=====================
Nim Tutorial (Part I)
=====================

:Author: Andreas Rumpf
:Version: |nimversion|

.. contents::

Introduction
============

.. raw:: html
  <blockquote><p>
  "Der Mensch ist doch ein Augentier -- sch&ouml;ne Dinge w&uuml;nsch ich mir."
  </p></blockquote>


This document is a tutorial for the programming language *Nim*.
This tutorial assumes that you are familiar with basic programming concepts
like variables, types or statements but is kept very basic. The `manual
<manual.html>`_ contains many more examples of the advanced language features.
All code examples in this tutorial, as well as the ones found in the rest of
Nim's documentation, follow the `Nim style guide <nep1.html>`.


The first program
=================

We start the tour with a modified "hello world" program:

.. code-block:: Nim
  # This is a comment
  echo("What's your name? ")
  var name: string = readLine(stdin)
  echo("Hi, ", name, "!")


Save this code to the file "greetings.nim". Now compile and run it::

  nim compile --run greetings.nim

With the ``--run`` `switch <nimc.html#command-line-switches>`_ Nim
executes the file automatically after compilation. You can give your program
command line arguments by appending them after the filename::

  nim compile --run greetings.nim arg1 arg2

Commonly used commands and switches have abbreviations, so you can also use::

  nim c -r greetings.nim

To compile a release version use::

  nim c -d:release greetings.nim

By default the Nim compiler generates a large amount of runtime checks
aiming for your debugging pleasure. With ``-d:release`` these checks are
`turned off and optimizations are turned on
<nimc.html#compile-time-symbols>`_.

Though it should be pretty obvious what the program does, I will explain the
syntax: statements which are not indented are executed when the program
starts. Indentation is Nim's way of grouping statements. Indentation is
done with spaces only, tabulators are not allowed.

String literals are enclosed in double quotes. The ``var`` statement declares
a new variable named ``name`` of type ``string`` with the value that is
returned by the `readLine <system.html#readLine,File>`_ procedure. Since the
compiler knows that `readLine <system.html#readLine,File>`_ returns a string,
you can leave out the type in the declaration (this is called `local type
inference`:idx:). So this will work too:

.. code-block:: Nim
  var name = readLine(stdin)

Note that this is basically the only form of type inference that exists in
Nim: it is a good compromise between brevity and readability.

The "hello world" program contains several identifiers that are already known
to the compiler: ``echo``, `readLine <system.html#readLine,File>`_, etc.
These built-ins are declared in the system_ module which is implicitly
imported by any other module.


Lexical elements
================

Let us look at Nim's lexical elements in more detail: like other
programming languages Nim consists of (string) literals, identifiers,
keywords, comments, operators, and other punctuation marks.


String and character literals
-----------------------------

String literals are enclosed in double quotes; character literals in single
quotes. Special characters are escaped with ``\``: ``\n`` means newline, ``\t``
means tabulator, etc. There are also *raw* string literals:

.. code-block:: Nim
  r"C:\program files\nim"

In raw literals the backslash is not an escape character.

The third and last way to write string literals are *long string literals*.
They are written with three quotes: ``""" ... """``; they can span over
multiple lines and the ``\`` is not an escape character either. They are very
useful for embedding HTML code templates for example.


Comments
--------

Comments start anywhere outside a string or character literal with the
hash character ``#``. Documentation comments start with ``##``:

.. code-block:: nim
  # A comment.

  var myVariable: int ## a documentation comment


Documentation comments are tokens; they are only allowed at certain places in
the input file as they belong to the syntax tree! This feature enables simpler
documentation generators.

You can also use the `discard statement`_ together with *long string
literals* to create block comments:

.. code-block:: nim
  discard """ You can have any Nim code text commented
  out inside this with no indentation restrictions.
        yes("May I ask a pointless question?") """


Numbers
-------

Numerical literals are written as in most other languages. As a special twist,
underscores are allowed for better readability: ``1_000_000`` (one million).
A number that contains a dot (or 'e' or 'E') is a floating point literal:
``1.0e9`` (one billion). Hexadecimal literals are prefixed with ``0x``,
binary literals with ``0b`` and octal literals with ``0o``. A leading zero
alone does not produce an octal.


The var statement
=================
The var statement declares a new local or global variable:

.. code-block::
  var x, y: int # declares x and y to have the type ``int``

Indentation can be used after the ``var`` keyword to list a whole section of
variables:

.. code-block::
  var
    x, y: int
    # a comment can occur here too
    a, b, c: string


The assignment statement
========================

The assignment statement assigns a new value to a variable or more generally
to a storage location:

.. code-block::
  var x = "abc" # introduces a new variable `x` and assigns a value to it
  x = "xyz"     # assigns a new value to `x`

``=`` is the *assignment operator*. The assignment operator cannot be
overloaded, overwritten or forbidden, but this might change in a future version
of Nim. You can declare multiple variables with a single assignment
statement and all the variables will have the same value:

.. code-block::
  var x, y = 3  # assigns 3 to the variables `x` and `y`
  echo "x ", x  # outputs "x 3"
  echo "y ", y  # outputs "y 3"
  x = 42        # changes `x` to 42 without changing `y`
  echo "x ", x  # outputs "x 42"
  echo "y ", y  # outputs "y 3"

Note that declaring multiple variables with a single assignment which calls a
procedure can have unexpected results: the compiler will *unroll* the
assignments and end up calling the procedure several times. If the result of
the procedure depends on side effects, your variables may end up having
different values! For safety use only constant values.


