
    Comparable(typ)
    
  DefineCurrency(TDollar, int)
  DefineCurrency(TEuro, int)



Type relations
--------------

The following section defines several relations on types that are needed to
describe the type checking done by the compiler.


Type equality
~~~~~~~~~~~~~
Nimrod uses structural type equivalence for most types. Only for objects,
enumerations and distinct types name equivalence is used. The following
algorithm (in pseudo-code) determines type equality:

.. code-block:: nimrod
  proc typeEqualsAux(a, b: PType,
                     s: var set[PType * PType]): bool =
    if (a,b) in s: return true
    incl(s, (a,b))
    if a.kind == b.kind:
      case a.kind
      of int, intXX, float, floatXX, char, string, cstring, pointer, bool, nil:
        # leaf type: kinds identical; nothing more to check
        result = true
      of ref, ptr, var, set, seq, openarray:
        result = typeEqualsAux(a.baseType, b.baseType, s)
      of range:
        result = typeEqualsAux(a.baseType, b.baseType, s) and
          (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB)
      of array:
        result = typeEqualsAux(a.baseType, b.baseType, s) and
                 typeEqualsAux(a.indexType, b.indexType, s)
      of tuple:
        if a.tupleLen == b.tupleLen:
          for i in 0..a.tupleLen-1:
            if not typeEqualsAux(a[i], b[i], s): return false
          result = true
      of object, enum, distinct:
        result = a == b
      of proc:
        result = typeEqualsAux(a.parameterTuple, b.parameterTuple, s) and
                 typeEqualsAux(a.resultType, b.resultType, s) and
                 a.callingConvention == b.callingConvention

  proc typeEquals(a, b: PType): bool =
    var s: set[PType * PType] = {}
    result = typeEqualsAux(a, b, s)

Since types are graphs which can have cycles, the above algorithm needs an
auxiliary set ``s`` to detect this case.


Subtype relation
~~~~~~~~~~~~~~~~
If object ``a`` inherits from ``b``, ``a`` is a subtype of ``b``. This subtype
relation is extended to the types ``var``, ``ref``, ``ptr``:

.. code-block:: nimrod
  proc isSubtype(a, b: PType): bool =
    if a.kind == b.kind:
      case a.kind
      of object:
        var aa = a.baseType
        while aa != nil and aa != b: aa = aa.baseType
        result = aa == b
      of var, ref, ptr:
        result = isSubtype(a.baseType, b.baseType)

.. XXX nil is a special value!


Convertible relation
~~~~~~~~~~~~~~~~~~~~
A type ``a`` is **implicitly** convertible to type ``b`` iff the following
algorithm returns true:

.. code-block:: nimrod
  # XXX range types?
  proc isImplicitlyConvertible(a, b: PType): bool =
    case a.kind
    of proc:
      if b.kind == proc:
        var x = a.parameterTuple
        var y = b.parameterTuple
        if x.tupleLen == y.tupleLen:
          for i in 0.. x.tupleLen-1:
            if not isSubtype(x[i], y[i]): return false
          result = isSubType(b.resultType, a.resultType)
    of int8:    result = b.kind in {int16, int32, int64, int}
    of int16:   result = b.kind in {int32, int64, int}
    of int32:   result = b.kind in {int64, int}
    of float:   result = b.kind in {float32, float64}
    of float32: result = b.kind in {float64, float}
    of float64: result = b.kind in {float32, float}
    of seq:
      result = b.kind == openArray and typeEquals(a.baseType, b.baseType)
    of array:
      result = b.kind == openArray and typeEquals(a.baseType, b.baseType)
      if a.baseType == char and a.indexType.rangeA == 0:
        result = b.kind = cstring
    of cstring, ptr:
      result = b.kind == pointer
    of string:
      result = b.kind == cstring

A type ``a`` is **explicitly** convertible to type ``b`` iff the following
algorithm returns true:
 
.. code-block:: nimrod
  proc isIntegralType(t: PType): bool =
    result = isOrdinal(t) or t.kind in {float, float32, float64}

  proc isExplicitlyConvertible(a, b: PType): bool =
    if isImplicitlyConvertible(a, b): return true
    if isIntegralType(a) and isIntegralType(b): return true
    if isSubtype(a, b) or isSubtype(b, a): return true
    if a.kind == distinct and typeEquals(a.baseType, b): return true
    if b.kind == distinct and typeEquals(b.baseType, a): return true
    return false
    

Assignment compatibility
~~~~~~~~~~~~~~~~~~~~~~~~

An expression ``b`` can be assigned to an expression ``a`` iff ``a`` is an
`l-value` and ``isImplicitlyConvertible(b.typ, a.typ)`` holds.


Overloading resolution
~~~~~~~~~~~~~~~~~~~~~~

To be written.


Statements and expressions
--------------------------
Nimrod uses the common statement/expression paradigm: `Statements`:idx: do not
produce a value in contrast to expressions. Call expressions are statements.
If the called procedure returns a value, it is not a valid statement
as statements do not produce values. To evaluate an expression for
side-effects and throw its value away, one can use the ``discard`` statement.

Statements are separated into `simple statements`:idx: and
`complex statements`:idx:.
Simple statements are statements that cannot contain other statements like
assignments, calls or the ``return`` statement; complex statements can
contain other statements. To avoid the `dangling else problem`:idx:, complex
statements always have to be intended::

  simpleStmt ::= returnStmt
             | yieldStmt
             | discardStmt
             | raiseStmt
             | breakStmt
             | continueStmt
             | pragma
             | importStmt
             | fromStmt
             | includeStmt
             | exprStmt
  complexStmt ::= ifStmt | whileStmt | caseStmt | tryStmt | forStmt
                   | blockStmt | asmStmt
                   | procDecl | iteratorDecl | macroDecl | templateDecl
                   | constSection | typeSection | whenStmt | varSection



Discard statement
~~~~~~~~~~~~~~~~~

Syntax::

  discardStmt ::= 'discard' expr

Example:

.. code-block:: nimrod

  discard proc_call("arg1", "arg2") # discard the return value of `proc_call`

The `discard`:idx: statement evaluates its expression for side-effects and
throws the expression's resulting value away. If the expression has no
side-effects, this generates a static error. Ignoring the return value of a
procedure without using a discard statement is a static error too.


