SWI-Prolog: Use operators from modules?

Question

I have a prolog-file (name definitions.pl) of this kind of type:

:- module(definitions,[]).

:-op(699, xfx, :=).
:-op(599, xfy, ∪).



C := (A ∪ B) :- union(A, B, C).

My aim is to use this file as module in another file test.pl. To integrate it I tried:

:-use_module(definitions). 

But for some reason its not possible to make statements like:

X:=[1,2]∪[3,4]. 

after loading test.pl into swipl. I also tried:

:- module(definitions,[:=/2, ∪/2]).

:-op(699, xfx, :=).
:-op(599, xfy, ∪).

but this gives some operator exprected error. What is the correct way to use operators in prolog-modules?

Answer 1

Move the operator definitions into the module export list:

:- module(definitions, [
    (:=)/2, op(699, xfx, :=),
    op(599, xfy, ?)
]).

:- use_module(library(lists), [union/3]).

C := (A ? B) :-
    union(A, B, C).

Usage example (assuming a definitions.pl file with the above contents in the current working directory):

?- use_module(definitions).
true.

?- X:=[1,2]?[3,4].
X = [1, 2, 3, 4].