Constants
=========

Constants 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:

.. code-block:: nim
  const x = "abc" # the constant x contains the string "abc"

Indentation can be used after the ``const`` keyword to list a whole section of
constants:

.. code-block::
  const
    x = 1
    # a comment can occur here too
    y = 2
    z = y + 5 # computations are possible


The let statement
=================
The ``let`` statement works like the ``var`` statement but the declared
symbols are *single assignment* variables: After the initialization their
value cannot change:

.. code-block::
  let x = "abc" # introduces a new variable `x` and binds a value to it
  x = "xyz"     # Illegal: assignment to `x`

The difference between ``let`` and ``const`` is: ``let`` introduces a variable
that can not be re-assigned, ``const`` means "enforce compile time evaluation
and put it into a data section":

.. code-block::
  const input = readLine(stdin) # Error: constant expression expected

.. code-block::
  let input = readLine(stdin)   # works


Control flow statements
=======================

The greetings program consists of 3 statements that are executed sequentially.
Only the most primitive programs can get away with that: branching and looping
are needed too.


If statement
------------

The if statement is one way to branch the control flow:

.. code-block:: nim
  let name = readLine(stdin)
  if name == "":
    echo("Poor soul, you lost your name?")
  elif name == "name":
    echo("Very funny, your name is name.")
  else:
    echo("Hi, ", name, "!")

There can be zero or more ``elif`` parts, and the ``else`` part is optional.
The keyword ``elif`` is short for ``else if``, and is useful to avoid
excessive indentation. (The ``""`` is the empty string. It contains no
characters.)


Case statement
--------------

Another way to branch is provided by the case statement. A case statement is
a multi-branch:

.. code-block:: nim
  let name = readLine(stdin)
  case name
  of "":
    echo("Poor soul, you lost your name?")
  of "name":
    echo("Very funny, your name is name.")
  of "Dave", "Frank":
    echo("Cool name!")
  else:
    echo("Hi, ", name, "!")

As it can be seen, for an ``of`` branch a comma separated list of values is also
allowed.

The case statement can deal with integers, other ordinal types and strings.
(What an ordinal type is will be explained soon.)
For integers or other ordinal types value ranges are also possible:

.. code-block:: nim
  # this statement will be explained later:
  from strutils import parseInt

  echo("A number please: ")
  let n = parseInt(readLine(stdin))
  case n
  of 0..2, 4..7: echo("The number is in the set: {0, 1, 2, 4, 5, 6, 7}")
  of 3, 8: echo("The number is 3 or 8")

However, the above code does not compile: the reason is that you have to cover
every value that ``n`` may contain, but the code only handles the values
``0..8``. Since it is not very practical to list every other possible integer
(though it is possible thanks to the range notation), we fix this by telling
the compiler that for every other value nothing should be done:

.. code-block:: nim
  ...
  case n
  of 0..2, 4..7: echo("The number is in the set: {0, 1, 2, 4, 5, 6, 7}")
  of 3, 8: echo("The number is 3 or 8")
  else: discard

The empty `discard statement`_ is a *do nothing* statement. The compiler knows
that a case statement with an else part cannot fail and thus the error
disappears. Note that it is impossible to cover all possible string values:
that is why string cases always need an ``else`` branch.

In general the case statement is used for subrange types or enumerations where
it is of great help that the compiler checks that you covered any possible
value.


While statement
---------------

The while statement is a simple looping construct:

.. code-block:: nim

  echo("What's your name? ")
  var name = readLine(stdin)
  while name == "":
    echo("Please tell me your name: ")
    name = readLine(stdin)
    # no ``var``, because we do not declare a new variable here

The example uses a while loop to keep asking the users for their name, as long
as the user types in nothing (only presses RETURN).


For statement
-------------

The ``for`` statement is a construct to loop over any element an *iterator*
provides. The example uses the built-in `countup <system.html#countup>`_
iterator:

.. code-block:: nim
  echo("Counting to ten: ")
  for i in countup(1, 10):
    echo($i)
  # --> Outputs 1 2 3 4 5 6 7 8 9 10 on different lines

The built-in `$ <system.html#$>`_ operator turns an integer (``int``) and many
other types into a string. The variable ``i`` is implicitly declared by the
``for`` loop and has the type ``int``, because that is what `countup
<system.html#countup>`_ returns. ``i`` runs through the values 1, 2, .., 10.
Each value is ``echo``-ed. This code does the same:

.. code-block:: nim
  echo("Counting to 10: ")
  var i = 1
  while i <= 10:
    echo($i)
    inc(i) # increment i by 1
  # --> Outputs 1 2 3 4 5 6 7 8 9 10 on different lines

Counting down can be achieved as easily (but is less often needed):

.. code-block:: nim
  echo("Counting down from 10 to 1: ")
  for i in countdown(10, 1):
    echo($i)
  # --> Outputs 10 9 8 7 6 5 4 3 2 1 on different lines

Since counting up occurs so often in programs, Nim also has a `..
<system.html#...i,S,T>`_ iterator that does the same:

.. code-block:: nim
  for i in 1..10:
    ...


Scopes and the block statement
------------------------------
Control flow statements have a feature not covered yet: they open a
new scope. This means that in the following example, ``x`` is not accessible
outside the loop:

.. code-block:: nim
  while false:
    var x = "hi"
  echo(x) # does not work

A while (for) statement introduces an implicit block. Identifiers
are only visible within the block they have been declared. The ``block``
statement can be used to open a new block explicitly:

.. code-block:: nim
  block myblock:
    var x = "hi"
  echo(x) # does not work either

The block's *label* (``myblock`` in the example) is optional.


