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The principles behind the module system of SWI-Prolog differ in a
number of aspects from the Quintus Prolog module system.
- The SWI-Prolog module system allows the user to redefine system
predicates.
- All predicates that are available in the
system and user
modules are visible in all other modules as well.
- Quintus has the `meta_predicate/1'
declaration were SWI-Prolog has the
module_transparent/1
declaration.
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:
- Changing a meta_predicate declaration implies all predicates
calling the predicate need to be reloaded. This can
cause serious consistency problems.
- It does not help for dynamically defined predicates calling module
sensitive predicates.
- It slows down the compiler (at least in the SWI-Prolog
architecture).
- At least within the SWI-Prolog architecture the run-time overhead is
larger than the overhead introduced by the transparent mechanism.
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) (24).
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 (25).
The example in figure 5 gives the
solution.
<![CDATA[
:- module(findall, [findall/3]).
:- dynamic
solution/1.
:- module_transparent
findall/3,
store/2.
findall(Var, Goal, Bag) :-
assert(findall:solution('$mark')),
store(Var, Goal),
collect(Bag).
store(Var, Goal) :-
Goal, % refers to context module of
% caller of findall/3
assert(findall:solution(Var)),
fail.
store(_, _).
collect(Bag) :-
...,
]]>
Figure 5 : findall/3
using modules
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