Nim/doc/tut2.txt

=========================
Nimrod Tutorial (Part II)
=========================

:Author: Andreas Rumpf
:Version: |nimrodversion|

.. contents::


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

  "Object-oriented programming is an exceptionally bad idea which could
  only have originated in California." --Edsger Dijkstra


This document is a tutorial for the advanced constructs of the *Nimrod*
programming language. **Note that this document is somewhat obsolete as
the `manual <manual.html>`_ contains many more examples of the advanced 
language features.**


Pragmas
=======
Pragmas are Nimrod'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 <manual.html>`_ 
or `user guide <nimrodc.html>`_ for a description of the available pragmas.


Object Oriented Programming
===========================

While Nimrod's support for object oriented programming (OOP) is minimalistic,
powerful OOP technics 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, prefering composition over inheritance
is often the better design.


Objects
-------

Like tuples, objects are a means to pack different values together in a
structured way. However, objects provide many features that tuples do not:
They provide inheritance and information hiding. Because objects encapsulate
data, the ``T()`` object constructor should only be used internally and the
programmer should provide a proc to initialize the object (this is called
a *constructor*).

Objects have access to their type at runtime. There is an
``of`` operator that can be used to check the object's type:

.. code-block:: nimrod
  type
    TPerson = object of TObject
      name*: string  # the * means that `name` is accessible from other modules
      age: int       # no * means that the field is hidden from other modules

    TStudent = object of TPerson # TStudent inherits from TPerson
      id: int                    # with an id field

  var
    student: TStudent
    person: TPerson
  assert(student of TStudent) # is true
  # object construction:
  student = TStudent(name: "Anton", age: 5, id: 2)

Object fields that should be visible from outside the defining module have to
be marked by ``*``. In contrast to tuples, different object types are
never *equivalent*. New object types can only be defined within a type
section.

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


**Note**: Composition (*has-a* relation) is often preferable to inheritance
(*is-a* relation) for simple code reuse. Since objects are value types in
Nimrod, 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 Nimrod
these types can only be declared within a single type section. (Anything else
would require arbitrary symbol lookahead which slows down compilation.)

Example:

.. code-block:: nimrod
  type
    PNode = ref TNode # a traced reference to a TNode
    TNode = object
      le, ri: PNode   # left and right subtrees
      sym: ref TSym   # leaves contain a reference to a TSym

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


Type conversions
----------------
Nimrod distinguishes between `type casts`:idx: and `type conversions`:idx:.
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):

.. code-block:: nimrod
  proc getID(x: TPerson): int =
    return TStudent(x).id

The ``EInvalidObjectConversion`` exception is raised if ``x`` is not a
``TStudent``.


Object variants
---------------
Often an object hierarchy is overkill in certain situations where simple
`variant`:idx: types are needed.

An example:

.. code-block:: nimrod

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

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

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


Methods
-------
In ordinary object oriented languages, procedures (also called *methods*) are
bound to a class. This has disadvantages:

* Adding a method to a class the programmer has no control over is
  impossible or needs ugly workarounds.
* Often it is unclear where the method should belong to: is
  ``join`` a string method or an array method?

Nimrod avoids these problems by not assigning methods to a class. All methods
in Nimrod are `multi-methods`:idx:. As we will see later, multi-methods are
distinguished from procs only for dynamic binding purposes.


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`:idx: is not restricted to objects, it can be used
for any type:

.. code-block:: nimrod
  
  echo("abc".len) # is the same as echo(len("abc"))
  echo("abc".toUpper())
  echo({'a', 'b', 'c'}.card)
  stdout.writeln("Hallo") # the same as writeln(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:

.. code-block:: nimrod
  import strutils
  
  stdout.writeln("Give a list of numbers (separated by spaces): ")
  stdout.write(stdin.readLine.split.map(parseInt).max.`$`)
  stdout.writeln(" is the maximum!")


Properties
----------
As the above example shows, Nimrod 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:

.. code-block:: nimrod
  
  type
    TSocket* = object of TObject
      FHost: int # cannot be accessed from the outside of the module
                 # the `F` prefix is a convention to avoid clashes since
                 # the accessors are named `host`

  proc `host=`*(s: var TSocket, value: int) {.inline.} =
    ## setter of hostAddr
    s.FHost = value
  
  proc host*(s: TSocket): int {.inline.} =
    ## getter of hostAddr
    return s.FHost

  var
    s: TSocket
  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`:idx:\ :

.. code-block:: nimrod
  type
    TVector* = object
      x, y, z: float

  proc `[]=`* (v: var TVector, 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: TVector, 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``:

.. code-block:: nimrod
  type
    PExpr = ref object of TObject ## abstract base class for an expression
    PLiteral = ref object of PExpr
      x: int
    PPlusExpr = ref object of PExpr
      a, b: PExpr

  # watch out: 'eval' relies on dynamic binding
  method eval(e: PExpr): int =
    # override this base method
    quit "to override!"
  
  method eval(e: PLiteral): int = e.x
  method eval(e: PPlusExpr): int = eval(e.a) + eval(e.b)
  
  proc newLit(x: int): PLiteral = PLiteral(x: x)
  proc newPlus(a, b: PExpr): PPlusExpr = PPlusExpr(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 they should use static binding, but ``eval`` is a method because it
requires dynamic binding.

