It is sometimes convenient to use an additional operator ‘;’, standing for disjunction (or). (The precedence of ‘;’ is such that it dominates ‘,’ but is dominated by ‘:-’.) An example is the clause (A), which can be read as (B).
grandfather(X, Z) :- ( mother(X, Y) ; father(X, Y) ), father(Y, Z). (A)
“For any X, Y, and Z, X has Z as a grandfather if either the mother of X is Y or the father of X is Y, and the father of Y is Z.” (B)
Such uses of disjunction can usually be eliminated by defining an extra predicate. For instance, (A) is equivalent to (C)
grandfather(X, Z) :- parent(X, Y), father(Y, Z). parent(X, Y) :- mother(X, Y). parent(X, Y) :- father(X, Y). (C)
For historical reasons, the token ‘|’, when used outside a list, is actually an alias for ‘;’. The aliasing is performed when terms are read in. Since release 4.3, however, ‘|’ can be defined as a proper infix operator, which then disables the aliasing. So the use of ‘|’ instead of ‘;’ for disjunction is not recommended in new code.