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causes that one goal implicates alternative goals.
+IfGoal -> +ThenGoal; +ElseGoal
;((IfGoal-> +ThenGoal), +ElseGoal)
If IfGoal succeeds then ThenGoal will be executed.
If IfGoal fails ElseGoal will be executed.
'->'/2 succeeds if either ThenGoal or ElseGoal succeeds.
There are two significant differences to ','/2:
- 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 1100 which is right-associative,
'->'/2 is a predefined infix-operator with priority 1050 which is right-associative (see also current_op/3).
Arguments
IfGoal goal ThenGoal goal ElseGoal goal
Examples
| Day=sunny -> Weather=sunshine; Weather=raining. | Succeeds. |
| X=b, X=a -> write(a); write(b). | Succeeds with substitution X <- b and outputs to the current outputstream 'b'. |
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)/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_else/print_de.html published 2008/7/7 update 2001/2/7 (c) 1996-2006 IF Computer Japan | ||||||||
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