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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) (23).
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 (24).
The example in figure 4 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 4 : findall/3
using modules
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).
<![CDATA[
:- op(1150, fx, (meta_predicate)).
meta_predicate((Head, More)) :- !,
meta_predicate1(Head),
meta_predicate(More).
meta_predicate(Head) :-
meta_predicate1(Head).
meta_predicate1(Head) :-
Head =.. [Name|Arguments],
member(Arg, Arguments),
module_expansion_argument(Arg), !,
functor(Head, Name, Arity),
module_transparent(Name/Arity).
meta_predicate1(_). % just a mode declaration
module_expansion_argument(:).
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.