=============
Nimrod Manual
=============
:Author: Andreas Rumpf
:Version: |nimrodversion|
.. contents::
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 supertype 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: ../data/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 prefered 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.
Literal strings
---------------
`Literal strings`:idx: can be delimited by matching double quotes, and can
contain the following `escape sequences`:idx:\ :
================== ===================================================
Escape sequence Meaning
================== ===================================================
``\n`` `newline`:idx:
``\r``, ``\c`` `carriage return`:idx:
``\l`` `line feed`:idx:
``\f`` `form feed`:idx:
``\t`` `tabulator`:idx:
``\v`` `vertical tabulator`:idx:
``\\`` `backslash`:idx:
``\"`` `quotation mark`:idx:
``\'`` `apostrophe`:idx:
``\d+`` `character with decimal value d`:idx:;
all decimal digits directly
following are used for the character
``\a`` `alert`:idx:
``\b`` `backspace`:idx:
``\e`` `escape`:idx: `[ESC]`:idx:
``\xHH`` `character with hex value HH`:idx:;
exactly two hex digits are allowed
================== ===================================================
Strings in Nimrod may contain any 8-bit value, except embedded zeros.
Literal strings can also be delimited by three double squotes
``"""`` ... ``"""``.
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.
There are also `raw string literals` that are preceded with the letter ``r``
(or ``R``) and are delimited by matching double quotes (just like ordinary
string literals) and do not interpret the escape sequences. This is especially
convenient for regular expressions or Windows paths:
.. code-block:: nimrod
var f = openFile(r"C:\texts\text.txt") # a raw string, so ``\t`` is no tab
Literal characters
------------------
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 unterscores
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 apostophe ('\'') is called a
`type suffix`:idx:. Literals without a type prefix are of the type ``int``,
unless the literal contains a dot or an ``E`` in which case it is of
type ``float``.
The type suffixes are:
================= =========================
Type Suffix Resulting type of literal
================= =========================
``'i8`` int8
``'i16`` int16
``'i32`` int32
``'i64`` int64
``'f32`` float32
``'f64`` float64
================= =========================
Floating point literals may also be in binary, octal or hexadecimal
notation:
``0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64``
is approximately 1.72826e35 according to the IEEE floating point standard.
Other tokens
------------
The following strings denote other tokens::
( ) { } [ ] , ; [. .] {. .} (. .)
: = ^ .. `
`..`:tok: takes precedence over other tokens that contain a dot: `{..}`:tok: are
the three tokens `{`:tok:, `..`:tok:, `}`:tok: and not the two tokens
`{.`:tok:, `.}`:tok:.
In Nimrod one can define his own operators. An `operator`:idx: is any
combination of the following characters that is not listed above::
+ - * / < >
= @ $ ~ & %
! ? ^ . | \
These keywords are also operators:
``and or not xor shl shr div mod in notin is isnot``.
Syntax
======
This section lists Nimrod's standard syntax in ENBF. How the parser receives
indentation tokens is already described in the Lexical Analysis section.
Nimrod allows user-definable operators.
Binary operators have 8 different levels of precedence. For user-defined
operators, the precedence depends on the first character the operator consists
of. All binary operators are left-associative.
================ ============================================== ================== ===============
Precedence level Operators First characters Terminal symbol
================ ============================================== ================== ===============
7 (highest) ``$`` OP7
6 ``* / div mod shl shr %`` ``* % \ /`` OP6
5 ``+ -`` ``+ ~ |`` OP5
4 ``&`` ``&`` OP4
3 ``== <= < >= > != in not_in is isnot`` ``= < > !`` OP3
2 ``and`` OP2
1 ``or xor`` OP1
0 (lowest) ``? @ ^ ` : .`` OP0
================ ============================================== ================== ===============
The grammar's start symbol is ``module``. The grammar is LL(1) and therefore
not ambigious.
.. include:: grammar.txt
:literal:
Semantics
=========
Constants
---------
`Constants`:idx: are symbols which are bound to a value. The constant's value
cannot change. The compiler must be able to evaluate the expression in a
constant declaration at compile time.
Nimrod contains a sophisticated compile-time evaluator, so procedures which
have no side-effect can be used in constant expressions too:
.. code-block:: nimrod
import strutils
const
constEval = contains("abc", 'b') # computed at compile time!
