=====================

Nim Tutorial (Part I)

:Author: Andreas Rumpf :Version: |nimversion|

.. default-role:: code .. include:: rstcommon.rst .. contents::

Introduction

"Der Mensch ist doch ein Augentier -- Schöne Dinge wünsch' ich mir."

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.

Here are several other resources for learning Nim:

Nim Basics tutorial - a gentle introduction of the concepts mentioned above
Learn Nim in 5 minutes - quick, five-minute introduction to Nim
The Nim manual - 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.

The first program

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

```Nim test = "nim c $1" # 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:
  ```cmd
  nim compile --run greetings.nim

With the --run switch 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

This is a debug version. To compile a release version use:

  nim c -d:release greetings.nim

By default, the Nim compiler generates a large number of runtime checks aiming for your debugging pleasure. With -d:release some checks are turned off and optimizations are turned on.

For benchmarking or production code, use the -d:release switch. For comparing the performance with unsafe languages like C, use the -d:danger switch in order to get meaningful, comparable results. Otherwise, Nim might be handicapped by checks that are not even available for C.

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 procedure. Since the compiler knows that readLine returns a string, you can leave out the type in the declaration (this is called local type inference:idx:). So this will work too:

```Nim test = "nim c $1" 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](syncio.html#readLine,File), etc.
These built-ins are declared in the [system](system.html) 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:

  ```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 is 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 ##:

```nim test = "nim c $1" # 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.

Multiline comments are started with `#[` and terminated with `]#`.  Multiline
comments can also be nested.

  ```nim  test = "nim c $1"
  #[
  You can have any Nim code text commented
  out inside this with no indentation restrictions.
        yes("May I ask a pointless question?")
    #[
       Note: these can be nested!!
    ]#
  ]#

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:

  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:

```nim test = "nim c $1" var

x, y: int
# a comment can occur here too
a, b, c: string



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:

  ```nim  test = "nim c $1"
  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:

```nim test = "nim c $1" 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:

  ```nim
  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":

  const input = readLine(stdin) # Error: constant expression expected

```nim test = "nim c $1" let input = readLine(stdin) # works



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

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

  ```nim
  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 can be overloaded. You can declare multiple variables with a single assignment statement and all the variables will have the same value:

``nim test = "nim c $1" var x, y = 3 # assigns 3 to the variablesxandyecho "x ", x # outputs "x 3" echo "y ", y # outputs "y 3" x = 42 # changesxto 42 without changingy` echo "x ", x # outputs "x 42" echo "y ", y # outputs "y 3"



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:

  ```nim  test = "nim c $1"
  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 allows for multiple branches:

```nim test = "nim c $1" 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:

  ```nim
  # this statement will be explained later:
  from std/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:

  ...
  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:

```nim test = "nim c $1" 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.i,T,T,Positive) iterator:

  ```nim  test = "nim c $1"
  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 variable i is implicitly declared by the for loop and has the type int, because that is what countup returns. i runs through the values 1, 2, .., 10. Each value is echo-ed. This code does the same:

  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

Since counting up occurs so often in programs, Nim also has a .. iterator that does the same:

  for i in 1 .. 10:
    ...

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

  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

Zero-indexed counting has two shortcuts ..< and .. ^1 (backward index operator) to simplify counting to one less than the higher index:

  for i in 0 ..< 10:
    ...  # the same as 0 .. 9

  var s = "some string"
  for i in 0 ..< s.len:
    ...

  var s = "some string"
  for idx, c in s[0 .. ^1]:
    ... # ^1 is the last element, ^2 would be one before it, and so on

Other useful iterators for collections (like arrays and sequences) are

items and mitems, which provides immutable and mutable elements respectively, and
pairs and mpairs which provides the element and an index number (immutable and mutable respectively)

```nim test = "nim c $1" for index, item in ["a","b"].pairs:

echo item, " at index ", index

# => a at index 0 # => b at index 1


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:

  ```nim  test = "nim c $1"  status = 1
  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:

