# Nim Tutorial (Part II)

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Author: | Andreas Rumpf
Version: | 1.0.2

# Introduction

> "Repetition renders the ridiculous reasonable." -- Norman Wildberger

This document is a tutorial for the advanced constructs of the Nim programming language. Note that this document is somewhat obsolete as the manual contains many more examples of the advanced language features.

# Pragmas

Pragmas are Nim's method to give the compiler additional information/ commands without introducing a massive number of new keywords. Pragmas are enclosed in the special `{.` and `.}` curly dot brackets. This tutorial does not cover pragmas. See the manual or user guide for a description of the available pragmas.

# Object Oriented Programming

While Nim's support for object oriented programming (OOP) is minimalistic, powerful OOP techniques can be used. OOP is seen as one way to design a program, not the only way. Often a procedural approach leads to simpler and more efficient code. In particular, preferring composition over inheritance is often the better design.

## Inheritance

Inheritance in Nim is entirely optional. To enable inheritance with runtime type information the object needs to inherit from `RootObj`. This can be done directly, or indirectly by inheriting from an object that inherits from `RootObj`. Usually types with inheritance are also marked as `ref` types even though this isn't strictly enforced. To check at runtime if an object is of a certain type, the `of` operator can be used.

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

      Student = ref object of Person # Student inherits from Person
        id: int                      # with an id field

    var
      student: Student
      person: Person
    assert(student of Student) # is true
    # object construction:
    student = Student(name: "Anton", age: 5, id: 2)
    echo student[]

Inheritance is done with the `object of` syntax. Multiple inheritance is currently not supported. If an object type has no suitable ancestor, `RootObj` can be used as its ancestor, but this is only a convention. Objects that have no ancestor are implicitly `final`. You can use the `inheritable` pragma to introduce new object roots apart from `system.RootObj`. (This is used in the GTK wrapper for instance.)

Ref objects should be used whenever inheritance is used. It isn't strictly necessary, but with non-ref objects assignments such as `let person: Person = Student(id: 123)` will truncate subclass fields.

Note: Composition (has-a relation) is often preferable to inheritance (is-a relation) for simple code reuse. Since objects are value types in Nim, composition is as efficient as inheritance.

## Mutually recursive types

Objects, tuples and references can model quite complex data structures which depend on each other; they are mutually recursive. In Nim these types can only be declared within a single type section. (Anything else would require arbitrary symbol lookahead which slows down compilation.)

Example:

    type
      Node = ref object  # a reference to an object with the following field:
        le, ri: Node     # left and right subtrees
        sym: ref Sym     # leaves contain a reference to a Sym

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

## Type conversions

Nim distinguishes between type casts and type conversions. Casts are done with the `cast` operator and force the compiler to interpret a bit pattern to be of another type.

Type conversions are a much more polite way to convert a type into another: They preserve the abstract value, not necessarily the bit-pattern. If a type conversion is not possible, the compiler complains or an exception is raised.

The syntax for type conversions is `destination_type(expression_to_convert)` (like an ordinary call):

    proc getID(x: Person): int =
      Student(x).id

The `InvalidObjectConversionError` exception is raised if `x` is not a `Student`.

## Object variants

Often an object hierarchy is overkill in certain situations where simple variant types are needed.

An example:

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

    var n = Node(kind: nkFloat, floatVal: 1.0)
    # the following statement raises an `FieldError` exception, because
    # n.kind's value does not fit:
    n.strVal = ""

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

## Method call syntax

There is a syntactic sugar for calling routines: The syntax `obj.method(args)` can be used instead of `method(obj, args)`. If there are no remaining arguments, the parentheses can be omitted: `obj.len` (instead of `len(obj)`).

This method call syntax is not restricted to objects, it can be used for any type:

    import strutils

    echo "abc".len # is the same as echo len("abc")
    echo "abc".toUpperAscii()
    echo({'a', 'b', 'c'}.card)
    stdout.writeLine("Hallo") # the same as writeLine(stdout, "Hallo")

(Another way to look at the method call syntax is that it provides the missing postfix notation.)