Types
-----
All expressions have a `type`:idx: which is known at compile time. Nimrod
is statically typed. One can declare new types, which is in essence defining
an identifier that can be used to denote this custom type.
These are the major type classes:
* ordinal types (consist of integer, bool, character, enumeration
(and subranges thereof) types)
* floating point types
* string type
* structured types
* reference (pointer) type
* procedural type
* generic type
Ordinal types
~~~~~~~~~~~~~
`Ordinal types`:idx: have the following characteristics:
- Ordinal types are countable and ordered. This property allows
the operation of functions as ``Inc``, ``Ord``, ``Dec`` on ordinal types to
be defined.
- Ordinal values have a smallest possible value. Trying to count further
down than the smallest value gives a checked runtime or static error.
- Ordinal values have a largest possible value. Trying to count further
than the largest value gives a checked runtime or static error.
Integers, bool, characters and enumeration types (and subrange of these
types) belong to ordinal types.
Pre-defined numerical types
~~~~~~~~~~~~~~~~~~~~~~~~~~~
These integer types are pre-defined:
``int``
the generic signed integer type; its size is platform dependant
(the compiler chooses the processor's fastest integer type)
this type should be used in general. An integer literal that has no type
suffix is of this type.
intXX
additional signed integer types of XX bits use this naming scheme
(example: int16 is a 16 bit wide integer).
The current implementation supports ``int8``, ``int16``, ``int32``, ``int64``.
Literals of these types have the suffix 'iXX.
There are no `unsigned integer`:idx: types, only `unsigned operations`:idx:
that treat their arguments as unsigned. Unsigned operations all wrap around;
they cannot lead to over- or underflow errors. Unsigned operations use the
``%`` suffix as convention:
====================== ======================================================
operation meaning
====================== ======================================================
``a +% b`` unsigned integer addition
``a -% b`` unsigned integer substraction
``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)
====================== ======================================================
The following floating point types are pre-defined:
``float``
the generic floating point type; its size is platform dependant
(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`:idx: is performed in expressions where different
kinds of integer types are used. However, if the type conversion
loses information, the `EOutOfRange`:idx: exception is raised (if the error
cannot be detected at compile time).
Automatic type conversion in expressions with different kinds
of floating point types is performed: The smaller type is
converted to the larger. Arithmetic performed on floating point types
follows the IEEE standard. Integer types are not converted to floating point
types automatically and vice versa.
Boolean type
~~~~~~~~~~~~
The `boolean`:idx: type is named ``bool`` in Nimrod and can be one of the two
pre-defined values ``true`` and ``false``. Conditions in while,
if, elif, when statements need to be of type bool.
This condition holds::
ord(false) == 0 and ord(true) == 1
The operators ``not, and, or, xor, <, <=, >, >=, !=, ==`` are defined
for the bool type. The ``and`` and ``or`` operators perform short-cut
evaluation. Example:
.. code-block:: nimrod
while p != nil and p.name != "xyz":
# p.name is not evaluated if p == nil
p = p.next
The size of the bool type is one byte.
Character type
~~~~~~~~~~~~~~
The `character type`:idx: is named ``char`` in Nimrod. Its size is one byte.
Thus it cannot represent an UTF-8 character, but a part of it.
The reason for this is efficiency: For the overwhelming majority of use-cases,
the resulting programs will still handle UTF-8 properly as UTF-8 was specially
designed for this.
Another reason is that Nimrod can support ``array[char, int]`` or
``set[char]`` efficiently as many algorithms rely on this feature. The
`TRune` type is used for Unicode characters, it can represent any Unicode
character. ``TRune`` is declared 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 *wholes*:
.. code-block:: nimrod
type
TTokenType = enum
a = 2, b = 4, c = 89 # wholes are valid
However, it is then not an ordinal anymore, so it is not possible to use these
enums as an index type for arrays. The procedures ``inc``, ``dec``, ``succ``
and ``pred`` are not available for them either.
Subrange types
~~~~~~~~~~~~~~
A `subrange`:idx: type is a range of values from an ordinal type (the base
type). To define a subrange type, one must specify it's limiting values: the
highest and lowest 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.