```nim test = "nim c $1" status = 1 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:

  ```nim  test = "nim c $1"
  block myblock:
    echo "entering block"
    while true:
      echo "looping"
      break # leaves the loop, but not the block
    echo "still in block"
  echo "outside the block"

  block myblock2:
    echo "entering block"
    while true:
      echo "looping"
      break myblock2 # leaves the block (and the loop)
    echo "still in block" # it won't be printed
  echo "outside the block"

Continue statement

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

```nim test = "nim c $1" for i in 1 .. 5:

if i <= 3: continue
echo i # will only print 4 and 5



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

Example:

  ```nim  test = "nim c $1"
  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, but with these differences:

Each condition must 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:c: construct in the C programming language.

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 are all simple statements. Complex statements like if, when, for, while can contain other statements. To avoid ambiguities, complex statements must always be indented, but single simple statements do not:

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

  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:

```nim test = "nim c $1" # 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,varargs[typed,])
and [readLine](syncio.html#readLine,File) in the examples, the concept of a
*procedure* is needed. You might be used to them being called *methods* or
*functions* in other languages, but Nim
[differentiates these concepts](tut1.html#procedures-funcs-and-methods). In
Nim, new procedures are defined with the `proc` keyword:

  ```nim  test = "nim c $1"
  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. The bool type is built-in: the only valid values for bool are true and false. The conditions in if or while statements must be of 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 shorthand for return result. The result value is always returned automatically at the end of a procedure if there is no return statement at the exit.

```nim test = "nim c $1" proc sumTillNegative(x: varargs[int]): int =

for i in x:
  if i < 0:
    return
  result = result + i

echo sumTillNegative() # echoes 0 echo sumTillNegative(3, 4, 5) # echoes 12 echo sumTillNegative(3, 4 , -1 , 6) # echoes 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
initialized 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
initialization.

A procedure that does not have any `return` statement and does not use the
special `result` variable returns the value of its last expression. For example,
this procedure

  ```nim  test = "nim c $1"
  proc helloWorld(): string =
    "Hello, World!"

returns the string "Hello, World!".

Parameters

Parameters are immutable 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:

```nim test = "nim c $1" 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:

  ```nim  test = "nim c $1"
  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 must be used. Nim does not allow silently throwing away a return value:

  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:

```nim test = "nim c $1" proc p(x, y: int): int {.discardable.} =

return x + y

p(3, 4) # now valid



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:

  ```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:

  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:

  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++:

  proc toString(x: int): string =
    result =
      if x < 0: "negative"
      elif x > 0: "positive"
      else: "zero"

  proc toString(x: bool): string =
    result =
      if x: "yep"
      else: "nope"

  assert toString(13) == "positive" # calls the toString(x: int) proc
  assert toString(true) == "yep"    # calls the toString(x: bool) proc

(Note that toString is usually the $ 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 -- see the manual for details. Ambiguous calls are reported as errors.

Operators

The Nim standard library makes heavy use of overloading - one reason for this is that each operator like + is 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 doing so may reduce readability.

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 "`":

  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:

``nim test = "nim c $1" 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 avoid this need in a
language that supports metaprogramming as extensively as Nim does.)
However, this cannot be done for mutually recursive procedures:

  ```nim
  # forward declaration:
  proc even(n: int): bool

  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.

Funcs and methods

As mentioned in the introduction, Nim differentiates between procedures, functions, and methods, defined by the proc, func, and method keywords respectively. In some ways, Nim is a bit more pedantic in its definitions than other languages.

Functions are closer to the concept of a pure mathematical function, which might be familiar to you if you've ever done functional programming. Essentially they are procedures with additional limitations set on them: they can't access global state (except const) and can't produce side-effects. The func keyword is basically an alias for proc tagged with {.noSideEffects.}. Functions can still change their mutable arguments however, which are those marked as var, along with any ref objects.

Unlike procedures, methods are dynamically dispatched. This sounds a bit complicated, but it is a concept closely related to inheritance and object-oriented programming. If you overload a procedure (two procedures with the same name but of different types or with different sets of arguments are said to be overloaded), the procedure to use is determined at compile-time. Methods, on the other hand, depend on objects that inherit from the RootObj. This is something that is covered in much greater depth in the second part of the tutorial.

