Category:Elena

Official website
Execution method:	Compiled (bytecode)
Garbage collected:	Yes
Type safety:	Safe
Type strength:	Strong;
Type expression:	Implicit
Type checking:	Dynamic
See Also:	Elena on the HOPL; Elena compared to other languages;

Overview

ELENA is a general-purpose, object-oriented, polymorphic language with late binding. It features message dispatching / manipulation, dynamic object mutation, a script engine / interpreter and group object support.

Any program or library consists of modules containing classes and symbols.

The main way to interact with objects in ELENA is sending a message. The message name is structured and consists of a verb, a signature and a parameter counter. The verb defines a message action, for example read or write some data. There are only limited set of possible verbs. The signature is user defined and describes the message parameters. If the signature is not provided the message is considered to be generic and can be qualified (by dispatching).

If the object wants to handle the message it has to contain the method with the same name. If no method mapping was found the flow is considered to be broken and the control goes to the next alternative flow (exception handler) or the program is stopped. It is possible to declare generic handler which will be called for all incoming messages.

Namespaces

Any ELENA program or library consists of modules (files with .NL extension) containing classes and symbols. Every class or symbol may be referred by its namespace (or to put it other way around a symbol namespace is a path to the symbol module).

All source files (files with .L extension) located in the same folder are compiled into the corresponding module. A project file (a file with .PRJ extension) defines the root namespace and the output type (stand-alone executable, VM executable or a library). The project may produce several modules if it contains the files located in sub folders (the new module namespace consists of the root one plus the folder relative path)

Messaging

As in the most of dynamic object-oriented languages the main way to interact with objects in ELENA is sending a message. Unlike others the message name is structured and consists of a verb, a signature and a parameter counter. The verb defines a message action, for example read or write some data. There are only limited set of possible verbs (e.g. eval[uate], add, set, get, run, seek and so on). In general the signature is user defined and describes the message parameters. It can be used to define some custom action as well (e.g. writeLine, which in fact is eval&writeLine(1)). If the signature is not provided the message is considered to be generic and can be qualified (for example by dispatching).

If the object wants to handle the message it has to contain the method with the same name. If no method mapping was found the flow is considered to be broken and the control goes to the next alternative flow (exception handler) or the program is stopped.

The simple code to send a message looks like this:

   console write:"Hello World".

Note: "write" is a generic message; a literal constant is a parameter.

Several messages can be send in one statement, the parameter itself may be result of object interactions:

   console write "2 + 2 =" write:(2 add:2).

We could use operators to have the shorter code:

   console << "2+2=" << 2 + 2.

Note: In most cases "<<" is a synonym to "write" and "+" to "add".

Several parameters can be passed in the message as well:

   control foreach: (1,2,3) do:printingLn.

The generic message can have several parameters as well:

   consoleEx writeLine:”a+b=”: (a + b).

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
   {
     object theField1.
     object theField2.
     
     field1 = theField1.
   
     field2 = theField.    
   }
   
   class DerivedClass :: BaseClass
   {
     constructor new field1:aField2 field2:aField2
     [  
        theField1 := aField1.
        theField2 := aField2.
     ]
   
     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

Singletons cannot have constructors and their symbols can be used directly

   class ClassHelper =
   {
      sumOf:a:b
         = a add field1:b field2:a.
   }
   
   ...
   
   var aSum := ClassHelper sumOf:1:2.

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.

Code blocks

ELENA code block consists of a sequence of statements. The block is enclosed in square brackets and may contain nested sub code blocks (which in fact are inline action classes). The statement terminator is a dot.

   printAckermann n:n m:m
   [
       control forrange int:0 int:n do: (&int:i)
       [
           control forrange int:0 int:m do: (&int:j)
           [
               ...
               
               console writeLine
           ].
       ].
   ]

When a method should return a result (other than self) return statement is used. It should be the last statement in the block.

   [
       ...
   
       ^ aRetVal / anArray length
   ]

If the code block contains only return statement the simplified syntax can be used:

   Number = convertor toReal:theValue.

It is possible to declare the block variable and assigns the value to it. The variable name must be unique within the code block scope.

   var aRetVal := Integer new:0.

Conditional branching

ELENA like Smalltalk does not support any special language constructs to implement the conditional branching. Instead special Boolean symbols (system’true and system’false) are used. All conditional operations should return these symbols as a result.

There are three branching methods : if[1] , if[2], else[1]

   (m == 0) if:
   [
      n + 1
   ]
   : [
      m + n
   ].

Note that code in square brackets are in fact nested action classes ( an action class is a class supporting evaluate message). So this code is can be written in this form:

   (m == 0) if: 
   {
      eval
      [
         ^ n + 1.
      ]
   }
   : {
      eval
      [
         ^ m + n.
      ]
   }.

This expression can be written using special operators

   (m == 0) 
     ? [ ^n + 1 ]
     ! [ ^m + n ].

Note: the main difference between using explicit messages and conditional operators is that the compiler may optimize the resulting code in the later case.

We could omit true or else part

   (m == 0) 
      ! [ ^m / n ].

It is possible to use *if* template code :

   if (m == 0) 
     [ n := n + 1. ];
     [ n := m + n. ].

In this case the compiler always use optimized code for branching

Boolean symbols supports basic logical operations (AND, OR, XOR and NOT), so several conditions can be checked

   if (aChar >= 48) and:(aChar < 58)
   [
       theToken append:aChar.
   ];
   [
      Exception new:"Invalid expression"; raise.
   ]

Note that in this case both condition will be evaluated even if the first one is false If we want to use short-circuit evaluation expression brackets should be used

   if ((x >= 0)and:$(array@x != 0))
   [
       ...
   ]

A switch statement can be implemented using => operator

   aBulls =>
        -1 [ consoleEx writeLine:"Not a valid guess.". ^ true ];
         4 [ 
                 consoleEx writeLine:"Congratulations! You have won!". 
                 ^ false
             ];
           ! [
                theAttempt append:1.
                
                consoleEx writeLine:
                     "Your Score is " : aBulls 
                     : " bulls and " : aCows : " cows".
                
                ^ true
           ].

Resources

- (Wiki:)[1] - **ELENA reddit:** <https://www.reddit.com/r/elena_lang/> - **Gitter:** <https://gitter.im/ELENA-LANG/elena-lang> - **Source code:** <https://github.com/ELENA-LANG/elena-lang> - **Git clone URL:** <git://github.com/ELENA-LANG/elena-lang.git> - **Twitter:** <https://twitter.com/elena_language> - **Homepage:** <http://elenalang.sourceforge.net/>