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=============
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
---------------
Terminal symbol in the grammar: ``STR_LIT``.
`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:
``\`` '0'..'9'+ `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:
``\x`` HH `character with hex value HH`:idx:;
exactly two hex digits are allowed
================== ===================================================
Strings in Nimrod may contain any 8-bit value, even embedded zeros. However
some operations may interpret the first binary zero as a terminator.
Triple quoted string literals
-----------------------------
Terminal symbol in the grammar: ``TRIPLESTR_LIT``.
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
-------------------
Terminal symbol in the grammar: ``RSTR_LIT``.
There are also `raw string literals`:idx: 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. ``r"""`` is the same
as ``"""`` since triple quoted string literals do not interpret escape
sequences either.
Generalized raw string literals
-------------------------------
Terminal symbols in the grammar: ``GENERALIZED_STR_LIT``,
``GENERALIZED_TRIPLESTR_LIT``.
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.
Operators
---------
In Nimrod one can define his own operators. An `operator`:idx: is any
combination of the following characters::
= + - * / < >
@ $ ~ & % |
! ? ^ . : \
These keywords are also operators:
``and or not xor shl shr div mod in notin is isnot of``.
`=`:tok:, `:`:tok:, `::`:tok: are not available as general operators; they
are used for other notational purposes.
``*:`` is as a special case the two tokens `*`:tok: and `:`:tok:
(to support ``var v*: T``).
Other tokens
------------
The following strings denote other tokens::
` ( ) { } [ ] , ; [. .] {. .} (. .)
The `slice`:idx: operator `..`: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:.
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 10 different levels of precedence.
All binary operators are left-associative, except binary operators starting
with (or only consisting of) ``^``.
For operators that are not keywords the precedence is determined by the
following rules:
An operator symbol's *relevant character* is its first
character unless the first character is ``\`` and its length is greater than 1
then it is the second character.
If the operator ends with ``=`` and its relevant character is none of
``<``, ``>``, ``!``, ``=``, ``~``, ``?``, it is an *assignment operator* which
has the lowest precedence.
Otherwise precedence is determined by the relevant character.
================ =============================================== ================== ===============
Precedence level Operators Relevant character Terminal symbol
================ =============================================== ================== ===============
9 (highest) ``$ ^`` OP9
8 ``* / div mod shl shr %`` ``* % \ /`` OP8
7 ``+ -`` ``+ ~ |`` OP7
6 ``&`` ``&`` OP6
5 ``..`` ``.`` OP5
4 ``== <= < >= > != in not_in is isnot not of`` ``= < > !`` OP4
3 ``and`` OP3
2 ``or xor`` OP2
1 ``@ : ?`` OP1
0 (lowest) *assignment operator* (like ``+=``, ``*=``) OP0
================ =============================================== ================== ===============
The grammar's start symbol is ``module``.
.. include:: grammar.txt
:literal:
Semantics
=========
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 and has the
same size as a pointer. 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`:idx: 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.
The compiler supports the built-in stringify operator ``$`` for enumerations.
The stringify's result can be controlled by explicitely giving the string
values to use:
.. code-block:: nimrod
type
TMyEnum = enum
valueA = (0, "my value A"),
valueB = "value B",
valueC = 2,
valueD = (3, "abc")
As can be seen from the example, it is possible to both specify a field's
ordinal value and its string value by using a tuple. It is also
possible to only specify one of them.
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.
CString type
~~~~~~~~~~~~
The `cstring`:idx: type represents a pointer to a zero-terminated char array
compatible to the type ``char*`` in Ansi C. Its primary purpose lies in easy
interfacing with C. The index operation ``s[i]`` means the i-th *char* of
``s``; however no bounds checking for ``cstring`` is performed making the
index operation unsafe.
