SWI-Prolog -- when/2

Availability::- use_module(library(when)).(can be autoloaded)

Execute Goal when Condition becomes true. Condition is one of ?=(X, Y), nonvar(X), ground(X), ,(Cond1, Cond2) or

;(Cond1, 
Cond2)

. See also freeze/2 and dif/2. The implementation can deal with cyclic terms in X and Y.

The when/2 predicate is realised using attributed variables associated with the module when. It is defined in the autoload library library(when).