How to export dynamically created predicate? - prolog

Question

Consider the following code:
:- module(my_export, [create/2]).
create(Predicate, Id) :-
assertz(Predicate),
export(Id).
Assuming that the predicated and the identifier match, I'd expect the newly asserted predicate to be available outside the module. However, I get this output:
?- create(c(A,B) :- A > B, c/2).
true.
?- c(1,2).
Correct to: "my_export:c(1,2)"?
How come the predicate isn't exported? What is the correct way to do this?

Answer 1

You have to import module by using use_module/1.
For example, if this is sample_module.pl:
:- module(my_export, [create/2]).
create(Predicate, Id) :-
assertz(Predicate),
export(Id).
Then this input and output is true (observe closely what's going on):
?- create(tmp(A,B) :- A > B, tmp/2).
ERROR: toplevel: Undefined procedure: create/2 (DWIM could not correct goal)
?- consult('c:\\Prolog\\pl\\bin\\sample_module.pl').
% c:\Prolog\pl\bin\sample_module.pl compiled into my_export 0.00 sec, 2 clauses
true.
?- create(tmp(A,B) :- A > B, tmp/2).
true.
?- tmp(1,2).
Correct to: "my_export:tmp(1,2)"? yes
false.
?- use_module('c:\\Prolog\\pl\\bin\\sample_module.pl').
true.
?- tmp(1,2).
false.
?- tmp(5,4).
true.
Now, when you "compile buffer" in SWI-Prolog what really happens is consult/1. You need to import your module manually.

Answer 2

Your code work as-is, as long as the module is initially imported, as Grzegorz explained. For example:
?- [user].
:- module(my_export, [create/2]).
|:
|: create(Predicate, Id) :-
|: assertz(Predicate),
|: export(Id).
|: % user://1 compiled into my_export 0.00 sec, 2 clauses
true.
?- module_property(my_export, P).
P = class(user) ;
P = file('user://1') ;
P = line_count(10) ;
P = exports([create/2]) ;
false.
?- my_export:create(c(A,B) :- A > B, c/2).
true.
?- module_property(my_export, P).
P = class(user) ;
P = file('user://1') ;
P = line_count(10) ;
P = exports([c/2, create/2]) ;
false.
?- create(tmp(A,B) :- A > B, tmp/2).
true.
?- module_property(my_export, P).
P = class(user) ;
P = file('user://1') ;
P = line_count(10) ;
P = exports([tmp/2, c/2, create/2]) ;
false.
Note, however, that export/1 is usually a directive, not a predicate. There might be portability issues to other Prolog dialects supporting a module system.