Break statement
---------------
A block can be left prematurely with a ``break`` statement. The break statement
can leave a ``while``, ``for``, or a ``block`` statement. It leaves the
innermost construct, unless a label of a block is given:

.. code-block:: nim
  block myblock:
    echo("entering block")
    while true:
      echo("looping")
      break # leaves the loop, but not the block
    echo("still in block")

  block myblock2:
    echo("entering block")
    while true:
      echo("looping")
      break myblock2 # leaves the block (and the loop)
    echo("still in block")


Continue statement
------------------
Like in many other programming languages, a ``continue`` statement starts
the next iteration immediately:

.. code-block:: nim
  while true:
    let x = readLine(stdin)
    if x == "": continue
    echo(x)


When statement
--------------

Example:

.. code-block:: nim

  when system.hostOS == "windows":
    echo("running on Windows!")
  elif system.hostOS == "linux":
    echo("running on Linux!")
  elif system.hostOS == "macosx":
    echo("running on Mac OS X!")
  else:
    echo("unknown operating system")

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

* Each condition has to be a constant expression since it is evaluated by the
  compiler.
* The statements within a branch do not open a new scope.
* The compiler checks the semantics and produces code *only* for the statements
  that belong to the first condition that evaluates to ``true``.

The ``when`` statement is useful for writing platform specific code, similar to
the ``#ifdef`` construct in the C programming language.

**Note**: To comment out a large piece of code, it is often better to use a
``when false:`` statement than to use real comments. This way nesting is
possible.


Statements and indentation
==========================

Now that we covered the basic control flow statements, let's return to Nim
indentation rules.

In Nim there is a distinction between *simple statements* and *complex
statements*. *Simple statements* cannot contain other statements:
Assignment, procedure calls or the ``return`` statement belong to the simple
statements. *Complex statements* like ``if``, ``when``, ``for``, ``while`` can
contain other statements. To avoid ambiguities, complex statements always have
to be indented, but single simple statements do not:

.. code-block:: nim
  # no indentation needed for single assignment statement:
  if x: x = false

  # indentation needed for nested if statement:
  if x:
    if y:
      y = false
    else:
      y = true

  # indentation needed, because two statements follow the condition:
  if x:
    x = false
    y = false


*Expressions* are parts of a statement which usually result in a value. The
condition in an if statement is an example for an expression. Expressions can
contain indentation at certain places for better readability:

.. code-block:: nim

  if thisIsaLongCondition() and
      thisIsAnotherLongCondition(1,
         2, 3, 4):
    x = true

As a rule of thumb, indentation within expressions is allowed after operators,
an open parenthesis and after commas.

With parenthesis and semicolons ``(;)`` you can use statements where only
an expression is allowed:

.. code-block:: nim
  # computes fac(4) at compile time:
  const fac4 = (var x = 1; for i in 1..4: x *= i; x)


Procedures
==========

To define new commands like `echo <system.html#echo>`_ and `readLine
<system.html#readLine,File>`_ in the examples, the concept of a `procedure`
is needed. (Some languages call them *methods* or *functions*.) In Nim new
procedures are defined with the ``proc`` keyword:

.. code-block:: nim
  proc yes(question: string): bool =
    echo(question, " (y/n)")
    while true:
      case readLine(stdin)
      of "y", "Y", "yes", "Yes": return true
      of "n", "N", "no", "No": return false
      else: echo("Please be clear: yes or no")

  if yes("Should I delete all your important files?"):
    echo("I'm sorry Dave, I'm afraid I can't do that.")
  else:
    echo("I think you know what the problem is just as well as I do.")

This example shows a procedure named ``yes`` that asks the user a ``question``
and returns true if they answered "yes" (or something similar) and returns
false if they answered "no" (or something similar). A ``return`` statement
leaves the procedure (and therefore the while loop) immediately. The
``(question: string): bool`` syntax describes that the procedure expects a
parameter named ``question`` of type ``string`` and returns a value of type
``bool``. ``Bool`` is a built-in type: the only valid values for ``bool`` are
``true`` and ``false``.
The conditions in if or while statements should be of the type ``bool``.

Some terminology: in the example ``question`` is called a (formal) *parameter*,
``"Should I..."`` is called an *argument* that is passed to this parameter.


Result variable
---------------
A procedure that returns a value has an implicit ``result`` variable declared
that represents the return value. A ``return`` statement with no expression is a
shorthand for ``return result``. The ``result`` value is always returned
automatically at the end a procedure if there is no ``return`` statement at
the exit.

.. code-block:: nim
  proc sumTillNegative(x: varargs[int]): int =
    for i in x:
      if i < 0:
        return
      result = result + i

  echo sumTillNegative() # echos 0
  echo sumTillNegative(3, 4, 5) # echos 12
  echo sumTillNegative(3, 4 , -1 , 6) # echos 7

The ``result`` variable is already implicitly declared at the start of the
function, so declaring it again with 'var result', for example, would shadow it
with a normal variable of the same name. The result variable is also already
initialised with the type's default value. Note that referential data types will
be ``nil`` at the start of the procedure, and thus may require manual
initialisation.


Parameters
----------
Parameters are constant in the procedure body. By default, their value cannot be
changed because this allows the compiler to implement parameter passing in the
most efficient way. If a mutable variable is needed inside the procedure, it has
to be declared with ``var`` in the procedure body. Shadowing the parameter name
is possible, and actually an idiom:

.. code-block:: nim
  proc printSeq(s: seq, nprinted: int = -1) =
    var nprinted = if nprinted == -1: s.len else: min(nprinted, s.len)
    for i in 0 .. <nprinted:
      echo s[i]

If the procedure needs to modify the argument for the
caller, a ``var`` parameter can be used:

.. code-block:: nim
  proc divmod(a, b: int; res, remainder: var int) =
    res = a div b        # integer division
    remainder = a mod b  # integer modulo operation

  var
    x, y: int
  divmod(8, 5, x, y) # modifies x and y
  echo(x)
  echo(y)

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. Note that the above example would better make use of
a tuple as a return value instead of using var parameters.