Var statement
~~~~~~~~~~~~~

Syntax::

  colonOrEquals ::= ':' typeDesc ['=' expr] | '=' expr
  varField ::= symbol ['*'] [pragma]
  varPart ::= symbol (comma symbol)* [comma] colonOrEquals [COMMENT | IND COMMENT]
  varSection ::= 'var' (varPart
                     | indPush (COMMENT|varPart)
                       (SAD (COMMENT|varPart))* DED indPop)


`Var`:idx: statements declare new local and global variables and
initialize them. A comma separated list of variables can be used to specify
variables of the same type:

.. code-block:: nimrod

  var
    a: int = 0
    x, y, z: int

If an initializer is given the type can be omitted: the variable is then of the
same type as the initializing expression. Variables are always initialized
with a default value if there is no initializing expression. The default
value depends on the type and is always a zero in binary.

============================    ==============================================
Type                            default value
============================    ==============================================
any integer type                0
any float                       0.0
char                            '\\0'
bool                            false
ref or pointer type             nil
procedural type                 nil
sequence                        nil (*not* ``@[]``)
string                          nil (*not* "")
tuple[x: A, y: B, ...]          (default(A), default(B), ...)
                                (analogous for objects)
array[0..., T]                  [default(T), ...]
range[T]                        default(T); this may be out of the valid range
T = enum                        cast[T](0); this may be an invalid value
============================    ==============================================


Const section
~~~~~~~~~~~~~

Syntax::

  colonAndEquals ::= [':' typeDesc] '=' expr

  constDecl ::= symbol ['*'] [pragma] colonAndEquals [COMMENT | IND COMMENT]
              | COMMENT
  constSection ::= 'const' indPush constDecl (SAD constDecl)* DED indPop


Example:

.. code-block:: nimrod

  const
    MyFilename = "/home/my/file.txt"
    debugMode: bool = false

The `const`:idx: section declares symbolic constants. A symbolic constant is
a name for a constant expression. Symbolic constants only allow read-access.


If statement
~~~~~~~~~~~~

Syntax::

  ifStmt ::= 'if' expr ':' stmt ('elif' expr ':' stmt)* ['else' ':' stmt]

Example:

.. code-block:: nimrod

  var name = readLine(stdin)

  if name == "Andreas":
    echo("What a nice name!")
  elif name == "":
    echo("Don't you have a name?")
  else:
    echo("Boring name...")

The `if`:idx: statement is a simple way to make a branch in the control flow:
The expression after the keyword ``if`` is evaluated, if it is true
the corresponding statements after the ``:`` are executed. Otherwise
the expression after the ``elif`` is evaluated (if there is an
``elif`` branch), if it is true the corresponding statements after
the ``:`` are executed. This goes on until the last ``elif``. If all
conditions fail, the ``else`` part is executed. If there is no ``else``
part, execution continues with the statement after the ``if`` statement.


Case statement
~~~~~~~~~~~~~~

Syntax::

  caseStmt ::= 'case' expr [':'] ('of' sliceExprList ':' stmt)*
                                 ('elif' expr ':' stmt)*
                                 ['else' ':' stmt]

Example:

.. code-block:: nimrod

  case readline(stdin)
  of "delete-everything", "restart-computer":
    echo("permission denied")
  of "go-for-a-walk":     echo("please yourself")
  else:                   echo("unknown command")

The `case`:idx: statement is similar to the if statement, but it represents
a multi-branch selection. The expression after the keyword ``case`` is
evaluated and if its value is in a *slicelist* the corresponding statements
(after the ``of`` keyword) are executed. If the value is not in any
given *slicelist* the ``else`` part is executed. If there is no ``else``
part and not all possible values that ``expr`` can hold occur in a 
``slicelist``, a static error occurs. This holds only for expressions of 
ordinal types.
If the expression is not of an ordinal type, and no ``else`` part is
given, control passes after the ``case`` statement.

To suppress the static error in the ordinal case an ``else`` part with a ``nil``
statement can be used.


When statement
~~~~~~~~~~~~~~

Syntax::

  whenStmt ::= 'when' expr ':' stmt ('elif' expr ':' stmt)* ['else' ':' stmt]

Example:

.. code-block:: nimrod

  when sizeof(int) == 2:
    echo("running on a 16 bit system!")
  elif sizeof(int) == 4:
    echo("running on a 32 bit system!")
  elif sizeof(int) == 8:
    echo("running on a 64 bit system!")
  else:
    echo("cannot happen!")

The `when`:idx: statement is almost identical to the ``if`` statement with some
exceptions:

* Each ``expr`` has to be a constant expression (of type ``bool``).
* The statements do not open a new scope.
* The statements that belong to the expression that evaluated to true are
  translated by the compiler, the other statements are not checked for
  semantics! However, each ``expr`` is checked for semantics.

The ``when`` statement enables conditional compilation techniques. As
a special syntactic extension, the ``when`` construct is also available
within ``object`` definitions.


Raise statement
~~~~~~~~~~~~~~~

Syntax::

  raiseStmt ::= 'raise' [expr]

Example:

.. code-block:: nimrod
  raise newEOS("operating system failed")

Apart from built-in operations like array indexing, memory allocation, etc.
the ``raise`` statement is the only way to raise an exception.

.. XXX document this better!

If no exception name is given, the current exception is `re-raised`:idx:. The
`ENoExceptionToReraise`:idx: exception is raised if there is no exception to
re-raise. It follows that the ``raise`` statement *always* raises an
exception.