A Nimrod ``string`` is implicitely convertible
to ``cstring`` for convenience. If a Nimrod string is passed to a C-style
variadic proc, it is implicitely converted to ``cstring`` too:
.. code-block:: nimrod
proc printf(formatstr: cstring) {.importc: "printf", varargs,
header: "<stdio.h>".}
printf("This works %s", "as expected")
Even though the conversion is implict, it is not *safe*: The garbage collector
does not consider a ``cstring`` to be a root and may collect the underlying
memory. However in practice this almost never happens as the GC considers
stack roots conservatively. One can use the builtin procs ``GC_ref`` and
``GC_unref`` to keep the string data alive for the rare cases where it does
not work.
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 ``of`` 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 of 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 (also called `aliasing`:idx:).
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.
An empty subscript ``[]`` notation 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; in fact n[].data is highly discouraged!
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:
.. code-block:: nimrod
type
TData = tuple[x, y: int, s: string]
# allocate memory for TData on the heap:
var d = cast[ptr TData](alloc0(sizeof(TData)))
# create a new string on the garbage collected heap:
d.s = "abc"
# tell the GC that the string is not needed anymore:
GCunref(d.s)
# free the memory:
dealloc(d)
Without the ``GCunref`` call the memory allocated for the ``d.s`` string would
never be freed. The example also demonstrates two important features for low
level programming: the ``sizeof`` proc returns the size of a type or value
in bytes. The ``cast`` operator can circumvent the type system: the compiler
is forced to treat the result of the ``alloc0`` call (which returns an untyped
pointer) as if it would have the type ``ptr TData``. Casting should only be
done if it is unavoidable: it breaks type safety and bugs can lead to
mysterious crashes.
**Note**: The example only works because the memory is initialized to zero
(``alloc0`` instead of ``alloc`` does this): ``d.s`` is thus initialized to
``nil`` which the string assignment can handle. You need to know low level
details like this when mixing garbage collected data with unmanaged memory.
.. 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.
Examples:
.. 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
.. code-block:: nimrod
type
TOnMouseMove = proc (x, y: int) {.closure.}
proc onMouseMove(mouseX, mouseY: int) =
# has default calling convention
echo "x: ", mouseX, " y: ", mouseY
proc setOnMouseMove(mouseMoveEvent: TOnMouseMove) = nil
# ok, 'onMouseMove' has the default calling convention, which is compatible
# to 'closure':
setOnMouseMove(onMouseMove)
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. As a special extension,
a procedure of the calling convention ``nimcall`` can be passed to a parameter
that expects a proc of the calling convention ``closure``.
Nimrod supports these `calling conventions`:idx:\:
`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; 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 has a hidden implicit parameter
(an *environment*). Proc vars that have the calling convention ``closure``
take up two machine words: One for the proc pointer and another one for
the pointer to implicitely passed environment.
`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.
The default calling convention is ``nimcall``, unless it is an inner proc (
a proc inside of a proc). For an inner proc an analysis is performed wether it
accesses its environment. If it does so, it has the calling convention
``closure``, otherwise it has the calling convention ``nimcall``.
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)
Void type
~~~~~~~~~
The `void`:idx: type denotes the absense of any type. Parameters of
type ``void`` are treated as non-existent, ``void`` as a return type means that
the procedure does not return a value:
.. code-block:: nimrod
proc nothing(x, y: void): void =
echo "ha"
nothing() # writes "ha" to stdout
The ``void`` type is particularly useful for generic code:
.. code-block:: nimrod
proc callProc[T](p: proc (x: T), x: T) =
when T is void:
p()
else:
p(x)
proc intProc(x: int) = nil
proc emptyProc() = nil
callProc[int](intProc, 12)
callProc[void](emptyProc)
However, a ``void`` type cannot be inferred in generic code:
.. code-block:: nimrod
callProc(emptyProc)
# Error: type mismatch: got (proc ())
# but expected one of:
# callProc(p: proc (T), x: T)
The ``void`` type is only valid for parameters and return types; other symbols
cannot have the type ``void``.