Strings are compared by their lexicographical order. All comparison operators
are available. Strings can be indexed like arrays (lower bound is 0). Unlike
arrays, they can be used in case statements:
.. code-block:: nimrod
case paramStr(i)
of "-v": incl(options, optVerbose)
of "-h", "-?": incl(options, optHelp)
else: write(stdout, "invalid command line option!\n")
Per convention, all strings are UTF-8 strings, but this is not enforced. For
example, when reading strings from binary files, they are merely a sequence of
bytes. The index operation ``s[i]`` means the i-th *char* of ``s``, not the
i-th *unichar*. The iterator ``runes`` from the ``unicode``
module can be used for iteration over all unicode characters.
Structured types
~~~~~~~~~~~~~~~~
A variable of a `structured type`:idx: can hold multiple values at the same
time. Stuctured 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 homogenous 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] # [] this 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.
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
``--bound_checks: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 heterogenous 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 heterogenous storage
types with no overhead and few abstraction possibilities. The constructor ``()``
can be used to construct tuples. The order of the fields in the constructor
must match the order of the tuple's definition. Different tuple-types are
*equivalent* if they specify the same fields of the same type in the same
order.
The assignment operator for tuples copies each component.
The default assignment operator for objects copies each component. Overloading
of the assignment operator for objects is not possible, but this may change in
future versions of the compiler.
.. code-block:: nimrod
type
TPerson = tuple[name: string, age: int] # type representing a person
# a person consists of a name
# and an age
var
person: TPerson
person = (name: "Peter", age: 30)
# the same, but less readable:
person = ("Peter", 30)
The implementation aligns the fields for best access performance. The alignment
is compatible with the way the C compiler does it.
Objects provide many features that tuples do not. Object provide inheritance
and information hiding. Objects have access to their type at runtime, so that
the ``is`` operator can be used to determine the object's type.
.. code-block:: nimrod
type
TPerson = object
name*: string # the * means that `name` is accessible from other modules
age: int # no * means that the field is hidden
TStudent = object of TPerson # a student is a person
id: int # with an id field
var
student: TStudent
person: TPerson
assert(student is TStudent) # is true
Object fields that should be visible from outside the defining module, have to
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: floavVal: 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 combatible 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 conains 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 (similiar to `pointers`:idx: in other programming languages) are a
way to introduce many-to-one relationships. This means different references can
point to and modify the same location in memory.
Nimrod distinguishes between `traced`:idx: and `untraced`:idx: references.
Untraced references are also called *pointers*. Traced references point to
objects of a garbage collected heap, untraced references point to
manually allocated objects or to objects somewhere else in memory. Thus
untraced references are *unsafe*. However for certain low-level operations
(accessing the hardware) untraced references are unavoidable.
Traced references are declared with the **ref** keyword, untraced references
are declared with the **ptr** keyword.
The ``^`` operator can be used to derefer a reference, the ``addr`` procedure
returns the address of an item. An address is always an untraced reference.
Thus the usage of ``addr`` is an *unsafe* feature.
The ``.`` (access a tuple/object field operator)
and ``[]`` (array/string/sequence index operator) operators perform implicit
dereferencing operations for reference types:
.. code-block:: nimrod
type
PNode = ref TNode
TNode = object
le, ri: PNode
data: int
var
n: PNode
new(n)
n.data = 9 # no need to write n^ .data
To allocate a new traced object, the built-in procedure ``new`` has to be used.
To deal with untraced memory, the procedures ``alloc``, ``dealloc`` and
``realloc`` can be used. The documentation of the system module contains
further information.
If a reference points to *nothing*, it has the value ``nil``.
Special care has to be taken if an untraced object contains traced objects like
traced references, strings or sequences: In order to free everything properly,
the built-in procedure ``GCunref`` has to be called before freeing the
untraced memory manually!
.. XXX finalizers for traced objects
Procedural type
~~~~~~~~~~~~~~~
A `procedural type`:idx: is internally a pointer to a procedure. ``nil`` is
an allowed value for variables of a procedural type. Nimrod uses procedural
types to achieve `functional`:idx: programming techniques. Dynamic dispatch
for OOP constructs can also be implemented with procedural types.