Discard statement
-----------------
To call a procedure that returns a value just for its side effects and ignoring
its return value, a ``discard`` statement **has** to be used. Nim does not
allow to silently throw away a return value:

.. code-block:: nim
  discard yes("May I ask a pointless question?")


The return value can be ignored implicitly if the called proc/iterator has
been declared with the ``discardable`` pragma:

.. code-block:: nim
  proc p(x, y: int): int {.discardable.} =
    return x + y

  p(3, 4) # now valid

The ``discard`` statement can also be used to create block comments as
described in the `Comments`_ section.


Named arguments
---------------

Often a procedure has many parameters and it is not clear in which order the
parameters appear. This is especially true for procedures that construct a
complex data type. Therefore the arguments to a procedure can be named, so
that it is clear which argument belongs to which parameter:

.. code-block:: nim
  proc createWindow(x, y, width, height: int; title: string;
                    show: bool): Window =
     ...

  var w = createWindow(show = true, title = "My Application",
                       x = 0, y = 0, height = 600, width = 800)

Now that we use named arguments to call ``createWindow`` the argument order
does not matter anymore. Mixing named arguments with ordered arguments is
also possible, but not very readable:

.. code-block:: nim
  var w = createWindow(0, 0, title = "My Application",
                       height = 600, width = 800, true)

The compiler checks that each parameter receives exactly one argument.


Default values
--------------
To make the ``createWindow`` proc easier to use it should provide `default
values`, these are values that are used as arguments if the caller does not
specify them:

.. code-block:: nim
  proc createWindow(x = 0, y = 0, width = 500, height = 700,
                    title = "unknown",
                    show = true): Window =
     ...

  var w = createWindow(title = "My Application", height = 600, width = 800)

Now the call to ``createWindow`` only needs to set the values that differ
from the defaults.

Note that type inference works for parameters with default values; there is
no need to write ``title: string = "unknown"``, for example.


Overloaded procedures
---------------------
Nim provides the ability to overload procedures similar to C++:

.. code-block:: nim
  proc toString(x: int): string = ...
  proc toString(x: bool): string =
    if x: result = "true"
    else: result = "false"

  echo(toString(13))   # calls the toString(x: int) proc
  echo(toString(true)) # calls the toString(x: bool) proc

(Note that ``toString`` is usually the `$ <system.html#$>`_ operator in
Nim.) The compiler chooses the most appropriate proc for the ``toString``
calls. How this overloading resolution algorithm works exactly is not
discussed here (it will be specified in the manual soon).  However, it does
not lead to nasty surprises and is based on a quite simple unification
algorithm. Ambiguous calls are reported as errors.


Operators
---------
The Nim library makes heavy use of overloading - one reason for this is that
each operator like ``+`` is a just an overloaded proc. The parser lets you
use operators in `infix notation` (``a + b``) or `prefix notation` (``+ a``).
An infix operator always receives two arguments, a prefix operator always one.
Postfix operators are not possible, because this would be ambiguous: does
``a @ @ b`` mean ``(a) @ (@b)`` or ``(a@) @ (b)``? It always means
``(a) @ (@b)``, because there are no postfix operators in Nim.

Apart from a few built-in keyword operators such as ``and``, ``or``, ``not``,
operators always consist of these characters:
``+  -  *  \  /  <  >  =  @  $  ~  &  %  !  ?  ^  .  |``

User defined operators are allowed. Nothing stops you from defining your own
``@!?+~`` operator, but readability can suffer.

The operator's precedence is determined by its first character. The details
can be found in the manual.

To define a new operator enclose the operator in backticks "``":

.. code-block:: nim
  proc `$` (x: myDataType): string = ...
  # now the $ operator also works with myDataType, overloading resolution
  # ensures that $ works for built-in types just like before

The "``" notation can also be used to call an operator just like any other
procedure:

.. code-block:: nim
  if `==`( `+`(3, 4), 7): echo("True")


Forward declarations
--------------------

Every variable, procedure, etc. needs to be declared before it can be used.
(The reason for this is that it is non-trivial to do better than that in a
language that supports meta programming as extensively as Nim does.)
However, this cannot be done for mutually recursive procedures:

.. code-block:: nim
  # forward declaration:
  proc even(n: int): bool

.. code-block:: nim
  proc odd(n: int): bool =
    assert(n >= 0) # makes sure we don't run into negative recursion
    if n == 0: false
    else:
      n == 1 or even(n-1)

  proc even(n: int): bool =
    assert(n >= 0) # makes sure we don't run into negative recursion
    if n == 1: false
    else:
      n == 0 or odd(n-1)

Here ``odd`` depends on ``even`` and vice versa. Thus ``even`` needs to be
introduced to the compiler before it is completely defined. The syntax for
such a forward declaration is simple: just omit the ``=`` and the
procedure's body. The ``assert`` just adds border conditions, and will be
covered later in `Modules`_ section.

Later versions of the language will weaken the requirements for forward
declarations.

The example also shows that a proc's body can consist of a single expression
whose value is then returned implicitly.


Iterators
=========

Let's return to the boring counting example:

.. code-block:: nim
  echo("Counting to ten: ")
  for i in countup(1, 10):
    echo($i)

Can a `countup <system.html#countup>`_ proc be written that supports this
loop? Lets try:

.. code-block:: nim
  proc countup(a, b: int): int =
    var res = a
    while res <= b:
      return res
      inc(res)

However, this does not work. The problem is that the procedure should not
only ``return``, but return and **continue** after an iteration has
finished. This *return and continue* is called a `yield` statement. Now
the only thing left to do is to replace the ``proc`` keyword by ``iterator``
and there it is - our first iterator:

.. code-block:: nim
  iterator countup(a, b: int): int =
    var res = a
    while res <= b:
      yield res
      inc(res)

Iterators look very similar to procedures, but there are several
important differences:

* Iterators can only be called from for loops.
* Iterators cannot contain a ``return`` statement and procs cannot contain a
  ``yield`` statement.
* Iterators have no implicit ``result`` variable.
* Iterators do not support recursion.
* Iterators cannot be forward declared, because the compiler must be able
  to inline an iterator. (This restriction will be gone in a
  future version of the compiler.)