Try statement
~~~~~~~~~~~~~

Syntax::

  qualifiedIdent ::= symbol ['.' symbol]
  exceptList ::= [qualifiedIdent (comma qualifiedIdent)* [comma]]
  tryStmt ::= 'try' ':' stmt
             ('except' exceptList ':' stmt)*
             ['finally' ':' stmt]

Example:

.. code-block:: nimrod
  # read the first two lines of a text file that should contain numbers
  # and tries to add them
  var
    f: TFile
  if open(f, "numbers.txt"):
    try:
      var a = readLine(f)
      var b = readLine(f)
      echo("sum: " & $(parseInt(a) + parseInt(b)))
    except EOverflow:
      echo("overflow!")
    except EInvalidValue:
      echo("could not convert string to integer")
    except EIO:
      echo("IO error!")
    except:
      echo("Unknown exception!")
    finally:
      close(f)

The statements after the `try`:idx: are executed in sequential order unless
an exception ``e`` is raised. If the exception type of ``e`` matches any
of the list ``exceptlist`` the corresponding statements are executed.
The statements following the ``except`` clauses are called
`exception handlers`:idx:.

The empty `except`:idx: clause is executed if there is an exception that is
in no list. It is similar to an ``else`` clause in ``if`` statements.

If there is a `finally`:idx: clause, it is always executed after the
exception handlers.

The exception is *consumed* in an exception handler. However, an
exception handler may raise another exception. If the exception is not
handled, it is propagated through the call stack. This means that often
the rest of the procedure - that is not within a ``finally`` clause -
is not executed (if an exception occurs).


Return statement
~~~~~~~~~~~~~~~~

Syntax::

  returnStmt ::= 'return' [expr]

Example:

.. code-block:: nimrod
  return 40+2

The `return`:idx: statement ends the execution of the current procedure.
It is only allowed in procedures. If there is an ``expr``, this is syntactic
sugar for:

.. code-block:: nimrod
  result = expr
  return result

``return`` without an expression is a short notation for ``return result`` if
the proc has a return type. The `result`:idx: variable is always the return
value of the procedure. It is automatically declared by the compiler. As all
variables, ``result`` is initialized to (binary) zero:

.. code-block:: nimrod
  proc returnZero(): int =
    # implicitly returns 0


Yield statement
~~~~~~~~~~~~~~~

Syntax::

  yieldStmt ::= 'yield' expr

Example:

.. code-block:: nimrod
  yield (1, 2, 3)

The `yield`:idx: statement is used instead of the ``return`` statement in
iterators. It is only valid in iterators. Execution is returned to the body
of the for loop that called the iterator. Yield does not end the iteration
process, but execution is passed back to the iterator if the next iteration
starts. See the section about iterators (`Iterators and the for statement`_)
for further information.


Block statement
~~~~~~~~~~~~~~~

Syntax::

  blockStmt ::= 'block' [symbol] ':' stmt

Example:

.. code-block:: nimrod
  var found = false
  block myblock:
    for i in 0..3:
      for j in 0..3:
        if a[j][i] == 7:
          found = true
          break myblock # leave the block, in this case both for-loops
  echo(found)

The block statement is a means to group statements to a (named) `block`:idx:.
Inside the block, the ``break`` statement is allowed to leave the block
immediately. A ``break`` statement can contain a name of a surrounding
block to specify which block is to leave.


Break statement
~~~~~~~~~~~~~~~

Syntax::

  breakStmt ::= 'break' [symbol]

Example:

.. code-block:: nimrod
  break

The `break`:idx: statement is used to leave a block immediately. If ``symbol``
is given, it is the name of the enclosing block that is to leave. If it is
absent, the innermost block is left.


While statement
~~~~~~~~~~~~~~~

Syntax::

  whileStmt ::= 'while' expr ':' stmt

Example:

.. code-block:: nimrod
  echo("Please tell me your password: \n")
  var pw = readLine(stdin)
  while pw != "12345":
    echo("Wrong password! Next try: \n")
    pw = readLine(stdin)


The `while`:idx: statement is executed until the ``expr`` evaluates to false.
Endless loops are no error. ``while`` statements open an `implicit block`,
so that they can be left with a ``break`` statement.


Continue statement
~~~~~~~~~~~~~~~~~~

Syntax::

  continueStmt ::= 'continue'

A `continue`:idx: statement leads to the immediate next iteration of the
surrounding loop construct. It is only allowed within a loop. A continue
statement is syntactic sugar for a nested block:

.. code-block:: nimrod
  while expr1:
    stmt1
    continue
    stmt2

Is equivalent to:

.. code-block:: nimrod
  while expr1:
    block myBlockName:
      stmt1
      break myBlockName
      stmt2


Assembler statement
~~~~~~~~~~~~~~~~~~~
Syntax::

  asmStmt ::= 'asm' [pragma] (STR_LIT | RSTR_LIT | TRIPLESTR_LIT)

The direct embedding of `assembler`:idx: code into Nimrod code is supported
by the unsafe ``asm`` statement. Identifiers in the assembler code that refer to
Nimrod identifiers shall be enclosed in a special character which can be
specified in the statement's pragmas. The default special character is ``'`'``.


If expression
~~~~~~~~~~~~~

An `if expression` is almost like an if statement, but it is an expression.
Example:

.. code-block:: nimrod
  p(if x > 8: 9 else: 10)

An if expression always results in a value, so the ``else`` part is
required. ``Elif`` parts are also allowed (but unlikely to be good
style).


Type conversions
~~~~~~~~~~~~~~~~
Syntactically a `type conversion` is like a procedure call, but a
type name replaces the procedure name. A type conversion is always
safe in the sense that a failure to convert a type to another
results in an exception (if it cannot be determined statically).


Type casts
~~~~~~~~~~
Example:

.. code-block:: nimrod
  cast[int](x)

Type casts are a crude mechanism to interpret the bit pattern of
an expression as if it would be of another type. Type casts are
only needed for low-level programming and are inherently unsafe.


The addr operator
~~~~~~~~~~~~~~~~~
The `addr` operator returns the address of an l-value. If the
type of the location is ``T``, the `addr` operator result is
of the type ``ptr T``. Taking the address of an object that resides
on the stack is **unsafe**, as the pointer may live longer than the
object on the stack and can thus reference a non-existing object.


