1.183 class when

When executed, a when object tests the condition and then evaluates either the then- or else-function, returning its result. This construct is similar to C and similar languages Condition ? Then : Else.

The example below illustrates how a method number<-absolute could be attached to class number to return its absolute value:

?- send(class(number), get_method,
        get_method(absolute, int, new(vector),
                   when(@receiver < 0,
                        -(@receiver),
                        @receiver?value))).

?- new(N, number(-4)),
   get(N, absolute, V).		==> V = 4.

See also

class if

1.183.1 Instance variables

when <-> condition: code

Condition object that determines whether the when<-then or when<-else branch is evaluated.

when <-> else: any|function

Either of when<-then or when<-else is evaluated depending on the result of testing when<-condition. If this slot contains a function, this function is evaluated and the result returned. Otherwise the plain value is returned.

when <-> then: any|function

Inherits description from: when-else

1.183.2 Send methods

when ->initialise: condition=code, then=any|function, else=any|function

Create a when object. The branches are either a concrete value or a function object.