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, void:
# 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.
Type equality modulo type distinction
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The following algorithm (in pseudo-code) determines whether two types
are equal with no respect to ``distinct`` types. For brevity the cycle check
with an auxiliary set ``s`` is omitted:
.. code-block:: nimrod
proc typeEqualsOrDistinct(a, b: PType): bool =
if a.kind == b.kind:
case a.kind
of int, intXX, float, floatXX, char, string, cstring, pointer,
bool, nil, void:
# leaf type: kinds identical; nothing more to check
result = true
of ref, ptr, var, set, seq, openarray:
result = typeEqualsOrDistinct(a.baseType, b.baseType)
of range:
result = typeEqualsOrDistinct(a.baseType, b.baseType) and
(a.rangeA == b.rangeA) and (a.rangeB == b.rangeB)
of array:
result = typeEqualsOrDistinct(a.baseType, b.baseType) and
typeEqualsOrDistinct(a.indexType, b.indexType)
of tuple:
if a.tupleLen == b.tupleLen:
for i in 0..a.tupleLen-1:
if not typeEqualsOrDistinct(a[i], b[i]): return false
result = true
of distinct:
result = typeEqualsOrDistinct(a.baseType, b.baseType)
of object, enum:
result = a == b
of proc:
result = typeEqualsOrDistinct(a.parameterTuple, b.parameterTuple) and
typeEqualsOrDistinct(a.resultType, b.resultType) and
a.callingConvention == b.callingConvention
elif a.kind == distinct:
result = typeEqualsOrDistinct(a.baseType, b)
elif b.kind == distinct:
result = typeEqualsOrDistinct(a, b.baseType)
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 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 typeEqualsOrDistinct(a, b): return true
if isIntegralType(a) and isIntegralType(b): return true
if isSubtype(a, b) or isSubtype(b, a): return true
return false
The convertible relation can be relaxed by a user-defined type
`converter`:idx:.
.. code-block:: nimrod
converter toInt(x: char): int = result = ord(x)
var
x: int
chr: char = 'a'
# implicit conversion magic happens here
x = chr
echo x # => 97
# you can use the explicit form too
x = chr.toInt
echo x # => 97
The type conversion ``T(a)`` is an L-value if ``a`` is an L-value and
``typeEqualsOrDistinct(T, type(a))`` holds.
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 | letSection
| typeSection | whenStmt | varSection
Discard statement
~~~~~~~~~~~~~~~~~
Syntax::
discardStmt ::= 'discard' expr
Example:
.. code-block:: nimrod
proc p(x, y: int): int =
return x + y
discard p(3, 4) # discard the return value of `p`
The `discard`:idx: statement evaluates its expression for side-effects and
throws the expression's resulting value away.
Ignoring the return value of a procedure without using a discard statement is
a static error.
The return value can be ignored implicitely if the called proc/iterator has
been declared with the `discardable`:idx: pragma:
.. code-block:: nimrod
proc p(x, y: int): int {.discardable.} =
return x + y
p(3, 4) # now valid
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
============================ ==============================================
The implicit initialization can be avoided for optimization reasons with the
`noinit`:idx: pragma:
.. code-block:: nimrod
var
a {.noInit.}: array [0..1023, char]
If a proc is annotated with the ``noinit`` pragma this refers to its implicit
``result`` variable:
.. code-block:: nimrod
proc returnUndefinedValue: int {.noinit.} = nil
let statement
~~~~~~~~~~~~~
A `Let`:idx: statement declares new local and global `single assignment`:idx:
variables and binds a value to them. The syntax is the of the ``var``
statement, except that the keyword ``var`` is replaced by the keyword ``let``.
Let variables are not l-values and can thus not be passed to ``var`` parameters
nor can their address be taken. They cannot be assigned new values.
For let variables the same pragmas are available as for ordinary variables.
Const section
~~~~~~~~~~~~~
Syntax::
colonAndEquals ::= [':' typeDesc] '=' expr
constDecl ::= symbol ['*'] [pragma] colonAndEquals [COMMENT | IND COMMENT]
| COMMENT
constSection ::= 'const' indPush constDecl (SAD constDecl)* DED indPop
`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!
The rules for compile-time computability are:
1. Literals are compile-time computable.
2. Type conversions are compile-time computable.
3. Procedure calls of the form ``p(X)`` are compile-time computable if
``p`` is a proc without side-effects (see the `noSideEffect pragma`_
for details) and if ``X`` is a (possibly empty) list of compile-time
computable arguments.