Procedures
~~~~~~~~~~
What most programming languages call `methods`:idx: or `functions`:idx: are
called `procedures`:idx: in Nimrod (which is the correct terminology). A
procedure declaration defines an identifier and associates it with a block
of code. 
A procedure may call itself recursively. A parameter may be given a default
value that is used if the caller does not provide a value for this parameter.
The syntax is::

  param ::= symbol (comma symbol)* (':' typeDesc ['=' expr] | '=' expr)
  paramList ::= ['(' [param (comma param)*] [SAD] ')'] [':' typeDesc]

  genericParam ::= symbol [':' typeDesc] ['=' expr]
  genericParams ::= '[' genericParam (comma genericParam)* [SAD] ']'

  procDecl ::= 'proc' symbol ['*'] [genericParams] paramList [pragma]
               ['=' stmt]

If the ``= stmt`` part is missing, it is a `forward`:idx: declaration. If
the proc returns a value, the procedure body can access an implicitly declared
variable named `result`:idx: that represents the return value. Procs can be
overloaded. The overloading resolution algorithm tries to find the proc that is
the best match for the arguments. Example:

.. code-block:: nimrod

  proc toLower(c: Char): Char = # toLower for characters
    if c in {'A'..'Z'}:
      result = chr(ord(c) + (ord('a') - ord('A')))
    else:
      result = c

  proc toLower(s: string): string = # toLower for strings
    result = newString(len(s))
    for i in 0..len(s) - 1:
      result[i] = toLower(s[i]) # calls toLower for characters; no recursion!

Calling a procedure can be done in many different ways:

.. code-block:: nimrod
  proc callme(x, y: int, s: string = "", c: char, b: bool = false) = ...

  # call with positional arguments # parameter bindings:
  callme(0, 1, "abc", '\t', true)  # (x=0, y=1, s="abc", c='\t', b=true)
  # call with named and positional arguments:
  callme(y=1, x=0, "abd", '\t')    # (x=0, y=1, s="abd", c='\t', b=false)
  # call with named arguments (order is not relevant):
  callme(c='\t', y=1, x=0)         # (x=0, y=1, s="", c='\t', b=false)
  # call as a command statement: no () needed:
  callme 0, 1, "abc", '\t'


A procedure cannot modify its parameters (unless the parameters have the type
`var`).

`Operators`:idx: are procedures with a special operator symbol as identifier:

.. code-block:: nimrod
  proc `$` (x: int): string =
    # converts an integer to a string; this is a prefix operator.
    return intToStr(x)

Operators with one parameter are prefix operators, operators with two
parameters are infix operators. (However, the parser distinguishes these from
the operator's position within an expression.) There is no way to declare
postfix operators: all postfix operators are built-in and handled by the
grammar explicitly.

Any operator can be called like an ordinary proc with the '`opr`'
notation. (Thus an operator can have more than two parameters):

.. code-block:: nimrod
  proc `*+` (a, b, c: int): int =
    # Multiply and add
    return a * b + c

  assert `*+`(3, 4, 6) == `*`(a, `+`(b, c))



Var parameters
~~~~~~~~~~~~~~
The type of a parameter may be prefixed with the ``var`` keyword:

.. code-block:: nimrod
  proc divmod(a, b: int,
              res, remainder: var int) =
    res = a div b
    remainder = a mod b

  var
    x, y: int

  divmod(8, 5, x, y) # modifies x and y
  assert x == 1
  assert y == 3

In the example, ``res`` and ``remainder`` are `var parameters`.
Var parameters can be modified by the procedure and the changes are
visible to the caller. The argument passed to a var parameter has to be
an l-value. Var parameters are implemented as hidden pointers. The
above example is equivalent to:

.. code-block:: nimrod
  proc divmod(a, b: int,
              res, remainder: ptr int) =
    res^ = a div b
    remainder^ = a mod b

  var
    x, y: int
  divmod(8, 5, addr(x), addr(y))
  assert x == 1
  assert y == 3

In the examples, var parameters or pointers are used to provide two
return values. This can be done in a cleaner way by returning a tuple:

.. code-block:: nimrod
  proc divmod(a, b: int): tuple[res, remainder: int] =
    return (a div b, a mod b)

  var t = divmod(8, 5)
  assert t.res == 1
  assert t.remainder = 3

One can use `tuple unpacking`:idx: to access the tuple's fields:

.. code-block:: nimrod
  var (x, y) = divmod(8, 5) # tuple unpacking
  assert x == 1
  assert y == 3


Overloading of the subscript operator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``[]`` subscript operator for arrays/openarrays/sequences can be overloaded.
Overloading support is only possible if the first parameter has no type that
already supports the built-in ``[]`` notation. Currently the compiler currently
does not check this. XXX Multiple indexes


