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yap-6.3/Logtalk/examples/metapredicates/metapredicates.lgt

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% example adopted from:
% Programming Language Prolog Part 2, Modules
% Committee Draft - January 14, 1998 X3J17/97/5
:- object(tracer).
:- info([
version is 2.0,
author is 'Paulo Moura',
date is 2000/7/24,
comment is 'Tracer for a goal call, exit, and fail ports.']).
:- public(trace/1).
:- metapredicate(trace(::)). % changes interpretation of meta-calls on trace/1 clauses
:- mode(trace(+callable), zero_or_more).
:- info(trace/1, [
comment is 'Traces goal execution.',
argnames is ['Goal']]).
trace(Goal) :-
write('call: '), writeq(Goal), nl,
call(Goal), % Goal is called in the context of the object sending the message trace/1
write('exit: '), writeq(Goal), nl.
trace(Goal) :-
write('fail: '), writeq(Goal), nl,
fail.
:- end_object.
% sort code adopted from an example on the SICStus Prolog User Manual
% metapredicate example taken from Prolog Part 2, Modules - Committee Draft
:- object(sort(_Type)).
:- info([
version is 1.0,
author is 'Paulo Moura',
date is 2000/7/24,
comment is 'List sorting parameterized by the type of the list elements.']).
:- uses(list, [append/3]). % calls to append(...) will be translated to list::append(...)
:- uses(tracer, [trace/1]). % calls to trace(...) will be translated to tracer::trace(...)
:- calls(comparingp).
:- public(sort/2).
:- mode(sort(+list, -list), one).
:- info(sort/2, [
comment is 'Sorts a list in ascending order (quicksort algorithm).',
argnames is ['List', 'Sorted']]).
:- private(partition/4).
:- mode(partition(+list, +nonvar, -list, -list), one).
:- info(partition/4, [
comment is 'Partition a list in two lists containing the elements smaller and larger than a pivot.',
argnames is ['List', 'Pivot', 'Small', 'Large']]).
sort([], []).
sort([Head| Tail], Sorted) :-
trace(partition(Tail, Head, Small, Large)),
trace(sort(Small, Sorted1)),
trace(sort(Large, Sorted2)),
append(Sorted1, [Head| Sorted2], Sorted).
partition([], _, [], []).
partition([Head| Tail], Pivot, Small, Large) :-
parameter(1, Type),
( Type::(Head < Pivot) ->
Small = [Head| Small1], Large = Large1
; Small = Small1, Large = [Head| Large1]
),
partition(Tail, Pivot, Small1, Large1).
:- end_object.