Constants cannot be of type ``ptr``, ``ref``, ``var`` or ``object``, nor can
they contain such a type.
Static statement/expression
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Syntax::
staticExpr ::= 'static' '(' optInd expr optPar ')'
staticStmt ::= 'static' ':' stmt
A `static`:idx: statement/expression can be used to enforce compile
time evaluation explicitely. Enforced compile time evaluation can even evaluate
code that has side effects:
.. code-block::
static:
echo "echo at compile time"
It's a static error if the compiler cannot perform the evaluation at compile time.
The current implementation poses some restrictions for compile time
evaluation: Code which contains ``cast`` or makes use of the foreign function
interface cannot be evaluated at compile time. Later versions of Nimrod will
support the FFI at compile time.
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.
As a special semantic extension, an expression in an ``of`` branch of a case
statement may evaluate to a set constructor; the set is then expanded into
a list of its elements:
.. code-block:: nimrod
const
SymChars: set[char] = {'a'..'z', 'A'..'Z', '\x80'..'\xFF'}
proc classify(s: string) =
case s[0]
of SymChars, '_': echo "an identifier"
of '0'..'9': echo "a number"
else: echo "other"
# is equivalent to:
proc classify(s: string) =
case s[0]
of 'a'..'z', 'A'..'Z', '\x80'..'\xFF', '_': echo "an identifier"
of '0'..'9': echo "a number"
else: echo "other"
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 ``'`'``:
.. code-block:: nimrod
proc addInt(a, b: int): int {.noStackFrame.} =
# a in eax, and b in edx
asm """
mov eax, `a`
add eax, `b`
jno theEnd
call `raiseOverflow`
theEnd:
"""
If expression
~~~~~~~~~~~~~
An `if expression` is almost like an if statement, but it is an expression.
Example:
.. code-block:: nimrod
var y = 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).
Table constructor
~~~~~~~~~~~~~~~~~
A `table constructor`:idx: is syntactic sugar for an array constructor:
.. code-block:: nimrod
{"key1": "value1", "key2": "value2"}
# is the same as:
[("key1", "value1"), ("key2", "value2")]
The empty table can be written ``{:}`` (in contrast to the empty set
which is ``{}``) which is thus another way to write as the empty array
constructor ``[]``. This slightly unusal way of supporting tables
has lots of advantages:
* The order of the (key,value)-pairs is preserved, thus it is easy to
support ordered dicts with for example ``{key: val}.newOrderedTable``.
* A table literal can be put into a ``const`` section and the compiler
can easily put it into the executable's data section just like it can
for arrays and the generated data section requires a minimal amount
of memory.
* Every table implementation is treated equal syntactically.
* Apart from the minimal syntactic sugar the language core does not need to
know about tables.
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
Var return type
~~~~~~~~~~~~~~~
A proc, converter or iterator may return a ``var`` type which means that the
returned value is an l-value and can be modified by the caller:
.. code-block:: nimrod
var g = 0
proc WriteAccessToG(): var int =
result = g
WriteAccessToG() = 6
assert g == 6
It is a compile time error if the implicitely introduced pointer could be
used to access a location beyond its lifetime:
.. code-block:: nimrod
proc WriteAccessToG(): var int =
var g = 0
result = g # Error!
For iterators, a component of a tuple return type can have a ``var`` type too:
.. code-block:: nimrod
iterator mpairs(a: var seq[string]): tuple[key: int, val: var string] =
for i in 0..a.high:
yield (i, a[i])
In the standard library every name of a routine that returns a ``var`` type
starts with the prefix ``m`` per convention.
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
does not check this restriction.
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 ':' 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 type of the i'th component.
Implict items/pairs invokations
+++++++++++++++++++++++++++++++
If the for loop expression ``e`` does not denote an iterator and the for loop
has exactly 1 variable, the for loop expression is rewritten to ``items(e)``;
ie. an ``items`` iterator is implicitely invoked:
.. code-block:: nimrod
for x in [1,2,3]: echo x
If the for loop has exactly 2 variables, a ``pairs`` iterator is implicitely
invoked.