Multi-methods
~~~~~~~~~~~~~

Procedures always use static dispatch. `Multi-methods`:idx: use dynamic
dispatch.

.. code-block:: nimrod
  type
    TExpr = object ## abstract base class for an expression
    TLiteral = object of TExpr
      x: int
    TPlusExpr = object of TExpr
      a, b: ref TExpr
      
  method eval(e: ref TExpr): int =
    # override this base method
    quit "to override!"
  
  method eval(e: ref TLiteral): int = return e.x

  method eval(e: ref TPlusExpr): int =
    # watch out: relies on dynamic binding
    return eval(e.a) + eval(e.b)
  
  proc newLit(x: int): ref TLiteral =
    new(result)
    result.x = x
    
  proc newPlus(a, b: ref TExpr): ref TPlusExpr =
    new(result)
    result.a = a
    result.b = b
  
  echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4)))
  
In the example the constructors ``newLit`` and ``newPlus`` are procs
because they should use static binding, but ``eval`` is a method because it
requires dynamic binding.

In a multi-method all parameters that have an object type are used for the
dispatching:

.. code-block:: nimrod
  type
    TThing = object
    TUnit = object of TThing
      x: int
      
  method collide(a, b: TThing) {.inline.} =
    quit "to override!"
    
  method collide(a: TThing, b: TUnit) {.inline.} =
    echo "1"
  
  method collide(a: TUnit, b: TThing) {.inline.} =
    echo "2"
  
  var
    a, b: TUnit
  collide(a, b) # output: 2


Invocation of a multi-method cannot be ambiguous: collide 2 is preferred over 
collide 1 because the resolution works from left to right. 
In the example ``TUnit, TThing`` is prefered over ``TThing, TUnit``.

**Performance note**: Nimrod does not produce a virtual method table, but
generates dispatch trees. This avoids the expensive indirect branch for method
calls and enables inlining. However, other optimizations like compile time
evaluation or dead code elimination do not work with methods.


Iterators and the for statement
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Syntax::

  forStmt ::= 'for' symbol (comma symbol)* [comma] 'in' expr ['..' expr] ':' stmt

  param ::= symbol (comma symbol)* [comma] ':' typeDesc
  paramList ::= ['(' [param (comma param)* [comma]] ')'] [':' typeDesc]

  genericParam ::= symbol [':' typeDesc]
  genericParams ::= '[' genericParam (comma genericParam)* [comma] ']'

  iteratorDecl ::= 'iterator' symbol ['*'] [genericParams] paramList [pragma]
               ['=' stmt]

The `for`:idx: statement is an abstract mechanism to iterate over the elements
of a container. It relies on an `iterator`:idx: to do so. Like ``while``
statements, ``for`` statements open an `implicit block`:idx:, so that they
can be left with a ``break`` statement. The ``for`` loop declares
iteration variables (``x`` in the example) - their scope reaches until the
end of the loop body. The iteration variables' types are inferred by the
return type of the iterator.

An iterator is similar to a procedure, except that it is always called in the
context of a ``for`` loop. Iterators provide a way to specify the iteration over
an abstract type. A key role in the execution of a ``for`` loop plays the
``yield`` statement in the called iterator. Whenever a ``yield`` statement is
reached the data is bound to the ``for`` loop variables and control continues
in the body of the ``for`` loop. The iterator's local variables and execution
state are automatically saved between calls. Example:

.. code-block:: nimrod
  # this definition exists in the system module
  iterator items*(a: string): char {.inline.} =
    var i = 0
    while i < len(a):
      yield a[i]
      inc(i)

  for ch in items("hello world"): # `ch` is an iteration variable
    echo(ch)

The compiler generates code as if the programmer would have written this:

.. code-block:: nimrod
  var i = 0
  while i < len(a):
    var ch = a[i]
    echo(ch)
    inc(i)

The current implementation always inlines the iterator code leading to zero
overhead for the abstraction. But this may increase the code size. Later
versions of the compiler will only inline iterators which have the calling
convention ``inline``.

If the iterator yields a tuple, there have to be as many iteration variables
as there are components in the tuple. The i'th iteration variable's type is
the one of the i'th component.


Type sections
~~~~~~~~~~~~~

Syntax::

  typeDef ::= typeDesc | objectDef | enumDef

  genericParam ::= symbol [':' typeDesc]
  genericParams ::= '[' genericParam (comma genericParam)* [comma] ']'

  typeDecl ::= COMMENT
             | symbol ['*'] [genericParams] ['=' typeDef] [COMMENT|IND COMMENT]

  typeSection ::= 'type' indPush typeDecl (SAD typeDecl)* DED indPop


Example:

.. code-block:: nimrod
  type # example demonstrates mutually recursive types
    PNode = ref TNode # a traced pointer to a TNode
    TNode = object
      le, ri: PNode   # left and right subtrees
      sym: ref TSym   # leaves contain a reference to a TSym

    TSym = object     # a symbol
      name: string    # the symbol's name
      line: int       # the line the symbol was declared in
      code: PNode     # the symbol's abstract syntax tree

A `type`:idx: section begins with the ``type`` keyword. It contains multiple
type definitions. A type definition binds a type to a name. Type definitions
can be recursive or even mutually recursive. Mutually recursive types are only
possible within a single ``type`` section.


Generics
~~~~~~~~

Example:

.. code-block:: nimrod
  type
    TBinaryTree[T] = object      # TBinaryTree is a generic type with
                                 # with generic param ``T``
      le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil
      data: T                    # the data stored in a node
    PBinaryTree[T] = ref TBinaryTree[T] # a shorthand for notational convenience

  proc newNode[T](data: T): PBinaryTree[T] = # constructor for a node
    new(result)
    result.dat = data

  proc add[T](root: var PBinaryTree[T], n: PBinaryTree[T]) =
    if root == nil:
      root = n
    else:
      var it = root
      while it != nil:
        var c = cmp(it.data, n.data) # compare the data items; uses
                                     # the generic ``cmd`` proc that works for
                                     # any type that has a ``==`` and ``<``
                                     # operator
        if c < 0:
          if it.le == nil:
            it.le = n
            return
          it = it.le
        else:
          if it.ri == nil:
            it.ri = n
            return
          it = it.ri

  iterator inorder[T](root: PBinaryTree[T]): T =
    # inorder traversal of a binary tree
    # recursive iterators are not yet implemented, so this does not work in
    # the current compiler!
    if root.le != nil: yield inorder(root.le)
    yield root.data
    if root.ri != nil: yield inorder(root.ri)

  var
    root: PBinaryTree[string] # instantiate a PBinaryTree with the type string
  add(root, newNode("hallo")) # instantiates generic procs ``newNode`` and
  add(root, newNode("world")) # ``add``
  for str in inorder(root):
    writeln(stdout, str)

`Generics`:idx: are Nimrod's means to parametrize procs, iterators or types with
`type parameters`:idx:. Depending on context, the brackets are used either to
introduce type parameters or to instantiate a generic proc, iterator or type.


Templates
~~~~~~~~~

A `template`:idx: is a simple form of a macro: It is a simple substitution
mechanism that operates on Nimrod's abstract syntax trees. It is processed in
the semantic pass of the compiler.

The syntax to *invoke* a template is the same as calling a procedure.

Example:

.. code-block:: nimrod
  template `!=` (a, b: expr): expr =
    # this definition exists in the System module
    not (a == b)

  assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6))

The ``!=``, ``>``, ``>=``, ``in``, ``notin``, ``isnot`` operators are in fact 
templates:

| ``a > b`` is transformed into ``b < a``.
| ``a in b`` is transformed into ``contains(b, a)``. 
| ``notin`` and ``isnot`` have the obvious meanings.

The "types" of templates can be the symbols ``expr`` (stands for *expression*), 
``stmt`` (stands for *statement*) or ``typedesc`` (stands for *type 
description*). These are no real types, they just help the compiler parsing.
Real types can be used too; this implies that expressions are expected.
However, for parameter type checking the arguments are semantically checked
before being passed to the template. Other arguments are not semantically
checked before being passed to the template.

