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4.9 Compatibility of the Module System

The principles behind the module system of SWI-Prolog differ in a number of aspects from the Quintus Prolog module system.

The meta_predicate/1 declaration causes the compiler to tag arguments that pass module sensitive information with the module using the :/2 operator. This approach has some disadvantages:

Unfortunately the transparent predicate approach also has some disadvantages. If a predicate A passes module sensitive information to a predicate B, passing the same information to a module sensitive system predicate both A and B should be declared transparent. Using the Quintus approach only A needs to be treated special (i.e., declared with meta_predicate/1) (30). A second problem arises if the body of a transparent predicate uses module sensitive predicates for which it wants to refer to its own module. Suppose we want to define findall/3 using assert/1 and retract/1 (31). The example in figure 3 gives the solution.

:- module(findall, [findall/3]).

:- dynamic

:- module_transparent

findall(Var, Goal, Bag) :-
        store(Var, Goal), 

store(Var, Goal) :-
        Goal,                   % refers to context module of
                                % caller of findall/3
store(_, _).

collect(Bag) :-

Figure 3 : findall/3 using modules

4.9.1 Emulating meta_predicate/1

The Quintus meta_predicate/1 directive can in many cases be replaced by the transparent declaration. Below is the definition of meta_predicate/1 as available from library(quintus).

:- op(1150, fx, (meta_predicate)).

meta_predicate((Head, More)) :- !, 
meta_predicate(Head) :-

meta_predicate1(Head) :-
        Head =.. [Name|Arguments], 
        member(Arg, Arguments), 
        module_expansion_argument(Arg), !, 
        functor(Head, Name, Arity), 
meta_predicate1(_).             % just a mode declaration

module_expansion_argument(N) :- integer(N).

The discussion above about the problems with the transparent mechanism show the two cases in which this simple transformation does not work.