Symbol lookup of the identifiers ``items``/``pairs`` is performed after
the rewriting step, so that all overloadings of ``items``/``pairs`` are taken
into account.
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.data = 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 ``cmp`` 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.
Is operator
~~~~~~~~~~~
The `is`:idx: operator checks for type equivalence at compile time. It is
therefore very useful for type specialization within generic code:
.. code-block:: nimrod
type
TTable[TKey, TValue] = object
keys: seq[TKey]
values: seq[TValue]
when not (TKey is string): # nil value for strings used for optimization
deletedKeys: seq[bool]
Type operator
~~~~~~~~~~~~~
The `type`:idx: (in many other languages called `typeof`:idx:) operator can
be used to get the type of an expression:
.. code-block:: nimrod
var x = 0
var y: type(x) # y has type int
If ``type`` is used to determine the result type of a proc/iterator/converter
call ``c(X)`` (where ``X`` stands for a possibly empty list of arguments), the
interpretation where ``c`` is an iterator is preferred over the
other interpretations:
.. code-block:: nimrod
import strutils
# strutils contains both a ``split`` proc and iterator, but since an
# an iterator is the preferred interpretation, `y` has the type ``string``:
var y: type("a b c".split)
Type constraints
~~~~~~~~~~~~~~~~
`Type constraints`:idx: can be used to restrict the instantiation of a generic
type parameter. Only the specified types are valid for instantiation:
.. code-block:: nimrod
proc onlyIntOrString[T: int|string](x, y: T): T = nil
onlyIntOrString(45, 66) # valid
onlyIntOrString(56.0, 0.0) # type mismatch
Apart from ordinary types, type constraints can also be of the
following *type classes*:
================== ===================================================
type class matches
================== ===================================================
``object`` any object type
``tuple`` any tuple type
``enum`` any enumeration
``proc`` any proc type
``ref`` any ``ref`` type
``ptr`` any ``ptr`` type
``var`` any ``var`` type
``distinct`` any distinct type
``array`` any array type
``set`` any set type
``seq`` any seq type
================== ===================================================
The following example is taken directly from the system module:
.. code-block:: nimrod
proc `==`*[T: tuple](x, y: T): bool =
## generic ``==`` operator for tuples that is lifted from the components
## of `x` and `y`.
for a, b in fields(x, y):
if a != b: return false
return true
Symbol lookup in generics
~~~~~~~~~~~~~~~~~~~~~~~~~
Symbols in generics are looked up in two different contexts: Both the context
at definition and the context at instantiation are considered for any symbol
occuring in a generic:
.. code-block:: nimrod
type
TIndex = distinct int
proc `==` (a, b: TIndex): bool {.borrow.}
var a = (0, 0.TIndex)
var b = (0, 0.TIndex)
echo a == b # works!
In the example the generic ``==`` for tuples (as defined in the system module)
uses the ``==`` operators of the tuple's components. However, the ``==`` for
the ``TIndex`` type is defined *after* the ``==`` for tuples; yet the example
compiles as the instantiation takes the currently defined symbols into account
too.
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:** The symbol binding rules for 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'
Bind statement
~~~~~~~~~~~~~~
Syntax::
bindStmt ::= 'bind' IDENT (comma IDENT)*
Exporting a template is a often a leaky abstraction as it can depend on
symbols that are not visible from a client module. However, to compensate for
this case, a `bind`:idx: statement can be used: It declares all identifiers
that should be bound early (i.e. when the template is parsed):
.. code-block:: nimrod
# Module A
var
lastId = 0
template genId*: expr =
bind lastId
inc(lastId)
lastId
.. code-block:: nimrod
# Module B
import A
echo genId() # Works
A ``bind`` statement can also be used in generics for the same purpose.
Identifier construction
~~~~~~~~~~~~~~~~~~~~~~~
In templates identifiers can be constructed with the backticks notation:
.. code-block:: nimrod
template typedef(name: expr, typ: typeDesc) =
type
`T name`* = typ
`P name`* = ref `T name`
typedef(myint, int)
var x: PMyInt
In the example ``name`` is instantiated with ``myint``, so \`T name\` becomes
``Tmyint``.
