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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.| document: http://www.ifcomputer.co.jp/MINERVA/Manual/Reference/Predicates/logic_and_control/if_then/print_de.html published 2008/6/30 update 2001/2/7 (c) 1996-2006 IF Computer Japan | ||||||||
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