I want to declare an operator (like :-op(500,xfx,-:-).) in a module in a way that I could use it from the application that uses this module.

In Ciao, operators declared in a module are local to that module, to avoid colateral effects. For the behavior you want, you need to create and use a Ciao package, which is simply a file which will be included in the client source with a :- use_package declaration. That is, if the code which implements the functionality is my_code.pl, you make another file my_package.pl including these two lines:

:- op(500, xfx, -:-). :- use_module(my_code).

and then use the declaration

:- use_package(.(my_package)).

in the client source, instead of use_module(...). Note that the dot in .(my_package) implies that files my_package.pl and my_code.pl are in the same directory as the client source. If you will use that package in several applications, the easier thing is to put those files in a library directory (such as for example the contrib directory of the distribution), change the second line of my_package.pl to

:- use_module(library(my_code)).

and use the declaration

:- use_package(my_package).

in the client source. Other solutions exist, but they are less simple.

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