Base semantics

    Xion is a case-sensitive language.

    Comments

    There are two kinds of comments. The inline comment starts with the string character sequence // and ends at the end of the line. Multi-lines comment starts with the character sequence /* and ends with */.

Example:

        //Let's create a hello variable
        HelloWriter hello/* = new HelloWriter()*/; /*Here, the variable initialization is ignored, but the ; is necessary*/

    Instruction separator

    A Xion program is made of Xion instruction sequence. Each instruction ends with the character ;, except instruction blocks. Control structures if , else,  while and for are separated by their instruction (just as C or Java).

    Blocks

    It is possible to group Xion instructions by placing them between characters { and }. This is usually done in control structures if, else, while, do and for  to group a sequence of instructions. The scope of  variables defined in a nested block is limited to this block. A block is not an expression.

    Variables

   Variables are declared with the same syntax as C/Java. Firstly is given the qualified type name, and then, separated by comas, the names of the variables may be given with an initialization expression after the character =. A variable declaration is not an expression. To call a variable, just type its name.

Example:    Integer i = 0, j = ++i, k = i + j + 3;

    Within the declaration of a method specified by a non static operation, the pseudo variables this and self contain the object on which the method is called.

    Assignations

   To affect a variable or an accessible field, use the symbol =. An assignation is an expression returning Void, so it is impossible to cascade them.

Example:    Integer i; i = 123456789;

    Control structures

       Xion defines some conditional instructions that control the code execution:

• if: allows the execution of an instruction (or a block) only if the given condition is true
      ex: if (opNeeded) op(); where opNeeded is a boolean variable
• else: allows the execution of an instruction (or a block) when the above described if has a condition evaluated to false
      ex: if (isEven) doEven(); else doOdd();
• while: repeats an instruction (or a block) while its condition evaluates to true (the condition is evaluated before the instruction on each iteration)
      ex: while(i > 10) i = i - 1; //if i evaluates to 10 the first time, it will not be decremented
• do: repeats an instruction (or a block) while its condition evaluates to true (the condition is evaluated after the instruction on each iteration)
      ex: do i = i - 1; while (i > 10); //if i evaluates to 10 the first time, it will be decremented once
      The above expression is equivalent to:
      i = i - 1; while (i > 10) i = i - 1;
• for: a while that defines pre-instructions, a condition and code that may modify the condition,
      ex: for (Integer i = 0; i < 10; ++i) do(); //Executes do() 10 times
      The above expression is equivalent to:
      Integer i = 0; while (i < 10) {do(); ++i;}

    Re-typing or "Cast"

       It may be necessary to change the type of an expression, when the return type is not of the programmer awaited type. Two methods exist (difference explained later). For instance to transform a Real into Integer.

    Notice that is is possible to cast only into a super or a sub-type (for instance, it is possible to cast a Real into Integer, but not into a String)... This follows the type conformance rule.

    As C/Java: the target type is indicated before the expression between parenthesis; ex: Real r = 1.0; Integer i = (Integer)r;

    The predefined operation oclAsType: Real r = 1.0; Integer i = r.oclAsType(Integer);

    Notice that cast from a type to one of its supertype is implicit. For instance, the predefined operation max(Real) of Real can be called as monReal.max(12), even if 12 is an Integer, sub type of Real. The contrary is false.

    Tracing

       For debugging reasons, it may be useful to show some values during script execution. This is possible thanks to write and writeRaw instructions. These prefix expressions and show the evaluated value. write, unlike writeRaw, will filter the output to be correctly showed in an HTML text. For instance write "toto<br>tutu"; shows toto<br>tutu, writeRaw "toto<br>"; shows on the first line toto,  tutu on the second line, <br> being the HTML carriage return.

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