The template body does not open a new scope. To open a new scope a ``block``
statement can be used:

.. code-block:: nimrod
  template declareInScope(x: expr, t: typeDesc): stmt = 
    var x: t
    
  template declareInNewScope(x: expr, t: typeDesc): stmt = 
    # open a new scope:
    block: 
      var x: t

  declareInScope(a, int)
  a = 42  # works, `a` is known here
  
  declareInNewScope(b, int)
  b = 42  # does not work, `b` is unknown


If there is a ``stmt`` parameter it should be the last in the template
declaration, because statements are passed to a template via a
special ``:`` syntax:

.. code-block:: nimrod

  template withFile(f, fn, mode: expr, actions: stmt): stmt =
    block:
      var f: TFile
      if open(f, fn, mode):
        try:
          actions
        finally:
          close(f)
      else:
        quit("cannot open: " & fn)
      
  withFile(txt, "ttempl3.txt", fmWrite):
    txt.writeln("line 1")
    txt.writeln("line 2")
  
In the example the two ``writeln`` statements are bound to the ``actions``
parameter. 

**Note:** Symbol binding rules in templates might change!

Symbol binding within templates happens after template instantation: 

.. code-block:: nimrod
  # Module A
  var 
    lastId = 0
  
  template genId*: expr =
    inc(lastId)
    lastId

.. code-block:: nimrod
  # Module B
  import A
  
  echo genId() # Error: undeclared identifier: 'lastId'

Exporting a template is a often a leaky abstraction. However, to compensate for
this case, the ``bind`` operator can be used: All identifiers within a ``bind``
context are bound early (i.e. when the template is parsed).
The affected identifiers are then always bound early even if the other
occurences are in no ``bind`` context: 

.. code-block:: nimrod
  # Module A
  var 
    lastId = 0
  
  template genId*: expr =
    inc(bind lastId)
    lastId

.. code-block:: nimrod
  # Module B
  import A
  
  echo genId() # Works


**Style note**: For code readability, it is the best idea to use the least
powerful programming construct that still suffices. So the "check list" is:

(1) Use an ordinary proc/iterator, if possible.
(2) Else: Use a generic proc/iterator, if possible.
(3) Else: Use a template, if possible.
(4) Else: Use a macro.


Macros
------

`Macros`:idx: are the most powerful feature of Nimrod. They can be used
to implement `domain specific languages`:idx:.

While macros enable advanced compile-time code transformations, they
cannot change Nimrod's syntax. However, this is no real restriction because
Nimrod's syntax is flexible enough anyway.

To write macros, one needs to know how the Nimrod concrete syntax is converted
to an abstract syntax tree.

There are two ways to invoke a macro:
(1) invoking a macro like a procedure call (`expression macros`)
(2) invoking a macro with the special ``macrostmt`` syntax (`statement macros`)


Expression Macros
~~~~~~~~~~~~~~~~~

The following example implements a powerful ``debug`` command that accepts a
variable number of arguments:

.. code-block:: nimrod
  # to work with Nimrod syntax trees, we need an API that is defined in the
  # ``macros`` module:
  import macros

  macro debug(n: expr): stmt =
    # `n` is a Nimrod AST that contains the whole macro invocation
    # this macro returns a list of statements:
    result = newNimNode(nnkStmtList, n)
    # iterate over any argument that is passed to this macro:
    for i in 1..n.len-1:
      # add a call to the statement list that writes the expression;
      # `toStrLit` converts an AST to its string representation:
      add(result, newCall("write", newIdentNode("stdout"), toStrLit(n[i])))
      # add a call to the statement list that writes ": "
      add(result, newCall("write", newIdentNode("stdout"), newStrLitNode(": ")))
      # add a call to the statement list that writes the expressions value:
      add(result, newCall("writeln", newIdentNode("stdout"), n[i]))

  var
    a: array [0..10, int]
    x = "some string"
  a[0] = 42
  a[1] = 45

  debug(a[0], a[1], x)

The macro call expands to:

.. code-block:: nimrod
  write(stdout, "a[0]")
  write(stdout, ": ")
  writeln(stdout, a[0])

  write(stdout, "a[1]")
  write(stdout, ": ")
  writeln(stdout, a[1])

  write(stdout, "x")
  write(stdout, ": ")
  writeln(stdout, x)


Statement Macros
~~~~~~~~~~~~~~~~

Statement macros are defined just as expression macros. However, they are
invoked by an expression following a colon::

  exprStmt ::= lowestExpr ['=' expr | [expr (comma expr)* [comma]] [macroStmt]]
  macroStmt ::= ':' [stmt] ('of' [sliceExprList] ':' stmt
                          | 'elif' expr ':' stmt
                          | 'except' exceptList ':' stmt )*
                           ['else' ':' stmt]

The following example outlines a macro that generates a lexical analyzer from
regular expressions:

.. code-block:: nimrod
  import macros

  macro case_token(n: stmt): stmt =
    # creates a lexical analyzer from regular expressions
    # ... (implementation is an exercise for the reader :-)
    nil

  case_token: # this colon tells the parser it is a macro statement
  of r"[A-Za-z_]+[A-Za-z_0-9]*":
    return tkIdentifier
  of r"0-9+":
    return tkInteger
  of r"[\+\-\*\?]+":
    return tkOperator
  else:
    return tkUnknown



Modules
-------
Nimrod supports splitting a program into pieces by a `module`:idx: concept.
Each module needs to be in its own file and has its own `namespace`:idx:.
Modules enable `information hiding`:idx: and `separate compilation`:idx:.
A module may gain access to symbols of another module by the `import`:idx:
statement. `Recursive module dependencies`:idx: are allowed, but slightly
subtle. Only top-level symbols that are marked with an asterisk (``*``) are
exported.

The algorithm for compiling modules is:

- compile the whole module as usual, following import statements recursively
- if there is a cycle only import the already parsed symbols (that are
  exported); if an unknown identifier occurs then abort

This is best illustrated by an example:

.. code-block:: nimrod
  # Module A
  type
    T1* = int  # Module A exports the type ``T1``
  import B     # the compiler starts parsing B

  proc main() =
    var i = p(3) # works because B has been parsed completely here

  main()


.. code-block:: nimrod
  # Module B
  import A  # A is not parsed here! Only the already known symbols
            # of A are imported.

  proc p*(x: A.T1): A.T1 =
    # this works because the compiler has already
    # added T1 to A's interface symbol table
    return x + 1


Scope rules
-----------
Identifiers are valid from the point of their declaration until the end of
the block in which the declaration occurred. The range where the identifier
is known is the `scope`:idx: of the identifier. The exact scope of an
identifier depends on the way it was declared.