Iterators

Let's return to the simple counting example:

```nim test = "nim c $1" echo "Counting to ten: " for i in countup(1, 10):

echo i


Can a [countup](system.html#countup.i,T,T,Positive) proc be written that
supports this loop? Let's try:

  ```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 here it is -- our first iterator:

```nim test = "nim c $1" 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#iterators-and-the-for-statement-firstminusclass-iterators)
for details. Iterators can have the same name and parameters as a proc since
essentially they have their own namespaces. Therefore, it is common 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
--------

Nim's boolean type is called `bool` and consists of the two
pre-defined values `true` and `false`. Conditions in `while`,
`if`, `elif`, and `when` statements must be of type bool.

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

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

Characters

The character type is called char. Its size is always one byte, so it cannot represent most UTF-8 characters, but it can represent one of the bytes that makes up a multibyte UTF-8 character. 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 especially 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 are mutable, so appending to a string is possible, and quite efficient. Strings in Nim are both zero-terminated and have a length field. A string's length can be retrieved with the builtin len procedure; the length never counts the terminating zero. Accessing the terminating zero is an error, it only exists so that a Nim string can be converted to a cstring without doing a copy.

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 using their lexicographical order. All the comparison operators are supported. By convention, all strings are UTF-8 encoded, 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.

A string variable is initialized with the empty string "".

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 specify a non-default integer type:

```nim test = "nim c $1" let

x = 0     # x is of type `int`
y = 0'i8  # y is of type `int8`
z = 0'i32 # z is of type `int32`
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 also defined for integers 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 under-flow
errors.

Lossless `Automatic type conversion`:idx: is performed in expressions where different
kinds of integer types are used. However, if the type conversion
would cause loss of information, the `RangeDefect`:idx: 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-bits.

Float literals can have a *type suffix* to specify a non-default float
type:

  ```nim  test = "nim c $1"
  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-754 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, nor vice versa. Use the toInt and toFloat procs for these conversions.

Type Conversion

Conversion between numerical types is performed by using the type as a function:

```nim test = "nim c $1" 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 [$](dollars.html) (stringify) operator
turns any basic type into a string, which you can then print to the console
using the `echo` proc. However, advanced types, and your own custom types,
won't work with the `$` operator until you define it for them.
Sometimes you just want to debug the current value of a complex type without
having to write its `$` operator.  You can use then the [repr](
system.html#repr,T) proc which works with any type and even complex data
graphs with cycles. The following example shows that even for basic types
there is a difference between the `$` and `repr` outputs:

  ```nim  test = "nim c $1"
  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)
  # --> 3.14:3.14

Advanced types

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

```nim test = "nim c $1" type

biggestInt = int64      # biggest integer type that is available
biggestFloat = float64  # biggest float type that is available


Enumeration and object types may only be defined within a
`type` statement.


Enumerations
------------

A variable of an enumeration type can only be assigned one of the enumeration's specified values.
These values are a set 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. For example:

  ```nim  test = "nim c $1"
  type
    Direction = enum
      north, east, south, west

  var x = south     # `x` is of type `Direction`; its value is `south`
  echo x            # prints "south"

All the 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, and the ord proc can convert it 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 must be in ascending order.

Ordinal types

Enumerations, 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, dec, succ and pred operations can fail by raising an RangeDefect or OverflowDefect. (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:

```nim test = "nim c $1" type

MySubrange = range[0..5]



`MySubrange` is a subrange of `int` which can only hold the values 0
to 5. Assigning any other value to a variable of type `MySubrange` 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,typedesc[T]) returns
the maximal value). Other programming languages may suggest the use of unsigned
integers for natural numbers. This is often **unwise**: 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
an array has the same type. The array's index type can be any ordinal type.

Arrays can be constructed using `[]`:

  ```nim  test = "nim c $1"
  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 proc returns the array's length. low(a) returns the lowest valid index for the array a and high(a) the highest valid index.

