Identifier Expressions

We call an identifier expression an unary or binary expression whose operands must be identifiers. There are height identifier operators: ::, isset, unset, & (identical to refof), * (identical to valof), scopeof, push and pop.

As all these operators take identifiers as their operands, we skip the second table (operand combinations) while introducing these operators.

:: Operator

The :: unary/binary operator (called scope operator) is used to define a global or particular scope for a variable.
The unary version of this operator denotes a global scope. For instance, ::alpha denotes the global variable alpha. In the body of a function, identifiers denote local variables; outside the body of a function identifiers denote global variables, that means that, in this context, the global scope operator is not mandatory. If one wants to use a global variable in the body of a function, the global scope operator is mandatory. Refer to the Function Definition Statement Section for more information about local function variables.

The binary version of this operator denotes a particular scope. For instance, Person::checkName denotes the class attribute or method of the class Person.

note: class (or static) attributes are not currently well supported by the OQL interpreter. Class attributes are only supported in some specific query expressions (refer to the Query Expression Section).

General Information
Operator	::
Syntax	:: identifier
	identifier::identifier
Type	unary and binary
Operand Types	identifier
Result Type	value of the identifier if used as a right value; identifier reference if used as a left value
Function	defines a global or particular scope for the identifier.

Expression Examples
expression	result
::a	the value of the global variable a
::alpha := 1	sets the value of the global variable alpha to 1, returns 1
Person::checkName("wayne")	calls the class method checkName in the class Person
2::alpha	raises an error

isset Operator

The isset operator is used to check whether a variable is already set or not. It returns true is the variable is set, false otherwise.

General Information
Operator	isset
Syntax	isset identifier
Type	unary
Operand Type	identifier
Result Type	boolean
Function	evaluated to true if the identifier is set, false otherwise

Expression Examples
expression	result
isset oql$variables	true
isset a	returns true if a is set, false otherwise
isset 1	raises an error

unset Operator

The unset operator is used to unset an variable. It returns the nil atom.

General Information
Operator	unset
Syntax	unset identifier
Type	unary
Operand Type	identifier
Result Type	nil
Function	unset the identifier

Expression Examples
expression	result
unset a	nil
unset ::a	nil
unset 2	raises an error

refof Operator

The & (identical to refof) operator is used to get the reference of an identifier. This operator is essentially used when one calls a function or method which updates one or more given parameters. For instance, let the function swap(x, y) which swaps the value of its two parameters. One needs to give the references of the variables that one wants to swap. For instance:

i := "ii";
j := "jj";

swap(&i, &j);

After the call to swap, the variable i equals jj while the variable j equals ii.
The reverse operator * (described following section) is used in the swap function.

General Information
Operator	refof
	&
Syntax	refof identifier
	& identifier
Type	unary
Operand Type	identifier
Result Type	identifier
Function	evaluates the expression to the identifier reference; returned an identifier atom

Expression Examples
expression	result
&alpha	alpha
refof alpha	alpha

valof Operator

The * (identical to valof) operator is used to get the value of the identifier pointed by a reference. For instance, after the two following expressions:

alpha := 1;
ralpha := α

*ralpha equals 1.

This operator may be used in the composition of a left value, for instance:

alpha := 1;
ralpha := α
*ralpha := 2; // now, alpha equals 2
*ralpha += 8; // now, alpha equals 10

But this operator is essentially used in the body of functions or methods which update one or more given parameters, for instance, the function swap described in the previous section is as follows:

function swap(x, y) {
    v := *x;
   *x := *y;
   *y := v;
}

General Information
Operator	valof
	*
Syntax	valof identifier
	* identifier
Type	unary
Operand Type	identifier
Result Type	value of the identifier
Function	returns the value of the identifier denotes by the operand

Expression Examples
expression	result
*alpha	if alpha value is an atom identifier x, returns the value of x, otherwise an error is thrown
x := 12; alpha := &x; *x	*x returns 12

scopeof Operator

The scopeof operator returns the string "global" or "local" depending whether the identifier is global or local.

General Information
Operator	scopeof
Syntax	scopeof identifier
Type	unary
Operand Type	identifier
Result Type	string
Function	returns the scope of the identifier.

Expression Examples
expression	result
scopeof ::alpha	returns "global" for any alpha if it set; otherwise an error is thrown
scopeof alpha	returns "global" or "local" depending on the context.

push Operator

The push operator is used to push an identifier on a new local table. This operator is rarely used.

General Information
Operator	push
Syntax	push identifier
	push identifier := expr
Type	unary and binary
Operand Types	first operand identifier, optionnal second operand: any type
Result Type	identifier or any type in case of an assignment
Function	push the identifier on to the symbol table stack. An assignment can be performed at the same time. Returns the identifier or the value of the expression assignment.

Expression Examples
expression	result
push a	pushes a on a new local symbol table.
push a := 10	pushes a on a new local symbol table and assigns its value to 10

pop Operator

The pop operator is used to pop an identifier from a local table. It is used after a push. For instance:

a := "hello";
a;             // a equals "hello"

push a := 10;
a;             // a equals 10

pop a;
a;             // a equals "hello"

General Information
Operator	pop
Syntax	pop identifier
Type	unary
Operand Type	identifier
Result Type	the type of the value of the identifier
Function	pop the identifier from the symbol table stack

Expression Examples
expression	result
pop a	returns the value of a if it is set; otherwise an error is returned