Block scope
~~~~~~~~~~~
The *scope* of a variable declared in the declaration part of a block
is valid from the point of declaration until the end of the block. If a
block contains a second block, in which the identifier is redeclared,
then inside this block, the second declaration will be valid. Upon
leaving the inner block, the first declaration is valid again. An
identifier cannot be redefined in the same block, except if valid for
procedure or iterator overloading purposes.


Tuple or object scope
~~~~~~~~~~~~~~~~~~~~~
The field identifiers inside a tuple or object definition are valid in the
following places:

* To the end of the tuple/object definition.
* Field designators of a variable of the given tuple/object type.
* In all descendant types of the object type.

Module scope
~~~~~~~~~~~~
All identifiers of a module are valid from the point of declaration until
the end of the module. Identifiers from indirectly dependent modules are *not* 
available. The `system`:idx: module is automatically imported in every other 
module.

If a module imports an identifier by two different modules, each occurrence of
the identifier has to be qualified, unless it is an overloaded procedure or
iterator in which case the overloading resolution takes place:

.. code-block:: nimrod
  # Module A
  var x*: string

.. code-block:: nimrod
  # Module B
  var x*: int

.. code-block:: nimrod
  # Module C
  import A, B
  write(stdout, x) # error: x is ambiguous
  write(stdout, A.x) # no error: qualifier used

  var x = 4
  write(stdout, x) # not ambiguous: uses the module C's x


Messages
========

The Nimrod compiler emits different kinds of messages: `hint`:idx:,
`warning`:idx:, and `error`:idx: messages. An *error* message is emitted if
the compiler encounters any static error.


Pragmas
=======

Syntax::

  colonExpr ::= expr [':' expr]
  colonExprList ::= [colonExpr (comma colonExpr)* [comma]]

  pragma ::= '{.' optInd (colonExpr [comma])* [SAD] ('.}' | '}')

Pragmas are Nimrod's method to give the compiler additional information /
commands without introducing a massive number of new keywords. Pragmas are
processed on the fly during semantic checking. Pragmas are enclosed in the
special ``{.`` and ``.}`` curly brackets. Pragmas are also often used as a
first implementation to play with a language feature before a nicer syntax
to access the feature becomes available.


noSideEffect pragma
-------------------
The `noSideEffect`:idx: pragma is used to mark a proc/iterator to have no side
effects. This means that the proc/iterator only changes locations that are
reachable from its parameters and the return value only depends on the
arguments. If none of its parameters have the type ``var T``
or ``ref T`` or ``ptr T`` this means no locations are modified. It is a static
error to mark a proc/iterator to have no side effect if the compiler cannot
verify this.

**Future directions**: ``func`` may become a keyword and syntactic sugar for a
proc with no side effects:

.. code-block:: nimrod
  func `+` (x, y: int): int


procvar pragma
--------------
The `procvar`:idx: pragma is used to mark a proc that it can be passed to a
procedural variable.


compileTime pragma
------------------
The `compileTime`:idx: pragma is used to mark a proc to be used at compile
time only. No code will be generated for it. Compile time procs are useful
as helpers for macros.


noReturn pragma
---------------
The `noreturn`:idx: pragma is used to mark a proc that it never returns. 


Acyclic pragma
--------------
The `acyclic`:idx: pragma can be used for object types to mark them as acyclic
even though they seem to be cyclic. This is an **optimization** for the garbage
collector to not consider objects of this type as part of a cycle:

.. code-block:: nimrod
  type
    PNode = ref TNode
    TNode {.acyclic, final.} = object
      left, right: PNode
      data: string

In the example a tree structure is declared with the ``TNode`` type. Note that
the type definition is recursive and the GC has to assume that objects of
this type may form a cyclic graph. The ``acyclic`` pragma passes the
information that this cannot happen to the GC. If the programmer uses the
``acyclic`` pragma for data types that are in reality cyclic, the GC may leak
memory, but nothing worse happens.

**Future directions**: The ``acyclic`` pragma may become a property of a
``ref`` type:

.. code-block:: nimrod
  type
    PNode = acyclic ref TNode
    TNode = object
      left, right: PNode
      data: string


Final pragma
------------
The `final`:idx: pragma can be used for an object type to specify that it
cannot be inherited from.


Pure pragma
-----------
The `pure`:idx: pragma serves two completely different purposes:
1) To mark a procedure that Nimrod should not generate any exit statements like
   ``return result;`` in the generated code. This is useful for procs that only
   consist of an assembler statement.
2) To mark an object type so that its type field should be omitted. This is
   necessary for binary compatibility with other compiled languages.


error pragma
------------
The `error`:idx: pragma is used to make the compiler output an error message
with the given content. Compilation currently aborts after an error, but this
may be changed in later versions.


fatal pragma
------------
The `fatal`:idx: pragma is used to make the compiler output an error message
with the given content. In contrast to the ``error`` pragma, compilation
is guaranteed to be aborted by this pragma.

warning pragma
--------------
The `warning`:idx: pragma is used to make the compiler output a warning message
with the given content. Compilation continues after the warning.

hint pragma
-----------
The `hint`:idx: pragma is used to make the compiler output a hint message with
the given content. Compilation continues after the hint.


compilation option pragmas
--------------------------
The listed pragmas here can be used to override the code generation options
for a section of code.

The implementation currently provides the following possible options (various
others may be added later).

===============  ===============  ============================================
pragma           allowed values   description
===============  ===============  ============================================
checks           on|off           Turns the code generation for all runtime
                                  checks on or off.
boundChecks      on|off           Turns the code generation for array bound
                                  checks on or off.
overflowChecks   on|off           Turns the code generation for over- or
                                  underflow checks on or off.
nilChecks        on|off           Turns the code generation for nil pointer
                                  checks on or off.
assertions       on|off           Turns the code generation for assertions