However, you can also use a ``closure`` iterator to get a different set of
restrictions. See `first class iterators <manual.html#first-class-iterators>`_
for details. Iterators can have the same name and parameters as a proc,
essentially they have their own namespace. Therefore it is common practice to
wrap iterators in procs of the same name which accumulate the result of the
iterator and return it as a sequence, like ``split`` from the `strutils module
<strutils.html>`_.


Basic types
===========

This section deals with the basic built-in types and the operations
that are available for them in detail.

Booleans
--------

The boolean type is named ``bool`` in Nim and consists of the two
pre-defined values ``true`` and ``false``. Conditions in while,
if, elif, when statements need to be of type bool.

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

.. code-block:: nim

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


Characters
----------
The `character type` is named ``char`` in Nim. 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.
Character literals are enclosed in single quotes.

Chars can be compared with the ``==``, ``<``, ``<=``, ``>``, ``>=`` operators.
The ``$`` operator converts a ``char`` to a ``string``. Chars cannot be mixed
with integers; to get the ordinal value of a ``char`` use the ``ord`` proc.
Converting from an integer to a ``char`` is done with the ``chr`` proc.


Strings
-------
String variables in Nim are **mutable**, so appending to a string
is quite efficient. Strings in Nim are both zero-terminated and have a
length field. One can retrieve a string's length with the builtin ``len``
procedure; the length never counts the terminating zero. Accessing the
terminating zero is no error and often leads to simpler code:

.. code-block:: nim
  if s[i] == 'a' and s[i+1] == 'b':
    # no need to check whether ``i < len(s)``!
    ...

The assignment operator for strings copies the string. You can use the ``&``
operator to concatenate strings and ``add`` to append to a string.

Strings are compared by their lexicographical order. All comparison operators
are available. 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*.

String variables are initialized with a special value, called ``nil``. However,
most string operations cannot deal with ``nil`` (leading to an exception being
raised) for performance reasons. One should use empty strings ``""``
rather than ``nil`` as the *empty* value. But ``""`` often creates a string
object on the heap, so there is a trade-off to be made here.


Integers
--------
Nim has these integer types built-in:
``int int8 int16 int32 int64 uint uint8 uint16 uint32 uint64``.

The default integer type is ``int``. Integer literals can have a *type suffix*
to mark them to be of another integer type:


.. code-block:: nim
  let
    x = 0     # x is of type ``int``
    y = 0'i8  # y is of type ``int8``
    z = 0'i64 # z is of type ``int64``
    u = 0'u   # u is of type ``uint``

Most often integers are used for counting objects that reside in memory, so
``int`` has the same size as a pointer.

The common operators ``+ - * div mod  <  <=  ==  !=  >  >=`` are defined for
integers. The ``and or xor not`` operators are defined for integers too and
provide *bitwise* operations. Left bit shifting is done with the ``shl``, right
shifting with the ``shr`` operator. Bit shifting operators always treat their
arguments as *unsigned*. For `arithmetic bit shifts`:idx: ordinary
multiplication or division can be used.

Unsigned operations all wrap around; they cannot lead to over- or underflow
errors.

`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).


Floats
------
Nim has these floating point types built-in: ``float float32 float64``.

The default float type is ``float``. In the current implementation,
``float`` is always 64 bit wide.

Float literals can have a *type suffix* to mark them to be of another float
type:

.. code-block:: nim
  var
    x = 0.0      # x is of type ``float``
    y = 0.0'f32  # y is of type ``float32``
    z = 0.0'f64  # z is of type ``float64``

The common operators ``+ - * /  <  <=  ==  !=  >  >=`` are defined for
floats and follow the IEEE standard.

Automatic type conversion in expressions with different kinds of floating
point types is performed: the smaller type is converted to the larger. Integer
types are **not** converted to floating point types automatically and vice
versa. The `toInt <system.html#toInt>`_ and `toFloat <system.html#toFloat>`_
procs can be used for these conversions.


Type Conversion
---------------
Conversion between basic types in nim is performed by using the
type as a function:

.. code-block:: nim
  var
    x: int32 = 1.int32   # same as calling int32(1)
    y: int8  = int8('a') # 'a' == 97'i8
    z: float = 2.5       # int(2.5) rounds down to 2
    sum: int = int(x) + int(y) + int(z) # sum == 100


Internal type representation
============================

As mentioned earlier, the built-in `$ <system.html#$>`_ (stringify) operator
turns any basic type into a string, which you can then print to the screen
with the ``echo`` proc. However, advanced types, or types you may define
yourself won't work with the ``$`` operator until you define one for them.
Sometimes you just want to debug the current value of a complex type without
having to write its ``$`` operator.  You can use then the `repr
<system.html#repr>`_ proc which works with any type and even complex data
graphs with cycles. The following example shows that even for basic types
there is a difference between the ``$`` and ``repr`` outputs:

.. code-block:: nim
  var
    myBool = true
    myCharacter = 'n'
    myString = "nim"
    myInteger = 42
    myFloat = 3.14
  echo($myBool, ":", repr(myBool))
  # --> true:true
  echo($myCharacter, ":", repr(myCharacter))
  # --> n:'n'
  echo($myString, ":", repr(myString))
  # --> nim:0x10fa8c050"nim"
  echo($myInteger, ":", repr(myInteger))
  # --> 42:42
  echo($myFloat, ":", repr(myFloat))
  # --> 3.1400000000000001e+00:3.1400000000000001e+00


Advanced types
==============

In Nim new types can be defined within a ``type`` statement:

.. code-block:: nim
  type
    biggestInt = int64      # biggest integer type that is available
    biggestFloat = float64  # biggest float type that is available

Enumeration and object types cannot be defined on the fly, but only within a
``type`` statement.


