term_expansion masks static predicate when file is loaded with "yap -l"

[Brian DeVries]

Howdy all,

I'm trying to write a lexer generator for Yap. I have a special term
type of the form @rule := <stuff>, and I am currently expanding this
(via `term_expansion`) into a clause of the predicate `match` for each
rule. I also have a few static `match` clauses in the same file. I
have a "Client" file that uses the lexer and loads it via `consult`.

The problem comes when loading the Client file. If I load it via
`consult` after starting the interpreter, everything works fine. If I
pre-load it from the command-line via `yap -l`, then the static
`match` clauses are no longer present. This behavior doesn't make
sense to me, so I'm thinking it's a bug, or a misunderstanding of some
nuance between command-line loading and `consult`.

Here's a stripped down example that demonstrates the behavior):

File: lexrule_simple.pl
===========================================
:- source.
% Define the operators
:- op(100, fx, '@').                % Indicates a lexer rule.
:- op(1200, xfx, ':=').                % Lexer rule neck

user:term_expansion( (@H := B), Expanded) :-
    NewPred = (
           match(H, H - L) -->
               (B, {format('Calling expanded rule: ~w~n', [H])})
          ),
    expand_term(NewPred, Expanded).

match(_, _ - C) -->
    [C].
===========================================



File: use_lexrule_simple.pl
===========================================
:- source.
:- consult(disappearing_match).

@p := bar.
bar --> [a].
===========================================


If the file is loaded via

===========================================
    ?- consult(use_lexrule_simple).
    ?- listing(match).
match(_,_-C,A,B) :-
   'C'(A,C,B).
match(p,p-_,A,B) :-
   bar(A,B),
   format('Calling expanded rule: ~w~n',user:[p]).
yes
    ?- match(X, X - T, [a], []).
T = a ? ;
Calling expanded rule: p
X = p ? ;
no
===========================================


If it is loaded via yap -l use_lexrule_simple.pl:

===========================================
   ?- listing(match).
match(p,p-_,A,B) :-
   bar(A,B),
   format('Calling expanded rule: ~w~n',user:[p]).
yes
   ?- match(X, X - T, [a], []).
Calling expanded rule: p
X = p ? ;
no
===========================================



Any ideas, suggestions? I could well be missing something, but the
documentation doesn't indicate that `yap -l` is any different from
consulting the file after the interpreter has been started, so I can't
see why Yap would behave in this way.


As a side question: I'm writing this lexer generator because searches
for such a system have turned up nothing and I'd like to wrap all of
the lexing logic (reading the file, tokenizing, etc.) into one
package. Is there a better approach, or is there an existing system
that I'm not aware of?


Thanks!
~Brian W. DeVries

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