apply_macros.pl -- Goal expansion rules to avoid meta-calling

This module defines goal_expansion/2 rules to deal with commonly used, but fundamentally slow meta-predicates. Notable maplist/2... defines a useful set of predicates, but its execution is considerable slower than a traditional Prolog loop. Using this library calls to maplist/2... are translated into an call to a generated auxiliary predicate that is compiled using compile_aux_clauses/1. Currently this module supports:

• maplist/2..
• forall/2
• once/1
• ignore/1
• phrase/2
• phrase/3
• call_dcg/2
• call_dcg/3

The idea for this library originates from ECLiPSe and came to SWI-Prolog through YAP.

author

- Jan Wielemaker

To be done

- Support more predicates

 expand_maplist(+Callable, +Lists, -Goal) is det[private]

Macro expansion for maplist/2 and higher arity. The first clause deals with code using maplist on fixed lists to reduce typing. Note that we only expand if all lists have fixed length. In theory we only need at least one of fixed length, but in that case the goal expansion instantiates variables in the clause, causing issues with the remainder of the clause expansion mechanism.

 maplist_extend_goal(+Closure, +Args, -Goal) is semidet[private]

Extend the maplist Closure with Args. This can be tricky. Notably library(yall) lambda expressions may instantiate the Closure while the real execution does not. We can solve that by using lambda_calls/3. The expand_goal_no_instantiate/2 ensures safe goal expansion.

 expand_closure_no_fail(+Goal, +Extra:integer, -GoalExt) is det[private]

Add Extra additional arguments to Goal.

 expand_apply(+GoalIn:callable, -GoalOut) is semidet[private]

Macro expansion for `apply' predicates.

 expand_apply(+GoalIn:callable, -GoalOut, +PosIn, -PosOut) is semidet[private]

Translation of simple meta calls to inline code while maintaining position information. Note that once(Goal) cannot be translated to (Goal->true) because this will break the compilation of (once(X) ; Y). A correct translation is to (Goal->true;fail). Abramo Bagnara suggested ((Goal->true),true), which is both faster and avoids warning if style_check(+var_branches) is used.

 expand_phrase(+PhraseGoal, -Goal) is semidet

 expand_phrase(+PhraseGoal, +Pos0, -Goal, -Pos) is semidet

Provide goal-expansion for PhraseGoal. PhraseGoal is either phrase/2,3 or call_dcg/2,3. The current version does not translate control structures, but only simple terminals and non-terminals.

For example:

?- expand_phrase(phrase(("ab", rule)), List), Goal).
Goal = (List=[97, 98|_G121], rule(_G121, [])).

throws

- Re-throws errors from dcg_translate_rule/2

 dcg_extend(+Callable, +Pos0, -Goal, -Pos, +Xs0, ?Xs) is semidet[private]

 qcall_instantiated(@Term) is semidet[private]

True if Term is instantiated sufficiently to call it.

To be done

- Shouldn't this be callable straight away?

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 expand_phrase(Arg1, Arg2, Arg3, Arg4)