Example:
.. code-block:: nimrod
type
TCallback = proc (x: int) {.cdecl.}
proc printItem(x: Int) = ...
proc forEach(c: TCallback) =
...
forEach(printItem) # this will NOT work because calling conventions differ
A subtle issue with procedural types is that the calling convention of the
procedure influences the type compability: Procedural types are only compatible
if they have the same calling convention.
Nimrod supports these `calling conventions`:idx:, which are all incompatible to
each other:
`stdcall`:idx:
This the stdcall convention as specified by Microsoft. The generated C
procedure is declared with the ``__stdcall`` keyword.
`cdecl`:idx:
The cdecl convention means that a procedure shall use the same convention
as the C compiler. Under windows the generated C procedure is declared with
the ``__cdecl`` keyword.
`safecall`:idx:
This is the safecall convention as specified by Microsoft. The generated C
procedure is declared with the ``__safecall`` keyword. The word *safe*
refers to the fact that all hardware registers shall be pushed to the
hardware stack.
`inline`:idx:
The inline convention means the the caller should not call the procedure,
but inline its code directly. Note that Nimrod does not inline, but leaves
this to the C compiler. Thus it generates ``__inline`` procedures. This is
only a hint for the compiler: It may completely ignore it and
it may inline procedures that are not marked as ``inline``.
`fastcall`:idx:
Fastcall means different things to different C compilers. One gets whatever
the C ``__fastcall`` means.
`nimcall`:idx:
Nimcall is the default convention used for Nimrod procedures. It is the
same as ``fastcall``, but only for C compilers that support ``fastcall``.
`closure`:idx:
indicates that the procedure expects a context, a closure that needs
to be passed to the procedure. The implementation is the
same as ``cdecl``, but with a hidden pointer parameter (the
*closure*). The hidden parameter is always the last one.
`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. This is unlikely to be needed by the user.
Most calling conventions exist only for the Windows 32-bit platform.
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 throwing its value away, one can use the ``discard``
statement.
Statements are separated into `simple statements`:idx: and
`complex statements`:idx:.
Simple statements are statements that cannot contain other statements like
assignments, calls or the ``return`` statement; complex statements can
contain other statements. To avoid the `dangling else problem`:idx:, complex
statements always have to be intended::
simpleStmt ::= returnStmt
| yieldStmt
| discardStmt
| raiseStmt
| breakStmt
| continueStmt
| pragma
| importStmt
| fromStmt
| includeStmt
| exprStmt
complexStmt ::= ifStmt | whileStmt | caseStmt | tryStmt | forStmt
| blockStmt | asmStmt
| procDecl | iteratorDecl | macroDecl | templateDecl
| constSection | typeSection | whenStmt | varSection
Discard statement
~~~~~~~~~~~~~~~~~
Syntax::
discardStmt ::= 'discard' expr
Example:
.. code-block:: nimrod
discard proc_call("arg1", "arg2") # discard the return value of `proc_call`
The `discard`:idx: statement evaluates its expression for side-effects and
throws the expression's resulting value away. If the expression has no
side-effects, this generates a static error. Ignoring the return value of a
procedure without using a discard statement is not allowed.
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 seperated 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 of the
same type as the initializing expression. Variables are always initialized
with a default value if there is no initializing expression. The default
value depends on the type and is always a zero in binary.
============================ ==============================================
Type default value
============================ ==============================================
any integer type 0
any float 0.0
char '\0'
bool false
ref or pointer type nil
procedural type nil
sequence nil (*not* ``@[]``)
string nil (*not* "")
tuple[x: A, y: B, ...] (default(A), default(B), ...)
(analogous for objects)
array[0..., T] [default(T), ...]
range[T] default(T); this may be out of the valid range
T = enum cast[T](0); this may be an invalid value
============================ ==============================================
Const section
~~~~~~~~~~~~~
Syntax::
colonAndEquals ::= [':' typeDesc] '=' expr
constDecl ::= symbol ['*'] [pragma] colonAndEquals [COMMENT | IND COMMENT]
| COMMENT
constSection ::= 'const' indPush constDecl (SAD constDecl)* DED indPop
Example:
.. code-block:: nimrod
const
MyFilename = "/home/my/file.txt"
debugMode: bool = false
The `const`:idx: section declares symbolic constants. A symbolic constant is
a name for a constant expression. Symbolic constants only allow read-access.
