Magik (programming language)

Magik is an object-oriented programming language that supports multiple inheritance, polymorphism and is dynamically typed. It was designed implemented in 1989 by Arthur Chance, of Smallworld Systems Ltd, as part of Smallworld Geographical Information System (GIS). Following Smallworld's acquisition in 2000, Magik is now is provided by GE Energy, still as part of its Smallworld technology platform.

Magik (Inspirational Magik) was originally introduced in 1990 and has been improved and updated over the years. Its current version is 4.0 or Magik SF (Small Footprint).

In July 2012, Magik developers announced that they were in the process of porting Magik language on the Java virtual machine. The successful porting was confirmed by Oracle Corporation in November of the same year.[1]

Similarities with Smalltalk

Magik itself shares some similarities with Smalltalk in terms of its language features and its architecture: the Magik language is compiled into byte codes interpreted by the Magik virtual machine. The Magik virtual machine is available on several platforms including Microsoft Windows, various flavours of Unix and Linux.

Magik is console based and code can be modified on the fly even when an application is running. The console can also be used to execute Magik code and to see the results.

Compiled code is stored in a single file called an image file. Each image file holds the compiled byte-codes and the state of the session (for example variable values) when the image was last saved.

Language features

Comments

Magik uses the # token to mark sections of code as comments:

 # This is a comment.

Assignments

Magik uses the << operator to make assignments:

  a << 1.234
  b << b + a
  c << "foo" + "bar" # Concat strings

For clarity, this notation is read as "a becomes 1.234" or "b becomes b plus a". This terminology separates assignment from comparison.

Magik also supports a compressed variation of this operator that works in a similar way to those found in C:

  b +<< a # Equivalent to b << b + a

To print a variable you can use the following command

 a << "hello"
 write(a)

Symbols

As well as conventional data types such as integers, floats and strings Magik also implements symbols. Symbols are a special token data type that are used extensively throughout Magik to uniquely identify objects. They are represented by a colon followed by a string of characters. Symbols can be escaped using the vertical bar character. For example:

  a << :hello  # whenever :hello is encountered, it is the same instance
  b << :|hello world|

Dynamic typing

Magik variables are not typed as they are in say C# and can reference different objects at runtime. Everything in Magik is an object (there is no distinction between objects and primitive types such as integers):

  a << 1.2     # a floating point number is assigned to variable 'a'
  a << "1.2"   # later, a string is assigned to variable 'a'
Objects

Objects are implemented in Magik using exemplars. Exemplars have similarities to classes in other programming languages such as Java, but with important differences. Magik supports multiple inheritance, and mixins (which implement functionality with no data). New instances are made by cloning an existing instance (which will typically be the exemplar but does not have to be).

New exemplars are created using the statement def_slotted_exemplar(), for example:

  def_slotted_exemplar(:my_object,
  {
    {:slot_a, 34},
    {:slot_b, "hello"}
  }, {:parent_object_a, :parent_object_b})

This code fragment will define a new exemplar called my_object that has two slots (or fields) called slot_a (pre-initialized to 34) and slot_b (pre-initialised to "hello") that inherits from two existing exemplars called parent_object_a and parent_object_b.

Comparison

Magik implements all usual logical operators (=, <, <=, >, >=, ~=/<>) for comparison, as well as a few unusual ones. The _is and _isnt operators are used for comparing specific instances of objects, or object references rather than values.

For example:

  a << "hello"
  b << "hello"

  a = b # returns True (_true) because the values of a and b are equal
  a _is b # returns False (_false) because a is not the same instance as b

  a << "hello"
  b << a = b # returns True (_true) because the values of a and b are equal
  a _is b # returns True (_true) because b was assigned the specific instance of the same object as a, rather than the value of a.

Methods

Methods are defined on exemplars using the statements _method and _endmethod:

  _method my_object.my_method(a, b)
    _return a + b
  _endmethod

It is convention to supply two methods new() (to create a new instance) and init() (to initialize an instance).

  # New method
  _method person.new(name, age)
    _return _clone.init(name, age)
  _endmethod

  # Initialize method.
  _private _method person.init(name, age)
     # Call the parent implementation.
     _super.init(name, age)
     # Initialise the slots.
     .name << name
     .age << age
    _return _self
  _endmethod

The _clone creates a physical copy of the person object. The _super statement allows objects to invoke an implementation of a method on the parent exemplar. Objects can reference themselves using the _self statement. An object's slots are accessed and assigned using a dot notation.

Methods that are not part of the public interface of the object can be marked private using the _private statement. Private methods can only be called by _self, _super and _clone.

Optional arguments can be declared using the _optional statement. Optional arguments that are not passed are assigned by Magik to the special object _unset (the equivalent of null). The _gather statement can be used to declare a list of optional arguments.

  _method my_object.my_method(_gather values)     
  _endmethod

Iteration

In Magik the _for, _over, _loop and _endloop statements allow iteration.

  _method my_object.my_method(_gather values)
    total << 0.0
    _for a _over values.elements()
    _loop
       total +<< a
    _endloop
    _return total
  _endmethod

  m << my_object.new()
  x << m.my_method(1.0, 2, 3.0, 4) # x = 10.0

Here values.elements() is an iterator which helps to iterate the values.

In Magik generator methods are called iterator methods. New iterator methods can be defined using the _iter and _loopbody statements:

  _iter _method my_object.even_elements()
    _for a _over _self.elements()
    _loop
      _if a.even? _is _true
      _then
         _loopbody(a)       
      _endif
    _endloop
  _endmethod

Procedures

Magik also supports functions called procedures. Procedures are also objects and are declared using the _proc and _endproc statements. Procedures are assigned to variables which may then be invoked:

  my_procedure << _proc @my_procedure(a, b, c)
    _return a + b + c
  _endproc

  x << my_procedure(1, 2, 3) # x = 6

Language Quirks

Because Magik was originally developed in England, methods in the core smallworld libraries are spelled using British English. For example:

  Use "initialise", not "initialize".

Collections

Like other programming language Magik too has collections. They include the following:

Hello World example

The following is an example of the Hello world program written in Magik:

 write("Hello World!")

References

  1. ↑ Jim Connors (2012-11-05). "Sprinkle Some Magik on that Java Virtual Machine". Retrieved 2012-11-06. With this new capability GE Energy has succeeded in hosting their Magik environment on top of the Java Virtual Machine

External links

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