| causes that one goal implicates another goal.
+IfGoal -> +ThenGoal
->(+IfGoal, +ThenGoal)
If IfGoal succeeds then ThenGoal will be executed.
If IfGoal fails ThenGoal will not be executed.
'->'/2 succeeds if IfGoal and ThenGoal both succeed.
There are two significant differences to ','/2:
- Only ThenGoal can be backtraced. IfGoal can succeed only
once.
- If IfGoal has subgoals and also a Cut ('!'/0) this
Cut has no effect outside of IfGoal.
'->'/2 is a predefined infix-operator with priority 1050
which is right-associative (see also current_op/3).
Arguments
IfGoal goal
ThenGoal goal
Examples
There exists a database with clauses:
colour(blue). colour(yellow).
| colour(X) -> write(X). |
Succeeds once with substitution X <- blue
and outputs 'blue' to the current outputstream. |
| colour(X) -> X = yellow. |
Fails. |
Standard
This predicate is part of the ISO-Prolog Standard.
See also
abort/0,
break/0,
call/1,
fcall/1,
catch/3,
','(conjunction)/2,
'!'(cut)/0,
';'(disjunction)/2,
fail/0,
true/0,
halt/0/1,
';'(if-then-else)/2,
'\+'(not provable)/1,
once/1,
repeat/0,
throw/1.
|