So "pure object oriented" code is easy to write:

    import strutils, sequtils

    stdout.writeLine("Give a list of numbers (separated by spaces): ")
    stdout.write(stdin.readLine.splitWhitespace.map(parseInt).max.`$`)
    stdout.writeLine(" is the maximum!")

## Properties

As the above example shows, Nim has no need for get-properties: Ordinary get-procedures that are called with the method call syntax achieve the same. But setting a value is different; for this a special setter syntax is needed:

    type
      Socket* = ref object of RootObj
        h: int # cannot be accessed from the outside of the module due to missing star

    proc `host=`*(s: var Socket, value: int) {.inline.} =
      ## setter of host address
      s.h = value

    proc host*(s: Socket): int {.inline.} =
      ## getter of host address
      s.h

    var s: Socket
    new s
    s.host = 34  # same as `host=`(s, 34)

(The example also shows `inline` procedures.)

The `[]` array access operator can be overloaded to provide array properties:

    type
      Vector* = object
        x, y, z: float

    proc `[]=`* (v: var Vector, i: int, value: float) =
      # setter
      case i
      of 0: v.x = value
      of 1: v.y = value
      of 2: v.z = value
      else: assert(false)

    proc `[]`* (v: Vector, i: int): float =
      # getter
      case i
      of 0: result = v.x
      of 1: result = v.y
      of 2: result = v.z
      else: assert(false)

The example is silly, since a vector is better modelled by a tuple which already provides `v[]` access.

## Dynamic dispatch

Procedures always use static dispatch. For dynamic dispatch replace the `proc` keyword by `method`:

    type
      Expression = ref object of RootObj ## abstract base class for an expression
      Literal = ref object of Expression
        x: int
      PlusExpr = ref object of Expression
        a, b: Expression

    # watch out: 'eval' relies on dynamic binding
    method eval(e: Expression): int {.base.} =
      # override this base method
      quit "to override!"

    method eval(e: Literal): int = e.x
    method eval(e: PlusExpr): int = eval(e.a) + eval(e.b)

    proc newLit(x: int): Literal = Literal(x: x)
    proc newPlus(a, b: Expression): PlusExpr = PlusExpr(a: a, b: b)

    echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4)))

Note that in the example the constructors `newLit` and `newPlus` are procs because it makes more sense for them to use static binding, but `eval` is a method because it requires dynamic binding.

Note: Starting from Nim 0.20, to use multi-methods one must explicitly pass `--multimethods:on` when compiling.

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

    type
      Thing = ref object of RootObj
      Unit = ref object of Thing
        x: int

    method collide(a, b: Thing) {.inline.} =
      quit "to override!"

    method collide(a: Thing, b: Unit) {.inline.} =
      echo "1"

    method collide(a: Unit, b: Thing) {.inline.} =
      echo "2"

    var a, b: Unit
    new a
    new b
    collide(a, b) # output: 2

As the example demonstrates, invocation of a multi-method cannot be ambiguous: Collide 2 is preferred over collide 1 because the resolution works from left to right. Thus `Unit, Thing` is preferred over `Thing, Unit`.

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

# Exceptions

In Nim exceptions are objects. By convention, exception types are suffixed with 'Error'. The system module defines an exception hierarchy that you might want to stick to. Exceptions derive from `system.Exception`, which provides the common interface.

Exceptions have to be allocated on the heap because their lifetime is unknown. The compiler will prevent you from raising an exception created on the stack. All raised exceptions should at least specify the reason for being raised in the `msg` field.

A convention is that exceptions should be raised in exceptional cases: For example, if a file cannot be opened, this should not raise an exception since this is quite common (the file may not exist).

