CODEWITHSUNDEEP
prolog
Olhovsky
Olhovsky

Reputation: 5549

A simple prolog syntax question

In user mode in SWI-Prolog I define a function as follows:

|: test1(+X,+Y) :- X >= 0, X =:= Y.

And now in query mode:

?- test1(1, 1).
false.

I was expecting this to return "true" since 1 is greater than 0 and 1 is equal to 1.

So where did I go wrong?

Upvotes: 1

Views: 284

Answers (1)

Etienne Laurin
Etienne Laurin

Reputation: 7184

This is what you want instead:

test1(X,Y) :- X >= 0, X =:= Y.

And then:

?- test1(1, 1).
true.

Adding +, - and ? in front of predicate arguments is not part of Prolog. It is just a convention for documenting how predicates should be used.

Here is what the GNU-prolog documentation has to say:

The mode specifies whether or not the argument must be instantiated when the built-in predicate is called. The mode is encoded with a symbol just before the type. Possible modes are:

  • +: the argument must be instantiated.
  • -: the argument must be a variable (will be instantiated if the built-in predicate succeeds).
  • ?: the argument can be instantiated or a variable.

In actual code, you should not prefix arguments with a +.

Upvotes: 5

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