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

.. code-block:: nimrod

  type
    TThing = object of TObject
    TUnit = object of TThing
      x: int
      
  method collide(a, b: TThing) {.inline.} =
    quit "to override!"
    
  method collide(a: TThing, b: TUnit) {.inline.} =
    echo "1"
  
  method collide(a: TUnit, b: TThing) {.inline.} =
    echo "2"
  
  var
    a, b: TUnit
  collide(a, b) # output: 2


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 ``TUnit, TThing`` is preferred over ``TThing, TUnit``.

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


Exceptions
==========

In Nimrod `exceptions`:idx: are objects. By convention, exception types are
prefixed with an 'E', not 'T'. The `system <system.html>`_ module defines an
exception hierarchy that you might want to stick to. Exceptions derive from
E_Base, 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:

.. code-block:: nimrod
  var
    e: ref EOS
  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:

.. code-block:: nimrod
  raise newException(EOS, "the request to the OS failed")


Try statement
-------------

The `try`:idx: statement handles exceptions:

.. code-block:: nimrod
  # read the first two lines of a text file that should contain numbers
  # and tries to add them
  var
    f: TFile
  if open(f, "numbers.txt"):
    try:
      let a = readLine(f)
      let b = readLine(f)
      echo "sum: ", parseInt(a) + parseInt(b)
    except EOverflow:
      echo "overflow!"
    except EInvalidValue:
      echo "could not convert string to integer"
    except EIO:
      echo "IO error!"
    except:
      echo "Unknown exception!"
      # 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 <system.html>`_ module.
Example:

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


Exception hierarchy
-------------------

If you want to create your own exceptions you can inherit from E_Base, but you
can also inherit from one of the existing exceptions if they fit your purpose.
The exception tree is::

  * E_Base
    * EAsynch
      * EControlC
    * ESynch
      * ESystem
        * EIO
        * EOS
          * EInvalidLibrary
      * EResourceExhausted
      * EOutOfMemory
      * EStackOverflow
    * EArithmetic
      * EDivByZero
      * EOverflow
    * EAccessViolation
    * EAssertionFailed
    * EInvalidValue
      * EInvalidKey
    * EInvalidIndex
    * EInvalidField
    * EOutOfRange
    * ENoExceptionToReraise
    * EInvalidObjectAssignment
    * EInvalidObjectConversion
    * EFloatingPoint
      * EFloatInvalidOp
      * EFloatDivByZero
      * EFloatOverflow
      * EFloatUnderflow
      * EFloatInexact
    * EDeadThread

See the `system <system.html>`_ module for a description of each exception.


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 ``EIO``, 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:

.. code-block:: nimrod
  proc complexProc() {.raises: [EIO, EArithmetic].} =
    ...

  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 ``{.effect.}`` pragma statement to your proc and
the compiler will output all inferred effects up to that point (exception
tracking is part of Nimrod's effect system). Another more roundabout way to
find out the list of exceptions raised by a proc is to use the Nimrod ``doc2``
command which generates documentation for a whole module and decorates all
procs with the list of raised exceptions. You can read more about Nimrod's
`effect system and related pragmas in the manual <manual.html#effect-system>`_.


Generics
========

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

.. code-block:: nimrod
  type
    TBinaryTree[T] = object      # TBinaryTree is a generic type with
                                 # with generic param ``T``
      le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil
      data: T                    # the data stored in a node
    PBinaryTree*[T] = ref TBinaryTree[T] # type that is exported

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

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

  iterator preorder*[T](root: PBinaryTree[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[PBinaryTree[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: PBinaryTree[string] # instantiate a PBinaryTree with ``string``
  add(root, newNode("hallo")) # instantiates ``newNode`` and ``add``
  add(root, "world")          # instantiates the second ``add`` proc
  for str in preorder(root):
    stdout.writeln(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 Nimrod'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:

.. code-block:: nimrod
  template `!=` (a, b: expr): expr =
    # this definition exists in the System module
    not (a == b)

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

The ``!=``, ``>``, ``>=``, ``in``, ``notin``, ``isnot`` operators are in fact
templates: 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:

.. code-block:: nimrod
  const
    debug = True

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

.. code-block:: nimrod
  const
    debug = True

  template log(msg: string) =
    if debug: stdout.writeln(msg)
  
  var
    x = 4
  log("x has the value: " & $x)

The parameters' types can be ordinary types or the meta types ``expr``
(stands for *expression*), ``stmt`` (stands for *statement*) or ``typedesc``
(stands for *type description*). If the template has no explicit return type,
``stmt`` is used for consistency with procs and methods.

