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11.3.150 ;/2   ISO


+Q ; +R

Disjunction: Succeeds if Q succeeds or R succeeds.

+P -> +Q ; +R

If P then Q else R, using first solution of P only.



callable, must be nonvar


callable, must be nonvar


callable, must be nonvar


These are normally regarded as part of the syntax of the language, but they are like a built-in predicate in that you can write call((Q ; R)) or call((P -> Q ; R)), with identical effect if they do not contain any cuts.

By default, the character ‘|’ (vertical bar) can be used as an alternative to the infix operator ‘;’. This equivalence does not hold when ‘|’ has been declared as an operator.

Using ‘|’ as an alternative to the infix operator ‘;’ is not recommended. A future version of the ISO Prolog standard is likely to define ‘|’ as an operator and with such an operator definition the ‘|’ will no longer be equivalent to ‘;’.

The operator precedences of the ‘;’ and ‘->’ are both greater than 1000, so that they dominate commas.

Cuts in P do not make sense, but are allowed, their scope being the goal P. The scope of cuts in Q and R extends to the containing clause.


For the if-then-else construct: if P succeeds and Q then fails, then backtracking into P does not occur. A cut in P does not make sense. ‘->’ acts like a cut except that its range is restricted to within the disjunction: it cuts away R and any choice points within P. ‘->’ may be thought of as a “local cut”.


Call errors (see ref-sem-exc).

See Also


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