Enumerations
------------
A variable of an enumeration type can only be assigned a value of a
limited set. This set consists of ordered symbols. Each symbol is mapped
to an integer value internally. The first symbol is represented
at runtime by 0, the second by 1 and so on. Example:

.. code-block:: nim

  type
    Direction = enum
      north, east, south, west

  var x = south      # `x` is of type `Direction`; its value is `south`
  echo($x)           # writes "south" to `stdout`

All comparison operators can be used with enumeration types.

An enumeration's symbol can be qualified to avoid ambiguities:
``Direction.south``.

The ``$`` operator can convert any enumeration value to its name, the ``ord``
proc to its underlying integer value.

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

An explicit ordered enum can have *holes*:

.. code-block:: nim
  type
    MyEnum = enum
      a = 2, b = 4, c = 89


Ordinal types
-------------
Enumerations without holes, integer types, ``char`` and ``bool`` (and
subranges) are called ordinal types. Ordinal types have quite
a few special operations:

-----------------     --------------------------------------------------------
Operation             Comment
-----------------     --------------------------------------------------------
``ord(x)``            returns the integer value that is used to
                      represent `x`'s value
``inc(x)``            increments `x` by one
``inc(x, n)``         increments `x` by `n`; `n` is an integer
``dec(x)``            decrements `x` by one
``dec(x, n)``         decrements `x` by `n`; `n` is an integer
``succ(x)``           returns the successor of `x`
``succ(x, n)``        returns the `n`'th successor of `x`
``pred(x)``           returns the predecessor of `x`
``pred(x, n)``        returns the `n`'th predecessor of `x`
-----------------     --------------------------------------------------------

The `inc <system.html#inc>`_, `dec <system.html#dec>`_, `succ
<system.html#succ>`_ and `pred <system.html#pred>`_ operations can fail by
raising an `EOutOfRange` or `EOverflow` exception. (If the code has been
compiled with the proper runtime checks turned on.)


Subranges
---------
A subrange type is a range of values from an integer or enumeration type
(the base type). Example:

.. code-block:: nim
  type
    Subrange = range[0..5]


``Subrange`` is a subrange of ``int`` which can only hold the values 0
to 5. Assigning any other value to a variable of type ``Subrange`` is a
compile-time or runtime error. Assignments from the base type to one of its
subrange types (and vice versa) are allowed.

The ``system`` module defines the important `Natural <system.html#Natural>`_
type as ``range[0..high(int)]`` (`high <system.html#high>`_ returns the
maximal value). Other programming languages mandate the usage of unsigned
integers for natural numbers. This is often **wrong**: you don't want unsigned
arithmetic (which wraps around) just because the numbers cannot be negative.
Nim's ``Natural`` type helps to avoid this common programming error.


Sets
----

.. include:: sets_fragment.txt

Arrays
------
An array is a simple fixed length container. Each element in
the array has the same type. The array's index type can be any ordinal type.

Arrays can be constructed via ``[]``:

.. code-block:: nim

  type
    IntArray = array[0..5, int] # an array that is indexed with 0..5
  var
    x: IntArray
  x = [1, 2, 3, 4, 5, 6]
  for i in low(x)..high(x):
    echo(x[i])

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

Arrays are value types, like any other Nim type. The assignment operator
copies the whole array contents.

The built-in `len <system.html#len,TOpenArray>`_ proc returns the array's
length. `low(a) <system.html#low>`_ returns the lowest valid index for the
array `a` and `high(a) <system.html#high>`_ the highest valid index.

.. code-block:: nim
  type
    Direction = enum
      north, east, south, west
    BlinkLights = enum
      off, on, slowBlink, mediumBlink, fastBlink
    LevelSetting = array[north..west, BlinkLights]
  var
    level: LevelSetting
  level[north] = on
  level[south] = slowBlink
  level[east] = fastBlink
  echo repr(level)  # --> [on, fastBlink, slowBlink, off]
  echo low(level)   # --> north
  echo len(level)   # --> 4
  echo high(level)  # --> west

The syntax for nested arrays (multidimensional) in other languages is a matter
of appending more brackets because usually each dimension is restricted to the
same index type as the others. In Nim you can have different dimensions with
different index types, so the nesting syntax is slightly different. Building on
the previous example where a level is defined as an array of enums indexed by
yet another enum, we can add the following lines to add a light tower type
subdivided in height levels accessed through their integer index:

.. code-block:: nim
  type
    LightTower = array[1..10, LevelSetting]
  var
    tower: LightTower
  tower[1][north] = slowBlink
  tower[1][east] = mediumBlink
  echo len(tower)     # --> 10
  echo len(tower[1])  # --> 4
  echo repr(tower)    # --> [[slowBlink, mediumBlink, ...more output..
  # The following lines don't compile due to type mismatch errors
  #tower[north][east] = on
  #tower[0][1] = on

Note how the built-in ``len`` proc returns only the array's first dimension
length.  Another way of defining the ``LightTower`` to show better its
nested nature would be to omit the previous definition of the ``LevelSetting``
type and instead write it embedded directly as the type of the first dimension:

.. code-block:: nim
  type
    LightTower = array[1..10, array[north..west, BlinkLights]]

It is quite frequent to have arrays start at zero, so there's a shortcut syntax
to specify a range from zero to the specified index minus one:

.. code-block:: nim
  type
    IntArray = array[0..5, int] # an array that is indexed with 0..5
    QuickArray = array[6, int]  # an array that is indexed with 0..5
  var
    x: IntArray
    y: QuickArray
  x = [1, 2, 3, 4, 5, 6]
  y = x
  for i in low(x)..high(x):
    echo(x[i], y[i])


Sequences
---------
Sequences are similar to arrays but of dynamic length which may change
during runtime (like strings). Since sequences are resizable they are always
allocated on the heap and garbage collected.