## Raise statement

Raising an exception is done with the `raise` statement:

    var
      e: ref OSError
    new(e)
    e.msg = "the request to the OS failed"
    raise e

If the `raise` keyword is not followed by an expression, the last exception is re-raised. For the purpose of avoiding repeating this common code pattern, the template `newException` in the `system` module can be used:

    raise newException(OSError, "the request to the OS failed")

## Try statement

The `try` statement handles exceptions:

    from strutils import parseInt

    # read the first two lines of a text file that should contain numbers
    # and tries to add them
    var
      f: File
    if open(f, "numbers.txt"):
      try:
        let a = readLine(f)
        let b = readLine(f)
        echo "sum: ", parseInt(a) + parseInt(b)
      except OverflowError:
        echo "overflow!"
      except ValueError:
        echo "could not convert string to integer"
      except IOError:
        echo "IO error!"
      except:
        echo "Unknown exception!"
        # reraise the unknown exception:
        raise
      finally:
        close(f)

The statements after the `try` are executed unless an exception is raised. Then the appropriate `except` part is executed.

The empty `except` part is executed if there is an exception that is not explicitly listed. It is similar to an `else` part in `if` statements.

If there is a `finally` part, it is always executed after the exception handlers.

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

If you need to access the actual exception object or message inside an `except` branch you can use the getCurrentException() and getCurrentExceptionMsg() procs from the system module. Example:

    try:
      doSomethingHere()
    except:
      let
        e = getCurrentException()
        msg = getCurrentExceptionMsg()
      echo "Got exception ", repr(e), " with message ", msg

## Annotating procs with raised exceptions

Through the use of the optional `{.raises.}` pragma you can specify that a proc is meant to raise a specific set of exceptions, or none at all. If the `{.raises.}` pragma is used, the compiler will verify that this is true. For instance, if you specify that a proc raises `IOError`, and at some point it (or one of the procs it calls) starts raising a new exception the compiler will prevent that proc from compiling. Usage example:

    proc complexProc() {.raises: [IOError, ArithmeticError].} =
      ...

    proc simpleProc() {.raises: [].} =
      ...

Once you have code like this in place, if the list of raised exception changes the compiler will stop with an error specifying the line of the proc which stopped validating the pragma and the raised exception not being caught, along with the file and line where the uncaught exception is being raised, which may help you locate the offending code which has changed.

If you want to add the `{.raises.}` pragma to existing code, the compiler can also help you. You can add the `{.effects.}` pragma statement to your proc and the compiler will output all inferred effects up to that point (exception tracking is part of Nim's effect system). Another more roundabout way to find out the list of exceptions raised by a proc is to use the Nim `doc2` command which generates documentation for a whole module and decorates all procs with the list of raised exceptions. You can read more about Nim's effect system and related pragmas in the manual.

# Generics

Generics are Nim's means to parametrize procs, iterators or types with type parameters. They are most useful for efficient type safe containers:

    type
      BinaryTree*[T] = ref object # BinaryTree is a generic type with
                                  # generic param ``T``
        le, ri: BinaryTree[T]     # left and right subtrees; may be nil
        data: T                   # the data stored in a node

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

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

    proc add*[T](root: var BinaryTree[T], data: T) =
      # convenience proc:
      add(root, newNode(data))

    iterator preorder*[T](root: BinaryTree[T]): T =
      # Preorder traversal of a binary tree.
      # Since recursive iterators are not yet implemented,
      # this uses an explicit stack (which is more efficient anyway):
      var stack: seq[BinaryTree[T]] = @[root]
      while stack.len > 0:
        var n = stack.pop()
        while n != nil:
          yield n.data
          add(stack, n.ri)  # push right subtree onto the stack
          n = n.le          # and follow the left pointer

    var
      root: BinaryTree[string] # instantiate a BinaryTree with ``string``
    add(root, newNode("hello")) # instantiates ``newNode`` and ``add``
    add(root, "world")          # instantiates the second ``add`` proc
    for str in preorder(root):
      stdout.writeLine(str)

The example shows a generic binary tree. Depending on context, the brackets are used either to introduce type parameters or to instantiate a generic proc, iterator or type. As the example shows, generics work with overloading: the best match of `add` is used. The built-in `add` procedure for sequences is not hidden and is used in the `preorder` iterator.

