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) 2019-2020, 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:- autoload(library(lists),[member/2]). 41:- autoload(library(pairs),[pairs_keys/2]). 42 43 44:- meta_predicate 45 wrap_predicate(, , , ), 46% unwrap_predicate(:, ?), 47 current_predicate_wrapper(, , , ). 48 49/** <module> Wrapping predicates 50 51This library allows adding _wrappers_ to predicates. The notion of 52wrappers is known in various languages under several names. For example 53Logtalk knows these as _before methods_ or _after methods_ and Python 54has _decorators_. A SWI-Prolog wrapper is a _body term_ that normally 55calls the original wrapped definition somewhere. 56*/ 57 58%! wrap_predicate(:Head, +Name, -Wrapped, +Body) is det. 59% 60% Wrap the predicate referenced by Head using Body. Subsequent calls 61% to Head call the given Body term. Body may call the original 62% definition through Wrapped. Wrapped is a term of the shape below, 63% where _Closure_ is an opaque _blob_. 64% 65% call(Closure(A1, ...)) 66% 67% Name names the wrapper for inspection using predicate_property/2 or 68% deletion using unwrap_predicate/2. If Head has a wrapper with Name 69% the Body of the existing wrapper is updated without changing the 70% order of the registered wrappers. The same predicate may be wrapped 71% multiple times. Multiple wrappers are executed starting with the 72% last registered (outermost). 73% 74% The predicate referenced by Head does not need to be defined at the 75% moment the wrapper is installed. If Head is undefined, the predicate 76% is created instead of _searched for_ using e.g., the auto loader. 77% 78% Registered wrappers __are not part of saved states__ (see 79% qsave_program/2) and thus need to be re-registered, for example 80% using initialization/1. 81% 82% An example of using wrap_predicate/4 for computing GCD: 83% == 84% :- wrap_predicate(gcd(A,B,Gcd), gcd_wrap, W, gcd_wrap(W, A, B, Gcd)). 85% 86% gcd(X, Y, Gcd), X < Y => gcd(X, Y-X, Gcd). 87% gcd(X, Y, Gcd), X > Y => gcd(Y, X-Y, Gcd). 88% gcd(X, _, Gcd) => Gcd = X. 89% 90% gcd_wrap(call(Closure), X, Y, Gcd) :- 91% functor(Closure, ClosureBlob, 3), 92% X_eval is X, 93% Y_eval is Y, 94% call(ClosureBlob, X_eval, Y_eval, Gcd). 95% == 96% or (less efficient): 97% == 98% gcd_wrap(call(Closure), X, Y, Gcd) :- 99% functor(Closure, ClosureBlob, 3), 100% call(ClosureBlob, X_eval, Y_eval, G), 101% Gcd is G. 102% == 103 104wrap_predicate(M:Head, WName, Wrapped, Body) :- 105 '$wrap_predicate'(M:Head, WName, _Closure, Wrapped, Body). 106 107%! unwrap_predicate(:PI, ?Name) is semidet. 108% 109% Remove the outermost wrapper whose name unifies with Name. Fails if 110% no matching wrapper exists. 111 112%! current_predicate_wrapper(:Head, -Name, -Wrapped, -Body) is nondet. 113% 114% True if Head is wrapped with Body. The arguments are compatible with 115% wrap_predicate/4 such that the result may be used to re-create the 116% wrapper. 117% 118% Wrappers are enumerated starting with the first registered 119% (innermost) wrapper. 120 121current_predicate_wrapper(M:Head, Name, Wrapped, Body) :- 122 '$wrapped_predicate'(M:Head, Pairs), 123 Head =.. [_|Args], 124 member(Name-CRef, Pairs), 125 clause(M:, Body0, CRef), 126 WHead =.. [_|Args], 127 body_closure(Body0, Wrapped, Body). 128 129body_closure(call(ClosureTerm), Wrapped, Wrapped) :- 130 callable(ClosureTerm), 131 functor(ClosureTerm, Closure, _Arity), 132 blob(Closure, closure), 133 !. 134body_closure(Body0, Wrapped, Body) :- 135 compound(Body0), 136 !, 137 compound_name_arity(Body0, Name, Arity), 138 compound_name_arity(Body, Name, Arity), 139 body_closure_args(0, Arity, Body0, Wrapped, Body). 140body_closure(Body, _, Body). 141 142body_closure_args(I, Arity, Body0, Closure, Body) :- 143 I < Arity, 144 !, 145 I2 is I+1, 146 arg(I2, Body0, Arg0), 147 arg(I2, Body, Arg), 148 body_closure(Arg0, Closure, Arg), 149 body_closure_args(I2, Arity, Body0, Closure, Body). 150body_closure_args(_, _, _, _, _). 151 152 153% Not for public docs! 154% '$syspreds':'$predicate_property'(?Property, +Value) is nondet. 155% 156% Extend predicate_property/2 to provide the `wrapped` property 157 158:- multifile 159 '$syspreds':'$predicate_property'/2. 160 161'$syspreds''$predicate_property'(wrapped(List), Pred) :- 162 '$wrapped_predicate'(Pred, Pairs), 163 pairs_keys(Pairs, List)