Sequences are always indexed with an ``int`` starting at position 0.  The `len
<system.html#len,seq[T]>`_, `low <system.html#low>`_ and `high
<system.html#high>`_ operations are available for sequences too.  The notation
``x[i]`` can be used to access the i-th element of ``x``.

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 <system.html#newSeq>`_ procedure.

A sequence may be passed to an openarray parameter.

Example:

.. code-block:: nim

  var
    x: seq[int] # a reference to a sequence of integers
  x = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence allocated on the heap

Sequence variables are initialized with ``nil``. However, most sequence
operations cannot deal with ``nil`` (leading to an exception being
raised) for performance reasons. Thus one should use empty sequences ``@[]``
rather than ``nil`` as the *empty* value. But ``@[]`` creates a sequence
object on the heap, so there is a trade-off to be made here.

The ``for`` statement can be used with one or two variables when used with a
sequence. When you use the one variable form, the variable will hold the value
provided by the sequence. The ``for`` statement is looping over the results
from the `items() <system.html#items.i,seq[T]>`_ iterator from the `system
<system.html>`_ module.  But if you use the two variable form, the first
variable will hold the index position and the second variable will hold the
value. Here the ``for`` statement is looping over the results from the
`pairs() <system.html#pairs.i,seq[T]>`_ iterator from the `system
<system.html>`_ module.  Examples:

.. code-block:: nim
  for i in @[3, 4, 5]:
    echo($i)
  # --> 3
  # --> 4
  # --> 5

  for i, value in @[3, 4, 5]:
    echo("index: ", $i, ", value:", $value)
  # --> index: 0, value:3
  # --> index: 1, value:4
  # --> index: 2, value:5


Open arrays
-----------
**Note**: Openarrays can only be used for parameters.

Often fixed size arrays turn out to be too inflexible; procedures should be
able to deal with arrays of different sizes. The `openarray`:idx: type allows
this. Openarrays are always indexed with an ``int`` starting at position 0.
The `len <system.html#len,TOpenArray>`_, `low <system.html#low>`_ and `high
<system.html#high>`_ operations are available for open arrays too.  Any array
with a compatible base type can be passed to an openarray parameter, the index
type does not matter.

.. code-block:: nim
  var
    fruits:   seq[string]       # reference to a sequence of strings that is initialized with 'nil'
    capitals: array[3, string]  # array of strings with a fixed size

  fruits = @[]                  # creates an empty sequence on the heap that will be referenced by 'fruits'

  capitals = ["New York", "London", "Berlin"]   # array 'capitals' allows only assignment of three elements
  fruits.add("Banana")          # sequence 'fruits' is dynamically expandable during runtime
  fruits.add("Mango")

  proc openArraySize(oa: openArray[string]): int =
    oa.len

  assert openArraySize(fruits) == 2     # procedure accepts a sequence as parameter
  assert openArraySize(capitals) == 3   # but also an array type

The openarray type cannot be nested: multidimensional openarrays are not
supported because this is seldom needed and cannot be done efficiently.


Varargs
-------

A ``varargs`` parameter is like an openarray parameter. However, it 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:: nim
  proc myWriteln(f: File, a: varargs[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 varargs parameter is the
last parameter in the procedure header. It is also possible to perform
type conversions in this context:

.. code-block:: nim
  proc myWriteln(f: File, a: varargs[string, `$`]) =
    for s in items(a):
      write(f, s)
    write(f, "\n")

  myWriteln(stdout, 123, "abc", 4.0)
  # is transformed by the compiler to:
  myWriteln(stdout, [$123, $"abc", $4.0])

In this example `$ <system.html#$>`_ is applied to any argument that is passed
to the parameter ``a``. Note that `$ <system.html#$>`_ applied to strings is a
nop.


Slices
------

Slices look similar to subranges types in syntax but are used in a different
context. A slice is just an object of type Slice which contains two bounds,
`a` and `b`. By itself a slice is not very useful, but other collection types
define operators which accept Slice objects to define ranges.

.. code-block:: nim

  var
    a = "Nim is a progamming language"
    b = "Slices are useless."

  echo a[7..12] # --> 'a prog'
  b[11..^2] = "useful"
  echo b # --> 'Slices are useful.'

In the previous example slices are used to modify a part of a string. The
slice's bounds can hold any value supported by
their type, but it is the proc using the slice object which defines what values
are accepted.


Tuples
------

A tuple type defines various named *fields* and an *order* of the fields.
The constructor ``()`` can be used to construct tuples. The order of the
fields in the constructor must match the order in the tuple's definition.
Different tuple-types are *equivalent* if they specify fields of
the same type and of the same name in the same order.

The assignment operator for tuples copies each component. The notation
``t.field`` is used to access a tuple's field. Another notation is
``t[i]`` to access the ``i``'th field. Here ``i`` needs to be a constant
integer.

.. code-block:: nim

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

  echo(person.name) # "Peter"
  echo(person.age)  # 30

  echo(person[0]) # "Peter"
  echo(person[1]) # 30

  # You don't need to declare tuples in a separate type section.
  var building: tuple[street: string, number: int]
  building = ("Rue del Percebe", 13)
  echo(building.street)

  # The following line does not compile, they are different tuples!
  #person = building
  # --> Error: type mismatch: got (tuple[street: string, number: int])
  #     but expected 'Person'

  # The following works because the field names and types are the same.
  var teacher: tuple[name: string, age: int] = ("Mark", 42)
  person = teacher

Even though you don't need to declare a type for a tuple to use it, tuples
created with different field names will be considered different objects despite
having the same field types.