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
**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.
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
Compiler 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 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.
shallow pragma
--------------
The `shallow`:idx: pragma affects the semantics of a type: The compiler is
allowed to make a shallow copy. This can cause serious semantic issues and
break memory safety! However, it can speed up assignments considerably,
because the semantics of Nimrod require deep copying of sequences and strings.
This can be expensive, especially if sequences are used to build a tree
structure:
.. code-block:: nimrod
type
TNodeKind = enum nkLeaf, nkInner
TNode {.final, shallow.} = object
case kind: TNodeKind
of nkLeaf:
strVal: string
of nkInner:
children: seq[TNode]
Pure pragma
-----------
An object type can be marked with the `pure`:idx: pragma so that its type
field which is used for runtime type identification is omitted. This is
necessary for binary compatibility with other compiled languages.
NoStackFrame pragma
-------------------
A proc can be marked with the `noStackFrame`:idx: pragma to tell the compiler
it should not generate a stack frame for the proc. There are also no exit
statements like ``return result;`` generated. This is useful for procs that
only consist of an assembler statement.
error pragma
------------
The `error`:idx: pragma is used to make the compiler output an error message
with the given content. Compilation does not necessarily abort after an error
though.
The ``error`` pragma can also be used to
annotate a symbol (like an iterator or proc). The *usage* of the symbol then
triggers a compile-time error. This is especially useful to rule out that some
operation is valid due to overloading and type conversions:
.. code-block:: nimrod
## check that underlying int values are compared and not the pointers:
proc `==`(x, y: ptr int): bool {.error.}
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.
line pragma
-----------
The `line`:idx: pragma can be used to affect line information of the annotated
statement as seen in stack backtraces:
.. code-block:: nimrod
template myassert*(cond: expr, msg = "") =
if not cond:
# change run-time line information of the 'raise' statement:
{.line: InstantiationInfo().}:
raise newException(EAssertionFailed, msg)
If the ``line`` pragma is used with a parameter, the parameter needs be a
``tuple[filename: string, line: int]``. If it is used without a parameter,
``system.InstantiationInfo()`` is used.
linearScanEnd pragma
--------------------
The `linearScanEnd`:idx: pragma can be used to tell the compiler how to
compile a Nimrod `case`:idx: statement. Syntactially it has to be used as a
statement:
.. code-block:: nimrod
case myInt
of 0:
echo "most common case"
of 1:
{.linearScanEnd.}
echo "second most common case"
of 2: echo "unlikely: use branch table"
else: echo "unlikely too: use branch table for ", myInt
In the example, the case branches ``0`` and ``1`` are much more common than
the other cases. Therefore the generated assembler code should test for these
values first, so that the CPU's branch predictor has a good chance to succeed
(avoiding an expensive CPU pipeline stall). The other cases might be put into a
jump table for O(1) overhead, but at the cost of a (very likely) pipeline
stall.
The ``linearScanEnd`` pragma should be put into the last branch that should be
tested against via linear scanning. If put into the last branch of the
whole ``case`` statement, the whole ``case`` statement uses linear scanning.
unroll pragma
-------------
The `unroll`:idx: pragma can be used to tell the compiler that it should unroll
a `for`:idx: or `while`:idx: loop for runtime efficiency:
.. code-block:: nimrod
proc searchChar(s: string, c: char): int =
for i in 0 .. s.high:
{.unroll: 4.}
if s[i] == c: return i
result = -1
In the above example, the search loop is unrolled by a factor 4. The unroll
factor can be left out too; the compiler then chooses an appropriate unroll
factor.
**Note**: Currently the compiler recognizes but ignores this pragma.
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.
**Note**: Passing variables to the ``dynlib`` pragma will fail at runtime
because of order of initialization problems.
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, exportc, dynlib.}
This is only useful if the program is compiled as a dynamic library via the
``--app:lib`` command line option.
