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How to use op/3 inside a file

I try to define operator in a file using op/3. Like so

is_in(Thing, Place):-
  location(Thing, Place).

op(35, xfx, is_in).

But when I consult/1 file in repl, I get an exception

No permission to modify static procedure `op/3'

I tried to use dynamic directive, but it results in the same error.

Executing op(35, xfx, is_in). in repl works.

Solution

  • The problem is that you are trying to redefine op/3 predicate instead of declaring a new operator.

    To declare a new operator when consulting a file you have to add a directive in your program:

    :-op(35, xfx, is_in).

    Note that the operator will not be available before the directive, so you should add the directive above its usage within the file.