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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