The assignment operator creates a copy of its right operand and assigns it to the left operand. This means that the left operand must be a variable (or a variable that has a value), any value that the left operand may have is overwritten by the copy of the right operand.
If the right operand contains a variable then the value of that variable is copied, not a reference to the variable. If the right operand contains an empty variable an exception is thrown. This operator is not part of the Prolog standard.
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The assignment operator does not create a copy of an instance of a class if a variable in the right operand refers to an instance. This can prevent a module from being closed because instances of a class may still exist, i.e. they exist in the expression copied by the :=/2 operator. To solve this problem the variable that contains the expression with the reference to the instance in it must receive another value. |
| Examples | |
| A := [B], B=c. | the variable A will have the value [B] |
| B=[b], A := [a|B]. | A will be the list [a, b] |
| B=[b], C=[], A := [B,a|C]. | A will be the list A=[[b], a] |
| B=a, B := [x|B]. | B will have the value [x| a] |
| Exceptions | |
| the left operand is not a variable | a type_error(variable, A) exception is thrown, the variable A is replaced by the left operand |
| the right operand is a single empty variable OR contains an empty variable | an instantiation_error exception is thrown |