                                  on or off.
warnings         on|off           Turns the warning messages of the compiler
                                  on or off.
hints            on|off           Turns the hint messages of the compiler
                                  on or off.
optimization     none|speed|size  Optimize the code for speed or size, or
                                  disable optimization.
callconv         cdecl|...        Specifies the default calling convention for
                                  all procedures (and procedure types) that
                                  follow.
===============  ===============  ============================================

Example:

.. code-block:: nimrod
  {.checks: off, optimization: speed.}
  # compile without runtime checks and optimize for speed


push and pop pragmas
--------------------
The `push/pop`:idx: pragmas are very similar to the option directive,
but are used to override the settings temporarily. Example:

.. code-block:: nimrod
  {.push checks: off.}
  # compile this section without runtime checks as it is
  # speed critical
  # ... some code ...
  {.pop.} # restore old settings


Register pragma
---------------
The `register`:idx: pragma is for variables only. It declares the variable as
``register``, giving the compiler a hint that the variable should be placed
in a hardware register for faster access. C compilers usually ignore this
though and for good reasons: Often they do a better job without it anyway.

In highly specific cases (a dispatch loop of an bytecode interpreter for
example) it may provide benefits, though.


DeadCodeElim pragma
-------------------
The `deadCodeElim`:idx: pragma only applies to whole modules: It tells the
compiler to activate (or deactivate) dead code elimination for the module the
pragma appers in.

The ``--deadCodeElim:on`` command line switch has the same effect as marking
every module with ``{.deadCodeElim:on}``. However, for some modules such as
the GTK wrapper it makes sense to *always* turn on dead code elimination -
no matter if it is globally active or not.

Example:

.. code-block:: nimrod
  {.deadCodeElim: on.}


Pragma pragma
-------------

The `pragma`:idx: pragma can be used to declare user defined pragmas. This is 
useful because Nimrod's templates and macros do not affect pragmas. User 
defined pragmas are in a different module-wide scope than all other symbols. 
They cannot be imported from a module.

Example:

.. code-block:: nimrod
  when appType == "lib":
    {.pragma: rtl, exportc, dynlib, cdecl.}
  else:
    {.pragma: rtl, importc, dynlib: "client.dll", cdecl.}
    
  proc p*(a, b: int): int {.rtl.} = 
    return a+b

In the example a new pragma named ``rtl`` is introduced that either imports
a symbol from a dynamic library or exports the symbol for dynamic library
generation.


Disabling certain messages
--------------------------
Nimrod generates some warnings and hints ("line too long") that may annoy the
user. A mechanism for disabling certain messages is provided: Each hint
and warning message contains a symbol in brackets. This is the message's
identifier that can be used to enable or disable it:

.. code-block:: Nimrod
  {.warning[LineTooLong]: off.} # turn off warning about too long lines

This is often better than disabling all warnings at once.


Foreign function interface
==========================

Nimrod's `FFI`:idx: (foreign function interface) is extensive and only the
parts that scale to other future backends (like the LLVM/EcmaScript backends)
are documented here.


Importc pragma
--------------
The `importc`:idx: pragma provides a means to import a proc or a variable
from C. The optional argument is a string containing the C identifier. If
the argument is missing, the C name is the Nimrod identifier *exactly as
spelled*:

.. code-block::
  proc printf(formatstr: cstring) {.importc: "printf", varargs.}

Note that this pragma is somewhat of a misnomer: Other backends will provide
the same feature under the same name.


Exportc pragma
--------------
The `exportc`:idx: pragma provides a means to export a type, a variable, or a
procedure to C. The optional argument is a string containing the C identifier.
If the argument is missing, the C name is the Nimrod
identifier *exactly as spelled*:

.. code-block:: Nimrod
  proc callme(formatstr: cstring) {.exportc: "callMe", varargs.}

Note that this pragma is somewhat of a misnomer: Other backends will provide
the same feature under the same name.


Varargs pragma
--------------
The `varargs`:idx: pragma can be applied to procedures only (and procedure 
types). It tells Nimrod that the proc can take a variable number of parameters 
after the last specified parameter. Nimrod string values will be converted to C
strings automatically:

.. code-block:: Nimrod
  proc printf(formatstr: cstring) {.nodecl, varargs.}

  printf("hallo %s", "world") # "world" will be passed as C string


Dynlib pragma for import
------------------------
With the `dynlib`:idx: pragma a procedure can be imported from
a dynamic library (``.dll`` files for Windows, ``lib*.so`` files for UNIX). The
non-optional argument has to be the name of the dynamic library:

.. code-block:: Nimrod
  proc gtk_image_new(): PGtkWidget {.cdecl, dynlib: "libgtk-x11-2.0.so", importc.}

In general, importing a dynamic library does not require any special linker
options or linking with import libraries. This also implies that no *devel*
packages need to be installed.

The ``dynlib`` import mechanism supports a versioning scheme: 

.. code-block:: nimrod 
  proc Tcl_Eval(interp: pTcl_Interp, script: cstring): int {.cdecl, 
    importc, dynlib: "libtcl(|8.5|8.4|8.3).so.(1|0)".}

At runtime the dynamic library is searched for (in this order)::
  
  libtcl.so.1
  libtcl.so.0
  libtcl8.5.so.1  
  libtcl8.5.so.0
  libtcl8.4.so.1
  libtcl8.4.so.0
  libtcl8.3.so.1
  libtcl8.3.so.0

The ``dynlib`` pragma supports not only constant strings as argument but also
string expressions in general:

.. code-block:: nimrod
  import os

  proc getDllName: string = 
    result = "mylib.dll"
    if ExistsFile(result): return
    result = "mylib2.dll"
    if ExistsFile(result): return
    quit("could not load dynamic library")
  
  proc myImport(s: cstring) {.cdecl, importc, dynlib: getDllName().}

**Note**: Patterns like ``libtcl(|8.5|8.4).so`` are only supported in constant
strings, because they are precompiled.

Dynlib pragma for export
------------------------

With the ``dynlib`` pragma a procedure can also be exported to
a dynamic library. The pragma then has no argument and has to be used in
conjunction with the ``exportc`` pragma:

.. code-block:: Nimrod
  proc exportme(): int {.cdecl, export, dynlib.}

This is only useful if the program is compiled as a dynamic library via the
``--app:lib`` command line option.