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Variables

Variables usually start with a capital letter. The only interesting exception is the special anonymous variable written _ and pronounced ``underscore''. In the rule

 
process(X,Y):-

		       generate(_,Z),

		       test(_,Z),

		       evaluate(Z,Y).

the underscores refer to different unnamed variables. For example, here are two versions of member/2.

 
member(X,[XY]).

member(X,[YZ]):-

		       member(X,Z).

member(X,[X_]).

member(X,[_Z]):-

		       member(X,Z).

Note that, in the clause,

 
know_both_parents(X):-

		       mother(_,X),

		       father(_,X).

the underscores do not refer to the same object. The reading is roughly that ``we know both the parents of X if someone(name unimportant) is the mother of X and someone else (unimportant) is the father''. Note that Prolog regards the two occurrences of the anonymous variable in the above as different variables.

Paul Brna
Mon May 24 20:14:48 BST 1999