/* Part of SWI-Prolog Author: Jan Wielemaker E-mail: J.Wielemaker@vu.nl WWW: http://www.swi-prolog.org Copyright (c) 2019-2023, VU University Amsterdam SWI-Prolog Solutions b.v. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ :- module(prolog_wrap, [ wrap_predicate/4, % :Head, +Name, -Wrapped, :Body unwrap_predicate/2, % :PI, ?Name current_predicate_wrapper/4 % :Head, -Name, -Wrapped, -Body ]). :- autoload(library(lists),[member/2]). :- autoload(library(pairs),[pairs_keys/2]). :- meta_predicate wrap_predicate(:, +, -, 0), % unwrap_predicate(:, ?), 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. % % 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. % == wrap_predicate(M:Head, WName, Wrapped, Body) :- '$wrap_predicate'(M:Head, WName, _Closure, Wrapped, Body). %! unwrap_predicate(:PI, ?Name) is semidet. % % Remove the outermost wrapper whose name unifies with Name. Fails if % no matching wrapper exists. %! current_predicate_wrapper(:Head, -Name, -Wrapped, -Body) is nondet. % % True if Head is wrapped with Body. The arguments are compatible with % wrap_predicate/4 such that the result may be used to re-create the % wrapper. % % Wrappers are enumerated starting with the first registered % (innermost) wrapper. current_predicate_wrapper(M:Head, Name, Wrapped, Body) :- '$wrapped_predicate'(M:Head, Pairs), Head =.. [_|Args], member(Name-CRef, Pairs), clause(M:WHead, Body0, CRef), WHead =.. [_|Args], body_closure(Body0, Wrapped, Body). body_closure(call(ClosureTerm), Wrapped, Wrapped) :- callable(ClosureTerm), functor(ClosureTerm, Closure, _Arity), blob(Closure, closure), !. body_closure(Body0, Wrapped, Body) :- compound(Body0), !, compound_name_arity(Body0, Name, Arity), compound_name_arity(Body, Name, Arity), body_closure_args(0, Arity, Body0, Wrapped, Body). body_closure(Body, _, Body). body_closure_args(I, Arity, Body0, Closure, Body) :- I < Arity, !, I2 is I+1, arg(I2, Body0, Arg0), arg(I2, Body, Arg), body_closure(Arg0, Closure, Arg), body_closure_args(I2, Arity, Body0, Closure, Body). body_closure_args(_, _, _, _, _). % Not for public docs! % '$syspreds':'$predicate_property'(?Property, +Value) is nondet. % % Extend predicate_property/2 to provide the `wrapped` property :- multifile '$syspreds':'$predicate_property'/2. '$syspreds':'$predicate_property'(wrapped(List), Pred) :- '$wrapped_predicate'(Pred, Pairs), pairs_keys(Pairs, List).