```nim test = "nim c $1" 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 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 into height levels accessed through their integer index:

  ```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 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 better illustrate 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:

  type
    LightTower = array[1..10, array[north..west, BlinkLights]]

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

```nim test = "nim c $1" 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,openArray[T]) and [high](
system.html#high,openArray[T]) 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:

  ```nim  test = "nim c $1"
  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 @[].

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() iterator from the system 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() iterator from the system module. Examples:

```nim test = "nim c $1" for value in @[3, 4, 5]:

echo value

# --> 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,openArray[T])
and [high](system.html#high,openArray[T]) 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.

  ```nim  test = "nim c $1"
  var
    fruits:   seq[string]       # reference to a sequence of strings that is initialized with '@[]'
    capitals: array[3, string]  # array of strings with a fixed size

  capitals = ["New York", "London", "Berlin"]   # array 'capitals' allows assignment of only 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:

```nim test = "nim c $1" 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:

  ```nim  test = "nim c $1"
  proc myWriteln(f: File, a: varargs[string, `$`]) =
    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 $ is applied to any argument that is passed to the parameter a. Note that $ 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.

```nim test = "nim c $1" var

a = "Nim is a programming 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.

To understand the different ways of specifying the indices of
strings, arrays, sequences, etc., it must be remembered that Nim uses
zero-based indices.

So the string `b` is of length 19, and two different ways of specifying the
indices are

  ```nim
  "Slices are useless."
   |          |     |
   0         11    17   using indices
  ^19        ^8    ^2   using ^ syntax

where b[0 .. ^1] is equivalent to b[0 .. b.len-1] and b[0 ..< b.len], and it can be seen that the ^1 provides a shorthand way of specifying the b.len-1. See the backwards index operator.

In the above example, because the string ends in a period, to get the portion of the string that is "useless" and replace it with "useful".

b[11 .. ^2] is the portion "useless", and b[11 .. ^2] = "useful" replaces the "useless" portion with "useful", giving the result "Slices are useful."

Note 1: alternate ways of writing this are b[^8 .. ^2] = "useful" or as b[11 .. b.len-2] = "useful" or as b[11 ..< b.len-1] = "useful".

Note 2: As the ^ template returns a distinct int of type BackwardsIndex, we can have a lastIndex constant defined as const lastIndex = ^1, and later used as b[0 .. lastIndex].

Objects

The default type to pack different values together in a single structure with a name is the object type. An object is a value type, which means that when an object is assigned to a new variable all its components are copied as well.

Each object type Foo has a constructor Foo(field: value, ...) where all of its fields can be initialized. Unspecified fields will get their default value.

  type
    Person = object
      name: string
      age: int

  var person1 = Person(name: "Peter", age: 30)

  echo person1.name # "Peter"
  echo person1.age  # 30

  var person2 = person1 # copy of person 1

  person2.age += 14

  echo person1.age # 30
  echo person2.age # 44


  # the order may be changed
  let person3 = Person(age: 12, name: "Quentin")

  # not every member needs to be specified
  let person4 = Person(age: 3)
  # unspecified members will be initialized with their default
  # values. In this case it is the empty string.
  doAssert person4.name == ""

Object fields that should be visible from outside the defining module have to be marked with *.

```nim test = "nim c $1" type

Person* = object # the type is visible from other modules
  name*: string  # the field of this type is visible from other modules
  age*: int


Tuples
------

Tuples are very much like what you have seen so far from objects. They
are value types where the assignment operator copies each component.
Unlike object types though, tuple types are structurally typed,
meaning different tuple-types are *equivalent* if they specify fields of
the same type and of the same name in the same order.

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. But unlike objects, a name for the tuple type may not be
used here.