Tuples can be *unpacked* during variable assignment (and only then!). This can
be handy to assign directly the fields of the tuples to individually named
variables. An example of this is the `splitFile <os.html#splitFile>`_ proc
from the `os module <os.html>`_ which returns the directory, name and
extension of a path at the same time. For tuple unpacking to work you have to
use parenthesis around the values you want to assign the unpacking to,
otherwise you will be assigning the same value to all the individual
variables! Example:

.. code-block:: nim

  import os

  let
    path = "usr/local/nimc.html"
    (dir, name, ext) = splitFile(path)
    baddir, badname, badext = splitFile(path)
  echo dir      # outputs `usr/local`
  echo name     # outputs `nimc`
  echo ext      # outputs `.html`
  # All the following output the same line:
  # `(dir: usr/local, name: nimc, ext: .html)`
  echo baddir
  echo badname
  echo badext

Tuple unpacking **only** works in ``var`` or ``let`` blocks. The following code
won't compile:

.. code-block:: nim

  import os

  var
    path = "usr/local/nimc.html"
    dir, name, ext = ""

  (dir, name, ext) = splitFile(path)
  # --> Error: '(dir, name, ext)' cannot be assigned to


Reference and pointer types
---------------------------
References (similar to pointers 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.

Nim 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 empty ``[]`` subscript notation can be used to *derefer* a reference,
meaning to retrieve the item the reference points to. The ``.`` (access a
tuple/object field operator) and ``[]`` (array/string/sequence index operator)
operators perform implicit dereferencing operations for reference types:

.. code-block:: nim

  type
    Node = ref NodeObj
    NodeObj = object
      le, ri: Node
      data: int
  var
    n: Node
  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 <system.html>`_
module contains further information.

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


Procedural type
---------------
A procedural type is a (somewhat abstract) pointer to a procedure.
``nil`` is an allowed value for a variable of a procedural type.
Nim uses procedural types to achieve `functional`:idx: programming
techniques.

Example:

.. code-block:: nim
  proc echoItem(x: int) = echo(x)

  proc forEach(action: proc (x: int)) =
    const
      data = [2, 3, 5, 7, 11]
    for d in items(data):
      action(d)

  forEach(echoItem)

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. The different calling conventions are
listed in the `manual <manual.html>`_.


Modules
=======
Nim supports splitting a program into pieces with a module concept.
Each module is 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. Only top-level symbols that are marked
with an asterisk (``*``) are exported:

.. code-block:: nim
  # Module A
  var
    x*, y: int

  proc `*` *(a, b: seq[int]): seq[int] =
    # allocate a new sequence:
    newSeq(result, len(a))
    # multiply two int sequences:
    for i in 0..len(a)-1: result[i] = a[i] * b[i]

  when isMainModule:
    # test the new ``*`` operator for sequences:
    assert(@[1, 2, 3] * @[1, 2, 3] == @[1, 4, 9])

The above module exports ``x`` and ``*``, but not ``y``.

The top-level statements of a module are executed at the start of the program.
This can be used to initialize complex data structures for example.

Each module has a special magic constant ``isMainModule`` that is true if the
module is compiled as the main file. This is very useful to embed tests within
the module as shown by the above example.

Modules that depend on each other are possible, but strongly discouraged,
because then one module cannot be reused without the other.

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:: nim
  # 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:: nim
  # 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
    result = x + 1


A symbol of a module *can* be *qualified* with the ``module.symbol`` syntax. If
the symbol is ambiguous, it even *has* to be qualified. A symbol is ambiguous
if it is defined in two (or more) different modules and both modules are
imported by a third one:

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

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

.. code-block:: nim
  # 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


But this rule does not apply to procedures or iterators. Here the overloading
rules apply:

.. code-block:: nim
  # Module A
  proc x*(a: int): string = $a

.. code-block:: nim
  # Module B
  proc x*(a: string): string = $a

.. code-block:: nim
  # Module C
  import A, B
  write(stdout, x(3))   # no error: A.x is called
  write(stdout, x(""))  # no error: B.x is called

  proc x*(a: int): string = nil
  write(stdout, x(3))   # ambiguous: which `x` is to call?


Excluding symbols
-----------------

The normal ``import`` statement will bring in all exported symbols.
These can be limited by naming symbols which should be excluded with
the ``except`` qualifier.

.. code-block:: nim
  import mymodule except y


From statement
--------------

We have already seen the simple ``import`` statement that just imports all
exported symbols. An alternative that only imports listed symbols is the
``from import`` statement:

.. code-block:: nim
  from mymodule import x, y, z

The ``from`` statement can also force namespace qualification on
symbols, thereby making symbols available, but needing to be qualified
to be used.

.. code-block:: nim
  from mymodule import x, y, z

  x()           # use x without any qualification

.. code-block:: nim
  from mymodule import nil

  mymodule.x()  # must qualify x with the module name as prefix

  x()           # using x here without qualification is a compile error

Since module names are generally long to be descriptive, you can also
define a shorter alias to use when qualifying symbols.

.. code-block:: nim
  from mymodule as m import nil

  m.x()         # m is aliasing mymodule


Include statement
-----------------
The ``include`` statement does something fundamentally different than
importing a module: it merely includes the contents of a file. The ``include``
statement is useful to split up a large module into several files:

.. code-block:: nim
  include fileA, fileB, fileC



Part 2
======

So, now that we are done with the basics, let's see what Nim offers apart
from a nice syntax for procedural programming: `Part II <tut2.html>`_


.. _strutils: strutils.html
.. _system: system.html
