Tuesday, May 6, 2014

ELENA 2.0:Classes, Roles and Symbols

ELENA is an object-oriented language, so to create a program we have to declare new classes.

A class encapsulates data (fields) with code (methods) to access it. In most cases it is not possible to get a direct access to the class content (it makes sense for dynamic languages when in the most cases code is generic and can be applied for different "types"). Usually the fields refer to another classes and so on until we reach "primitive" ones which content are considered as raw data (e.g. numeric or literal values).

To work with the class we have to create its instance with the help of the special methods - constructors. A constructor is used mostly to initialize the class fields. There are special type of classes which do not have fields and constructors and can be used directly (roles).

Classes form the inheritance tree. There is the common super class - system'Object. ELENA does not support multiple inheritance, though it is possible to inherit the code using redirect handler (so called "horizontal inheritance"). When the parent is not provided the class inherits directly system'Object (the super class).

#class BaseClass
  #field theField1.
  #field theField2.
  #method field1 = theField1.

  #method field2 = theField.


#class DerivedClass : BaseClass
  #constructor new &field1:aField2 &field2:aField2
     theField1 := aField1.
     theField2 := aField2.

  #method add &field1:aField2 &field2:aField2
     = MyClass new &Field1:(theField1 + aField1) 
                   &Field2:(theField2 + aField2).

To create a class instance we have to send a message (usually new) to its symbol (a class symbol is declared implicitly for every class and can be used as a normal one)

#var anObject := DerivedClass new &field1:1 
                   &field2:1. // DerivedClass is a symbol

Roles cannot have constructors and their symbols can be used directly

   #method sumOf:anObject1:anObject2
      = anObject1 add &field1::anObject2 &field2::anObject1.


#var aSum := ClassHelper sumOf:anObject1:anObject2.

In general the symbol is a named expression and can be used to declare initialized objects, constants, reusable expressions and so on.

#symbol ZeroClass = DerivedClass new &field:0 &field:0.

A static symbol is the class instance which state is preserved. There could be only one instance of static symbol.

#static SingletonClass = DerivedClass new &field:0 &field:0.

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