Like the object type the notation `t.field` is used to access a
tuple's field. Another notation that is not available for objects is
`t[i]` to access the `i`'th field. Here `i` must be a constant
integer.

  ```nim  test = "nim c $1"
  type
    # type representing a person:
    # A person consists of a name and an age.
    Person = tuple
      name: string
      age: int

    # Alternative syntax for an equivalent type.
    PersonX = tuple[name: string, age: int]

    # anonymous field syntax
    PersonY = (string, int)

  var
    person: Person
    personX: PersonX
    personY: PersonY

  person = (name: "Peter", age: 30)
  # Person and PersonX are equivalent
  personX = person

  # Create a tuple with anonymous fields:
  personY = ("Peter", 30)

  # A tuple with anonymous fields is compatible with a tuple that has
  # field names.
  person = personY
  personY = person

  # Usually used for short tuple initialization syntax
  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'

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. This can be handy to assign directly the fields of the tuples to individually named variables. An example of this is the splitFile proc from the os module which returns the directory, name, and extension of a path at the same time. For tuple unpacking to work you must use parentheses around the values you want to assign the unpacking to, otherwise, you will be assigning the same value to all the individual variables! For example:

```nim test = "nim c $1" import std/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 is also supported in for-loops:

  ```nim  test = "nim c $1"
  let a = [(10, 'a'), (20, 'b'), (30, 'c')]

  for (x, c) in a:
    echo x
  # This will output: 10; 20; 30

  # Accessing the index is also possible:
  for i, (x, c) in a:
    echo i, c
  # This will output: 0a; 1b; 2c

Fields of tuples are always public, they don't need to be explicitly marked to be exported, unlike for example fields in an object type.

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 in a garbage-collected heap, untraced references point to manually allocated objects or objects elsewhere in memory. Thus, untraced references are unsafe. However, for certain low-level operations (e.g. accessing the hardware), untraced references are necessary.

Traced references are declared with the ref keyword; untraced references are declared with the ptr keyword.

The empty [] subscript notation can be used to de-refer 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:

```nim test = "nim c $1" type

Node = ref object
  le, ri: Node
  data: int

var n = Node(data: 9) echo n.data # no need to write n[].data; in fact n[].data is highly discouraged!


To allocate a new traced object, the built-in procedure `new` can be used:

  ```nim
  var n: Node
  new(n)

To deal with untraced memory, the procedures alloc, dealloc and realloc can be used. The system module's documentation contains further details.

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:

```nim test = "nim c $1" proc greet(name: string): string =

"Hello, " & name & "!"

proc bye(name: string): string =

"Goodbye, " & name & "."

proc communicate(greeting: proc (x: string): string, name: string) =

echo greeting(name)

communicate(greet, "John") communicate(bye, "Mary")


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#types-procedural-type).

Distinct type
-------------

A Distinct type allows for the creation of a new type that "does not imply a
subtype relationship between it and its base type".
You must **explicitly** define all behavior for the distinct type.
To help with this, both the distinct type and its base type can cast from one
type to the other.
Examples are provided in the [manual](manual.html#types-distinct-type).

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 the symbols of another
module by using the `import`:idx: statement. Only top-level symbols that are marked
with an asterisk (`*`) are exported:

  ```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): 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.

A module's top-level statements 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.

A symbol of a module can be qualified with the module.symbol syntax. And if a symbol is ambiguous, it must 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:

  # Module A
  var x*: string

  # Module B
  var x*: int

  # Module C
  import A, B
  write(stdout, x) # error: x is ambiguous
  write(stdout, A.x) # okay: 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:

  # Module A
  proc x*(a: int): string = $a

  # Module B
  proc x*(a: string): string = $a

  # 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 = discard
  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 that should be excluded using the except qualifier.

  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:

  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 in order to be used.

  from mymodule import x, y, z

  x()           # use x without any qualification

  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.

  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:

  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

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

tut1.md 59 KB History Raw

Nim Tutorial (Part I)

Introduction

The first program

Comments

Numbers

The var statement

Case statement

While statement

Continue statement

Statements and indentation

Result variable

Parameters

Discard statement

Default values

Overloaded procedures

Operators

Funcs and methods

Iterators

Characters

Strings

Integers

Type Conversion

Advanced types

Ordinal types

Subranges

Varargs

Slices

Objects

Reference and pointer types

Procedural type

Excluding symbols

From statement

Include statement

Part 2

tut1.md 59 KB

History Raw