Threads
=======
Even though Nimrod's `thread`:idx: support and semantics are preliminary,
they should be quite usable already. To enable thread support
the ``--threads:on`` command line switch needs to be used. The ``system``
module then contains several threading primitives.
See the `threads <threads.html>`_ and `channels <channels.html>`_ modules
for the thread API.
Nimrod's memory model for threads is quite different than that of other common
programming languages (C, Pascal, Java): Each thread has its own (garbage
collected) heap and sharing of memory is restricted to global variables. This
helps to prevent race conditions. GC efficiency is improved quite a lot,
because the GC never has to stop other threads and see what they reference.
Memory allocation requires no lock at all! This design easily scales to massive
multicore processors that will become the norm in the future.
Thread pragma
-------------
A proc that is executed as a new thread of execution should be marked by the
`thread pragma`:idx:. The compiler checks procedures marked as ``thread`` for
violations of the `no heap sharing restriction`:idx:\: This restriction implies
that it is invalid to construct a data structure that consists of memory
allocated from different (thread local) heaps.
Since the semantic checking of threads requires whole program analysis,
it is quite expensive and can be turned off with ``--threadanalysis:off`` to
improve compile times.
A thread proc is passed to ``createThread`` and invoked indirectly; so the
``thread`` pragma implies ``procvar``.
Threadvar pragma
----------------
A global variable can be marked with the `threadvar`:idx: pragma; it is
a `thead-local`:idx: variable then:
.. code-block:: nimrod
var checkpoints* {.threadvar.}: seq[string] = @[]
Actor model
-----------
**Caution**: This section is already outdated! XXX
Nimrod supports the `actor model`:idx: of concurrency natively:
.. code-block:: nimrod
type
TMsgKind = enum
mLine, mEof
TMsg = object {.pure, final.}
case k: TMsgKind
of mEof: nil
of mLine: data: string
var
thr: TThread[TMsg]
printedLines = 0
m: TMsg
proc print() {.thread.} =
while true:
var x = recv[TMsg]()
if x.k == mEof: break
echo x.data
discard atomicInc(printedLines)
createThread(thr, print)
var input = open("readme.txt")
while not endOfFile(input):
m.data = input.readLine()
thr.send(m)
close(input)
m.k = mEof
thr.send(m)
joinThread(thr)
echo printedLines
In the actor model threads communicate only over sending messages (`send`:idx:
and `recv`:idx: built-ins), not by sharing memory. Every thread has
an `inbox`:idx: that keeps incoming messages until the thread requests a new
message via the ``recv`` operation. The inbox is an unlimited FIFO queue.
In the above example the ``print`` thread also communicates with its
parent thread over the ``printedLines`` global variable. In general, it is
highly advisable to only read from globals, but not to write to them. In fact
a write to a global that contains GC'ed memory is always wrong, because it
violates the *no heap sharing restriction*:
.. code-block:: nimrod
var
global: string
t: TThread[string]
proc horrible() {.thread.} =
global = "string in thread local heap!"
createThread(t, horrible)
joinThread(t)
For the above code the compiler produces "Warning: write to foreign heap". This
warning might become an error message in future versions of the compiler.
Creating a thread is an expensive operation, because a new stack and heap needs
to be created for the thread. It is therefore highly advisable that a thread
handles a large amount of work. Nimrod prefers *coarse grained*
over *fine grained* concurrency.
Threads and exceptions
----------------------
The interaction between threads and exceptions is simple: A *handled* exception
in one thread cannot affect any other thread. However, an *unhandled*
exception in one thread terminates the whole *process*!
Taint mode
==========
The Nimrod compiler and most parts of the standard library support
a `taint mode`:idx:. Input strings are declared with the `TaintedString`:idx:
string type declared in the ``system`` module.
If the taint mode is turned on (via the ``--taintMode:on`` command line
option) it is a distinct string type which helps to detect input
validation errors:
.. code-block:: nimrod
echo "your name: "
var name: TaintedString = stdin.readline
# it is safe here to output the name without any input validation, so
# we simply convert `name` to string to make the compiler happy:
echo "hi, ", name.string
If the taint mode is turned off, ``TaintedString`` is simply an alias for
``string``.
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