A simple prolog syntax question -

in user mode in swi-prolog define function follows:

|: test1(+x,+y) :- x >= 0, x =:= y. 

and in query mode:

?- test1(1, 1). false. 

i expecting return "true" since 1 greater 0 , 1 equal 1.

so did go wrong?

this want instead:

test1(x,y) :- x >= 0, x =:= y. 

and then:

?- test1(1, 1). true. 

adding +, - , ? in front of predicate arguments not part of prolog. convention documenting how predicates should used.

here gnu-prolog documentation has say:

the mode specifies whether or not argument must instantiated when built-in predicate called. mode encoded symbol before type. possible modes are:

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

in actual code, should not prefix arguments +.