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    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, VU University Amsterdam
    7    All rights reserved.
    8
    9    Redistribution and use in source and binary forms, with or without
   10    modification, are permitted provided that the following conditions
   11    are met:
   12
   13    1. Redistributions of source code must retain the above copyright
   14       notice, this list of conditions and the following disclaimer.
   15
   16    2. Redistributions in binary form must reproduce the above copyright
   17       notice, this list of conditions and the following disclaimer in
   18       the documentation and/or other materials provided with the
   19       distribution.
   20
   21    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   22    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   23    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   24    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   25    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   26    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   27    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   28    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   29    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   30    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   31    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   32    POSSIBILITY OF SUCH DAMAGE.
   33*/
   34
   35:- module(prolog_wrap,
   36          [ wrap_predicate/4,                   % :Head, +Name, -Wrapped, +Body
   37            unwrap_predicate/2,                 % :PI, ?Name
   38            current_predicate_wrapper/4		% :Head, -Name, -Wrapped, -Body
   39          ]).   40:- use_module(library(error)).   41:- use_module(library(apply)).   42:- use_module(library(lists)).   43
   44:- meta_predicate
   45    wrap_predicate(:, +, -, +),
   46%   unwrap_predicate(:, ?),
   47    current_predicate_wrapper(:, -, -, -).

Wrapping predicates

This library allows adding wrappers to predicates. The notion of wrappers is known in various languages under several names. For example Logtalk knows these as before methods or after methods and Python has decorators. A SWI-Prolog wrapper is a body term that normally calls the original wrapped definition somewhere. */

 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.

   82wrap_predicate(M:Head, WName, Wrapped, Body) :-
   83    '$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.

   99current_predicate_wrapper(M:Head, Name, Wrapped, Body) :-
  100    '$wrapped_predicate'(M:Head, Pairs),
  101    Head =.. [_|Args],
  102    member(Name-CRef, Pairs),
  103    clause(M:WHead, Body0, CRef),
  104    WHead =.. [_|Args],
  105    body_closure(Body0, Wrapped, Body).
  106
  107body_closure(call(ClosureTerm), Wrapped, Wrapped) :-
  108    callable(ClosureTerm),
  109    functor(ClosureTerm, Closure, _Arity),
  110    blob(Closure, closure),
  111    !.
  112body_closure(Body0, Wrapped, Body) :-
  113    compound(Body0),
  114    !,
  115    compound_name_arity(Body0, Name, Arity),
  116    compound_name_arity(Body,  Name, Arity),
  117    body_closure_args(0, Arity, Body0, Wrapped, Body).
  118body_closure(Body, _, Body).
  119
  120body_closure_args(I, Arity, Body0, Closure, Body) :-
  121    I < Arity,
  122    !,
  123    I2 is I+1,
  124    arg(I2, Body0, Arg0),
  125    arg(I2, Body, Arg),
  126    body_closure(Arg0, Closure, Arg),
  127    body_closure_args(I2, Arity, Body0, Closure, Body).
  128body_closure_args(_, _, _, _, _).
 $syspreds:$predicate_property(?Property, +Value) is nondet
Extend predicate_property/2 to provide the wrapped property
  135:- multifile
  136    '$syspreds':'$predicate_property'/2.  137
  138'$syspreds':'$predicate_property'(wrapped(List), Pred) :-
  139    '$wrapped_predicate'(Pred, Pairs),
  140    pairs_keys(Pairs, List)