The template body does not open a new scope. To open a new scope use a ``block``
statement:

.. code-block:: nimrod
  template declareInScope(x: expr, t: typeDesc): stmt {.immediate.} =
    var x: t

  template declareInNewScope(x: expr, t: typeDesc): stmt {.immediate.} =
    # open a new scope:
    block:
      var x: t

  declareInScope(a, int)
  a = 42  # works, `a` is known here
  
  declareInNewScope(b, int)
  b = 42  # does not work, `b` is unknown

(The manual explains why the ``immediate`` pragma is needed for these 
templates.)

If there is a ``stmt`` parameter it should be the last in the template
declaration. The reason is that statements can be passed to a template
via a special ``:`` syntax:

.. code-block:: nimrod

  template withFile(f: expr, filename: string, mode: TFileMode,
                    body: stmt): stmt {.immediate.} =
    block:
      let fn = filename
      var f: TFile
      if open(f, fn, mode):
        try:
          body
        finally:
          close(f)
      else:
        quit("cannot open: " & fn)
      
  withFile(txt, "ttempl3.txt", fmWrite):
    txt.writeln("line 1")
    txt.writeln("line 2")
  
In the example the two ``writeln`` statements are bound to the ``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.


Macros
======

Macros enable advanced compile-time code transformations, but they
cannot change Nimrod's syntax. However, this is no real restriction because
Nimrod's syntax is flexible enough anyway.

To write a macro, one needs to know how the Nimrod concrete syntax is converted
to an abstract syntax tree (AST). The AST is documented in the
`macros <macros.html>`_ module.

There are two ways to invoke a macro:
(1) invoking a macro like a procedure call (`expression macros`:idx:)
(2) invoking a macro with the special ``macrostmt``
    syntax (`statement macros`:idx:)


Expression Macros
-----------------

The following example implements a powerful ``debug`` command that accepts a
variable number of arguments:

.. code-block:: nimrod
  # to work with Nimrod syntax trees, we need an API that is defined in the
  # ``macros`` module:
  import macros

  macro debug(n: varargs[expr]): stmt =
    # `n` is a Nimrod AST that contains a list of expressions;
    # this macro returns a list of statements:
    result = newNimNode(nnkStmtList, n)
    # iterate over any argument that is passed to this macro:
    for i in 0..n.len-1:
      # add a call to the statement list that writes the expression;
      # `toStrLit` converts an AST to its string representation:
      result.add(newCall("write", newIdentNode("stdout"), toStrLit(n[i])))
      # add a call to the statement list that writes ": "
      result.add(newCall("write", newIdentNode("stdout"), newStrLitNode(": ")))
      # add a call to the statement list that writes the expressions value:
      result.add(newCall("writeln", newIdentNode("stdout"), n[i]))

  var
    a: array[0..10, int]
    x = "some string"
  a[0] = 42
  a[1] = 45

  debug(a[0], a[1], x)

The macro call expands to:

.. code-block:: nimrod
  write(stdout, "a[0]")
  write(stdout, ": ")
  writeln(stdout, a[0])

  write(stdout, "a[1]")
  write(stdout, ": ")
  writeln(stdout, a[1])

  write(stdout, "x")
  write(stdout, ": ")
  writeln(stdout, x)



Statement Macros
----------------

Statement macros are defined just as expression macros. However, they are
invoked by an expression following a colon.

The following example outlines a macro that generates a lexical analyzer from
regular expressions:

.. code-block:: nimrod

  macro case_token(n: stmt): stmt =
    # creates a lexical analyzer from regular expressions
    # ... (implementation is an exercise for the reader :-)
    nil

  case_token: # this colon tells the parser it is a macro statement
  of r"[A-Za-z_]+[A-Za-z_0-9]*":
    return tkIdentifier
  of r"0-9+":
    return tkInteger
  of r"[\+\-\*\?]+":
    return tkOperator
  else:
    return tkUnknown


Term rewriting macros
---------------------

Term rewriting macros can be used to enhance the compilation process
with user defined optimizations; see this `document <trmacros.html>`_ for 
further information.