If statement
~~~~~~~~~~~~
Syntax::
ifStmt ::= 'if' expr ':' stmt ('elif' expr ':' stmt)* ['else' ':' stmt]
Example:
.. code-block:: nimrod
var name = readLine(stdin)
if name == "Andreas":
echo("What a nice name!")
elif name == "":
echo("Don't you have a name?")
else:
echo("Boring name...")
The `if`:idx: statement is a simple way to make a branch in the control flow:
The expression after the keyword ``if`` is evaluated, if it is true
the corresponding statements after the ``:`` are executed. Otherwise
the expression after the ``elif`` is evaluated (if there is an
``elif`` branch), if it is true the corresponding statements after
the ``:`` are executed. This goes on until the last ``elif``. If all
conditions fail, the ``else`` part is executed. If there is no ``else``
part, execution continues with the statement after the ``if`` statement.
Case statement
~~~~~~~~~~~~~~
Syntax::
caseStmt ::= 'case' expr [':'] ('of' sliceExprList ':' stmt)*
('elif' expr ':' stmt)*
['else' ':' stmt]
Example:
.. code-block:: nimrod
case readline(stdin)
of "delete-everything", "restart-computer":
echo("permission denied")
of "go-for-a-walk": echo("please yourself")
else: echo("unknown command")
The `case`:idx: statement is similar to the if statement, but it represents
a multi-branch selection. The expression after the keyword ``case`` is
evaluated and if its value is in a *vallist* 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 ``vallist``,
a static error is given. This holds only for expressions of ordinal types.
If the expression is not of an ordinal type, and no ``else`` part is
given, control just passes after the ``case`` statement.
To suppress the static error in the ordinal case the programmer needs
to write an ``else`` part with a ``nil`` statement.
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 if they introduce new identifiers.
* 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 syntatic 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 openFile(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:
closeFile(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 similiar 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 =
# implicitely 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 leaved 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:
while expr1:
block myBlockName:
stmt1
break myBlockName
stmt2
Assembler statement
~~~~~~~~~~~~~~~~~~~
Syntax::
asmStmt ::= 'asm' [pragma] (STR_LIT | RSTR_LIT | TRIPLESTR_LIT)
The direct embedding of `assembler`:idx: code into Nimrod code is supported
by the unsafe ``asm`` statement. Identifiers in the assembler code that refer to
Nimrod identifiers shall be enclosed in a special character which can be
specified in the statement's pragmas. The default special character is ``'`'``.
If expression
~~~~~~~~~~~~~
An `if expression` is almost like an if statement, but it is an expression.
Example:
.. code-block:: nimrod
p(if x > 8: 9 else: 10)
An if expression always results in a value, so the ``else`` part is
required. ``Elif`` parts are also allowed (but unlikely to be good
style).
Type convertions
~~~~~~~~~~~~~~~~
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 `funtions`: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. The syntax is::
param ::= symbol (comma symbol)* [comma] ':' typeDesc
paramList ::= ['(' [param (comma param)* [comma]] ')'] [':' typeDesc]
genericParam ::= symbol [':' typeDesc]
genericParams ::= '[' genericParam (comma genericParam)* [comma] ']'
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 implicit 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. A parameter may be given a default value that
is used if the caller does not provide a value for this parameter. 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. There is no way to declare postfix
operators: All postfix operators are built-in and handled by the
grammar explicitely.
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
Even more elegant is to use `tuple unpacking` to access the tuple's fields:
.. code-block:: nimrod
var (x, y) = divmod(8, 5) # tuple unpacking
assert x == 1
assert y == 3
Unfortunately, this form of tuple unpacking is not yet implemented.
..
XXX remove this as soon as tuple unpacking is implemented
Iterators and the for statement
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Syntax::
forStmt ::= 'for' symbol (comma symbol)* [comma] 'in' expr ['..' expr] ':' stmt
param ::= symbol (comma symbol)* [comma] ':' typeDesc
paramList ::= ['(' [param (comma param)* [comma]] ')'] [':' typeDesc]
genericParam ::= symbol [':' typeDesc]
genericParams ::= '[' genericParam (comma genericParam)* [comma] ']'
iteratorDecl ::= 'iterator' symbol ['*'] [genericParams] paramList [pragma]
['=' stmt]
The `for`:idx: statement is an abstract mechanism to iterate over the elements
of a container. It relies on an `iterator`:idx: to do so. Like ``while``
statements, ``for`` statements open an `implicit block`:idx:, so that they
can be leaved with a ``break`` statement. The ``for`` loop declares
iteration variables (``x`` in the example) - their scope reaches until the
end of the loop body. The iteration variables' types are inferred by the
return type of the iterator.
