Not very relational, indeed.Ī better construct, often called a "soft cut" (don't believe the name, a cut is a cut is a cut), gives slightly better results (this is in YAP): ?- ( ( Ch = a Ch = b ) *-> Class = ab Class = other ).Īlternatively, SICStus has if/3 with very similar semantics: ?- if( ( Ch = a Ch = b ), Class = ab, Class = other ). Just the first answer from ( Ch = a Ch = b ) is given. While this answer is sound, it is incomplete. Prolog has a built-in one: ?- ( ( Ch = a Ch = b ) -> Class = ab Class = other ).
To write things more compactly, an if-then-else construct is needed.
One could write this clumsily like so: char_class(a, ab). For a and b the class should be ab and other for all other terms. There are essentially three different ways how to express something like if-then-else in Prolog. We can use if_/3 together with the reified list-membership predicate memberd_t/3 like so: (P *-> Q R) is "less" incomplete than (P -> Q R), but still has similar woes.Įnter the logically monotone control construct if_/3! (P -> Q R) loses declarative semantics when instantiation is insufficient. (P,Q non_P,R) is correct, but needs a discrete implementation of non_P. Let's check out different ways of expressing "if-then-else" in Prolog! To express list (non-)membership in a pure way, we define: negated condition non_P is non_member(,X),.We can match above pattern (" If P then Q else R") if. If X is a member of list then X equals 2 else X equals 4.
Let's take the following concrete example: How can we express "if-then-else" like that in Prolog? " If P then Q else R" is equivalent to "(P and Q) or (non_P and R)". First, let's recall some classical first order logic:
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