1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2019-2023, VU University Amsterdam
    7                              SWI-Prolog Solutions b.v.
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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   31    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   32    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   33    POSSIBILITY OF SUCH DAMAGE.
   34*/
   35
   36:- module(prolog_wrap,
   37          [ wrap_predicate/4,                   % :Head, +Name, -Wrapped, :Body
   38            unwrap_predicate/2,                 % :PI, ?Name
   39            current_predicate_wrapper/4		% :Head, -Name, -Wrapped, -Body
   40          ]).   41:- autoload(library(lists),[member/2]).   42:- autoload(library(pairs),[pairs_keys/2]).   43
   44
   45:- meta_predicate
   46    wrap_predicate(:, +, -, 0),
   47%   unwrap_predicate(:, ?),
   48    current_predicate_wrapper(:, -, -, -).

 wrap_predicate(:Head, +Name, -Wrapped, +Body) is det

Wrap the predicate referenced by Head using Body. Subsequent calls to Head call the given Body term. Body may call the original definition through Wrapped. Wrapped is a term of the shape below, where Closure is an opaque blob.

call(Closure(A1, ...))

Name names the wrapper for inspection using predicate_property/2 or deletion using unwrap_predicate/2. If Head has a wrapper with Name the Body of the existing wrapper is updated without changing the order of the registered wrappers. The same predicate may be wrapped multiple times. Multiple wrappers are executed starting with the last registered (outermost).

The predicate referenced by Head does not need to be defined at the moment the wrapper is installed. If Head is undefined, the predicate is created instead of searched for using e.g., the auto loader.

Registered wrappers are not part of saved states (see qsave_program/2) and thus need to be re-registered, for example using initialization/1.

An example of using wrap_predicate/4 for computing GCD:

    :- wrap_predicate(gcd(A,B,Gcd), gcd_wrap, W, gcd_wrap(W, A, B, Gcd)).

    gcd(X, Y, Gcd), X < Y => gcd(X, Y-X, Gcd).
    gcd(X, Y, Gcd), X > Y => gcd(Y, X-Y, Gcd).
    gcd(X, _, Gcd) => Gcd = X.

    gcd_wrap(call(Closure), X, Y, Gcd) :-
        functor(Closure, ClosureBlob, 3),
        X_eval is X,
        Y_eval is Y,
        call(ClosureBlob, X_eval, Y_eval, Gcd).

or (less efficient):

    gcd_wrap(call(Closure), X, Y, Gcd) :-
        functor(Closure, ClosureBlob, 3),
        call(ClosureBlob, X_eval, Y_eval, G),
        Gcd is G.

  105wrap_predicate(M:Head, WName, Wrapped, Body) :-
  106    '$wrap_predicate'(M:Head, WName, _Closure, Wrapped, Body).

 unwrap_predicate(:PI, ?Name) is semidet

Remove the outermost wrapper whose name unifies with Name. Fails if no matching wrapper exists.

 current_predicate_wrapper(:Head, -Name, -Wrapped, -Body) is nondet

True if Head is wrapped with Body. The arguments are compatible with wrap_predicate/4 such that the result may be used to re-create the wrapper.

Wrappers are enumerated starting with the first registered (innermost) wrapper.

  122current_predicate_wrapper(M:Head, Name, Wrapped, Body) :-
  123    '$wrapped_predicate'(M:Head, Pairs),
  124    Head =.. [_|Args],
  125    member(Name-CRef, Pairs),
  126    clause(M:WHead, Body0, CRef),
  127    WHead =.. [_|Args],
  128    body_closure(Body0, Wrapped, Body).
  129
  130body_closure(call(ClosureTerm), Wrapped, Wrapped) :-
  131    callable(ClosureTerm),
  132    functor(ClosureTerm, Closure, _Arity),
  133    blob(Closure, closure),
  134    !.
  135body_closure(Body0, Wrapped, Body) :-
  136    compound(Body0),
  137    !,
  138    compound_name_arity(Body0, Name, Arity),
  139    compound_name_arity(Body,  Name, Arity),
  140    body_closure_args(0, Arity, Body0, Wrapped, Body).
  141body_closure(Body, _, Body).
  142
  143body_closure_args(I, Arity, Body0, Closure, Body) :-
  144    I < Arity,
  145    !,
  146    I2 is I+1,
  147    arg(I2, Body0, Arg0),
  148    arg(I2, Body, Arg),
  149    body_closure(Arg0, Closure, Arg),
  150    body_closure_args(I2, Arity, Body0, Closure, Body).
  151body_closure_args(_, _, _, _, _).
  152
  153
  154%   Not for public docs!
  155%   '$syspreds':'$predicate_property'(?Property, +Value) is nondet.
  156%
  157%   Extend predicate_property/2 to provide the `wrapped` property
  158
  159:- multifile
  160    '$syspreds':'$predicate_property'/2.  161
  162'$syspreds':'$predicate_property'(wrapped(List), Pred) :-
  163    '$wrapped_predicate'(Pred, Pairs),
  164    pairs_keys(Pairs, List)