An iterator is similar to a procedure, except that it is always called in the
context of a ``for`` loop. Iterators provide a way to specify the iteration over
an abstract type. A key role in the execution of a ``for`` loop plays the
``yield`` statement in the called iterator. Whenever a ``yield`` statement is
reached the data is bound to the ``for`` loop variables and control continues
in the body of the ``for`` loop. The iterator's local variables and execution
state are automatically saved between calls. Example:
.. code-block:: nimrod
# this definition exists in the system module
iterator items*(a: string): char {.inline.} =
var i = 0
while i < len(a):
yield a[i]
inc(i)
for ch in items("hello world"): # `ch` is an iteration variable
echo(ch)
The compiler generates code as if the programmer would have written this:
.. code-block:: nimrod
var i = 0
while i < len(a):
var ch = a[i]
echo(ch)
inc(i)
The current implementation always inlines the iterator code leading to zero
overhead for the abstraction. But this may increase the code size. Later
versions of the compiler will only inline iterators which have the calling
convention ``inline``.
If the iterator yields a tuple, there have to be as many iteration variables
as there are components in the tuple. The i'th iteration variable's type is
the one of the i'th component.
Type sections
~~~~~~~~~~~~~
Syntax::
typeDef ::= typeDesc | objectDef | enumDef
genericParam ::= symbol [':' typeDesc]
genericParams ::= '[' genericParam (comma genericParam)* [comma] ']'
typeDecl ::= COMMENT
| symbol ['*'] [genericParams] ['=' typeDef] [COMMENT|IND COMMENT]
typeSection ::= 'type' indPush typeDecl (SAD typeDecl)* DED indPop
Example:
.. code-block:: nimrod
type # example demonstrates mutually recursive types
PNode = ref TNode # a traced pointer to a TNode
TNode = object
le, ri: PNode # left and right subtrees
sym: ref TSym # leaves contain a reference to a TSym
TSym = object # a symbol
name: string # the symbol's name
line: int # the line the symbol was declared in
code: PNode # the symbol's abstract syntax tree
A `type`:idx: section begins with the ``type`` keyword. It contains multiple
type definitions. A type definition binds a type to a name. Type definitions
can be recursive or even mutually recursive. Mutually recursive types are only
possible within a single ``type`` section.
Generics
~~~~~~~~
`Version 0.7.6: Generic types like in the example do not work.`:red:
Example:
.. code-block:: nimrod
type
TBinaryTree[T] = object # TBinaryTree is a generic type with
# with generic param ``T``
le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil
data: T # the data stored in a node
PBinaryTree[T] = ref TBinaryTree[T] # a shorthand for notational convenience
proc newNode[T](data: T): PBinaryTree[T] = # constructor for a node
new(result)
result.dat = data
proc add[T](root: var PBinaryTree[T], n: PBinaryTree[T]) =
if root == nil:
root = n
else:
var it = root
while it != nil:
var c = cmp(it.data, n.data) # compare the data items; uses
# the generic ``cmd`` proc that works for
# any type that has a ``==`` and ``<``
# operator
if c < 0:
if it.le == nil:
it.le = n
return
it = it.le
else:
if it.ri == nil:
it.ri = n
return
it = it.ri
iterator inorder[T](root: PBinaryTree[T]): T =
# inorder traversal of a binary tree
# recursive iterators are not yet implemented, so this does not work in
# the current compiler!
if root.le != nil:
yield inorder(root.le)
yield root.data
if root.ri != nil:
yield inorder(root.ri)
var
root: PBinaryTree[string] # instantiate a PBinaryTree with the type string
add(root, newNode("hallo")) # instantiates generic procs ``newNode`` and
add(root, newNode("world")) # ``add``
for str in inorder(root):
writeln(stdout, str)
`Generics`:idx: are Nimrod's means to parametrize procs, iterators or types with
`type parameters`:idx:. Depending on context, the brackets are used either to
introduce type parameters or to instantiate a generic proc, iterator or type.