# Templates

Templates are a simple substitution mechanism that operates on Nim's abstract syntax trees. Templates are processed in the semantic pass of the compiler. They integrate well with the rest of the language and share none of C's preprocessor macros flaws.

To invoke a template, call it like a procedure.

Example:

    template `!=` (a, b: untyped): untyped =
      # this definition exists in the System module
      not (a == b)

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

The `!=`, `>`, `>=`, `in`, `notin`, `isnot` operators are in fact templates: this has the benefit that if you overload the `==` operator, the `!=` operator is available automatically and does the right thing. (Except for IEEE floating point numbers - NaN breaks basic boolean logic.)

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

Templates are especially useful for lazy evaluation purposes. Consider a simple proc for logging:

    const
      debug = true

    proc log(msg: string) {.inline.} =
      if debug: stdout.writeLine(msg)

    var
      x = 4
    log("x has the value: " & $x)

This code has a shortcoming: if `debug` is set to false someday, the quite expensive `$` and `&` operations are still performed! (The argument evaluation for procedures is eager).

Turning the `log` proc into a template solves this problem:

    const
      debug = true

    template log(msg: string) =
      if debug: stdout.writeLine(msg)

    var
      x = 4
    log("x has the value: " & $x)

The parameters' types can be ordinary types or the meta types `untyped`, `typed`, or `type`. `type` suggests that only a type symbol may be given as an argument, and `untyped` means symbol lookups and type resolution is not performed before the expression is passed to the template.

If the template has no explicit return type, `void` is used for consistency with procs and methods.

To pass a block of statements to a template, use `untyped` for the last parameter:

    template withFile(f: untyped, filename: string, mode: FileMode,
                      body: untyped) =
      let fn = filename
      var f: File
      if open(f, fn, mode):
        try:
          body
        finally:
          close(f)
      else:
        quit("cannot open: " & fn)

    withFile(txt, "ttempl3.txt", fmWrite):
      txt.writeLine("line 1")
      txt.writeLine("line 2")

In the example the two `writeLine` statements are bound to the `body` parameter. The `withFile` template contains boilerplate code and helps to avoid a common bug: to forget to close the file. Note how the `let fn = filename` statement ensures that `filename` is evaluated only once.

## Example: Lifting Procs

    import math

    template liftScalarProc(fname) =
      ## Lift a proc taking one scalar parameter and returning a
      ## scalar value (eg ``proc sssss[T](x: T): float``),
      ## to provide templated procs that can handle a single
      ## parameter of seq[T] or nested seq[seq[]] or the same type
      ##
      ## .. code-block:: Nim
      ##  liftScalarProc(abs)
      ##  # now abs(@[@[1,-2], @[-2,-3]]) == @[@[1,2], @[2,3]]
      proc fname[T](x: openarray[T]): auto =
        var temp: T
        type outType = type(fname(temp))
        result = newSeq[outType](x.len)
        for i in 0..<x.len:
          result[i] = fname(x[i])

    liftScalarProc(sqrt)   # make sqrt() work for sequences
    echo sqrt(@[4.0, 16.0, 25.0, 36.0])   # => @[2.0, 4.0, 5.0, 6.0]

# Compilation to JavaScript

Nim code can be compiled to JavaScript. However in order to write JavaScript-compatible code you should remember the following:

- `addr` and `ptr` have slightly different semantic meaning in JavaScript. It is recommended to avoid those if you're not sure how they are translated to JavaScript.
- `cast[T](x)` in JavaScript is translated to `(x)`, except for casting between signed/unsigned ints, in which case it behaves as static cast in C language.
- `cstring` in JavaScript means JavaScript string. It is a good practice to use `cstring` only when it is semantically appropriate. E.g. don't use `cstring` as a binary data buffer.

# Part 3

Next part will be entirely about metaprogramming via macros: Part III