when/2 delays a predicate until some condition in its variable is met. For example, we may want to find out the maximum of two numbers, but we are not sure when they will be instantiated. We can write the standard max/3 predicate (but changing its name to gmax/3 to denote that the first and second arguments must be ground) as
gmax(X, Y, X):- X > Y, !. gmax(X, Y, Y):- X =< Y.
and then define a 'safe' max/3 as
max(X, Y, Z):- when((ground(X),ground(Y)), gmax(X, Y, Z)).
which can be called as follows:
?- max(X, Y, Z) , Y = 0, X = 8. X = 8, Y = 0, Z = 8 ? yes
Alternatively, max/3 could have been defined as
max(X, Y, Z):- when(ground((X, Y)), gmax(X, Y, Z)).
with the same effects as above. More complex implementations are possible. Look, for example, at the max.pl implementation under the when library directory, where a max/3 predicate is implemented which waits on all the arguments until there is enough information to determine their values:
?- use_module(library(when/max)). yes ?- max(X, Y, Z), Z = 5, Y = 4. X = 5, Y = 4, Z = 5 ? yes
Delays / executes Goal according to WakeupCond given. The WakeupConds now acceptable are ground(T) (Goal is delayed until T is ground), nonvar(T) (Goal is delayed until T is not a variable), and conjunctions and disjunctions of conditions:
wakeup_exp(ground(_1)). wakeup_exp(nonvar(_1)). wakeup_exp((C1,C2)) :- wakeup_exp(C1), wakeup_exp(C2). wakeup_exp((C1;C2)) :- wakeup_exp(C1), wakeup_exp(C2).
when/2 only fails it the WakeupCond is not legally formed. If WakeupCond is met at the time of the call no delay mechanism is involved --- but there exists a time penalty in the condition checking.
In case that an instantiation fires the execution of several predicates, the order in which these are executed is not defined.
T is a legal expression for delaying goals.