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hooks.lgt
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hooks.lgt
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% This file is part of Logtalk <https://logtalk.org/>
% SPDX-FileCopyrightText: 1998-2023 Paulo Moura <pmoura@logtalk.org>
% SPDX-License-Identifier: Apache-2.0
%
% Licensed under the Apache License, Version 2.0 (the "License");
% you may not use this file except in compliance with the License.
% You may obtain a copy of the License at
%
% http://www.apache.org/licenses/LICENSE-2.0
%
% Unless required by applicable law or agreed to in writing, software
% distributed under the License is distributed on an "AS IS" BASIS,
% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
% See the License for the specific language governing permissions and
% limitations under the License.
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% we start by defining three hook objects; each hook object defines a
% term_expansion/2 clause that expands a fact, a term_expansion/2 clause
% that adds a new clause to the expanded object, and a goal_expansion/2
% that rewrites a goal
:- object(ha,
% hook objects should always reference the "expanding" built-in protocol
implements(expanding)).
term_expansion(a, [aa]).
term_expansion((:- end_object), [ha, (:- end_object)]).
goal_expansion(start, first).
:- end_object.
:- object(hb,
implements(expanding)).
% term_expansion/2 clauses can return a list of terms
term_expansion(b, [bb]).
term_expansion((:- end_object), [hb, (:- end_object)]).
goal_expansion(first, second).
:- end_object.
:- object(hc,
implements(expanding)).
% term_expansion/2 clauses may also return a single term
term_expansion(c, cc).
term_expansion((:- end_object), [hc, (:- end_object)]).
goal_expansion(second, true).
:- end_object.
% we can also combine different hook objects by defining a new hook object
% whose term_expansion/2 and goal_expansion/2 clauses reference the
% term_expansion/2 and goal_expansion/2 clauses in other hook objects
:- object(hh,
implements(expanding)).
% the term-expansion mechanism tries term_expansion/2 clauses until
% it finds one that succeeds (or until all term_expansion/2 clauses
% are tried); thus more specific clauses should be listed before more
% general ones
% rename the object and set the context_switching_calls to ensure that the unit tests work
term_expansion((:- object(raw)), [(:- object(cooked)), (:- set_logtalk_flag(context_switching_calls,allow))]).
% the next term_expansion/2 clause returns the result of all hook
% expansions, assumed to be independent of each other, as the final
% expansion of a single term
term_expansion((:- end_object), Expansion) :-
ha::term_expansion((:- end_object), HA0),
list::append(HA, [(:- end_object)], HA0),
hb::term_expansion((:- end_object), HB0),
list::append(HB, [(:- end_object)], HB0),
hc::term_expansion((:- end_object), HC0),
list::append(HC, [(:- end_object)], HC0),
list::append([HA, HB, HC, [(:- end_object)]], Expansion).
% here we just try each individual hook object in succession but more
% elaborate combining schemes could be implemented if necessary
term_expansion(Term, Expansion) :-
ha::term_expansion(Term, Expansion).
term_expansion(Term, Expansion) :-
hb::term_expansion(Term, Expansion).
term_expansion(Term, Expansion) :-
hc::term_expansion(Term, Expansion).
% the following goal_expansion/2 clause defines a pipeline where
% the result of one hook expansion is feed into the next hook
goal_expansion(start, Expansion) :-
ha::goal_expansion(start, Expansion0),
hb::goal_expansion(Expansion0, Expansion1),
hc::goal_expansion(Expansion1, Expansion).
:- end_object.