Templates
~~~~~~~~~
A `template`:idx: is a simple form of a macro. It operates on parse trees and is
processed in the semantic pass of the compiler. So they integrate well with the
rest of the language and share none of C's preprocessor macros flaws. However,
they may lead to code that is harder to understand and maintain. So one ought
to use them sparingly. The usage of ordinary procs, iterators or generics is
preferred to the usage of templates.
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))
Macros
------
`Macros`:idx: are the most powerful feature of Nimrod. They can be used
to implement `domain specific languages`:idx:. But they may lead to code
that is harder to understand and maintain. So one ought to use them sparingly.
While macros enable advanced compile-time code tranformations, 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. (Unfortunately the AST is not yet documented.)
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 expression
# 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 analyser from
regular expressions:
.. code-block:: nimrod
import macros
macro case_token(n: stmt): stmt =
# creates a lexical analyser from regular expressions
# ... (implementation is an exercise for the reader :-)
nil
case_token: # this colon tells the parser it is a macro statement
of r"[A-Za-z_]+[A-Za-z_0-9]*":
return tkIdentifier
of r"0-9+":
return tkInteger
of r"[\+\-\*\?]+":
return tkOperator
else:
return tkUnknown
Modules
-------
Nimrod supports splitting a program into pieces by a `module`:idx: concept.
Each module needs to be in its own file. 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 dependancies`: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()
# 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 descendent 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 occurance 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
# Module B
var x*: int
# Module C
import A, B
write(stdout, x) # error: x is ambigious
write(stdout, A.x) # no error: qualifier used
var x = 4
write(stdout, x) # not ambigious: uses the module C's x
Messages
========
The Nimrod compiler emits different kinds of messages: `hint`:idx:,
`warning`:idx:, and `error`:idx: messages. An *error* message is emitted if
the compiler encounters any static error.
Pragmas
=======
Syntax::
colonExpr ::= expr [':' expr]
colonExprList ::= [colonExpr (comma colonExpr)* [comma]]
pragma ::= '{.' optInd (colonExpr [comma])* [SAD] ('.}' | '}')
Pragmas are Nimrod's method to give the compiler additional information/
commands without introducing a massive number of new keywords. Pragmas are
processed on the fly during semantic checking. Pragmas are enclosed in the
special ``{.`` and ``.}`` curly brackets.
define pragma
-------------
The `define`:idx: pragma defines a conditional symbol. This symbol may only be
used in other pragmas and in the ``defined`` expression and not in ordinary
Nimrod source code. The conditional symbols go into a special symbol table.
The compiler defines the target processor and the target operating
system as conditional symbols.
Warning: The ``define`` pragma is deprecated as it conflicts with separate
compilation! One should use boolean constants as a replacement - this is
cleaner anyway.
undef pragma
------------
The `undef`:idx: pragma the counterpart to the define pragma. It undefines a
conditional symbol.
Warning: The ``undef`` pragma is deprecated as it conflicts with separate
compilation!
error pragma
------------
The `error`:idx: pragma is used to make the compiler output an error message
with the given content. Compilation currently aborts after an error, but this
may be changed in later versions.
fatal pragma
------------
The `fatal`:idx: pragma is used to make the compiler output an error message
with the given content. In contrast to the ``error`` pragma, compilation
is guaranteed to be aborted by this pragma.
warning pragma
--------------
The `warning`:idx: pragma is used to make the compiler output a warning message
with the given content. Compilation continues after the warning.
hint pragma
-----------
The `hint`:idx: pragma is used to make the compiler output a hint message with
the given content. Compilation continues after the hint.
compilation option pragmas
--------------------------
The listed pragmas here can be used to override the code generation options
for a section of code.
The implementation currently provides the following possible options (later
various others may be added).
=============== =============== ============================================
pragma allowed values description
=============== =============== ============================================
checks on|off Turns the code generation for all runtime
checks on or off.
bound_checks on|off Turns the code generation for array bound
checks on or off.
overflow_checks on|off Turns the code generation for over- or
underflow checks on or off.
nil_checks 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