/* Part of SWI-Prolog Author: Jan Wielemaker E-mail: J.Wielemaker@vu.nl WWW: http://www.swi-prolog.org Copyright (c) 2010-2012, University of Amsterdam 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(block_directive, [ (block)/1, % +Heads op(1150, fx, (block)) ]). :- use_module(library(prolog_wrap), [wrap_predicate/4]). /** Block: declare suspending predicates This module provides SICStus Prolog-compatible =|:- block BlockSpec, ...|= declarations for delaying predicate calls if certain arguments are unbound. @see https://sicstus.sics.se/sicstus/docs/3.12.11/html/sicstus/Block-Declarations.html */ :- op(1150, fx, user:(block)). :- multifile user:term_expansion/2, block_declaration/2. % Head, Module head(Var, _) :- var(Var), !, fail. head((H:-_B), Head) :- !, head(H, Head). head(H, Head) :- ( H = _:_ -> Head = H ; prolog_load_context(module, M), Head = M:H ). %% block(+Heads). % % Declare predicates to suspend on certain modes. The argument is, % like meta_predicate/1, a comma-separated list of modes % (_BlockSpecs_). Calls to the predicate is suspended if at least % one of the conditions implies by a blockspec evaluated to % =true=. A blockspec evaluated to =true= iff all arguments % specified as `-' are unbound. % % Multiple BlockSpecs for a single predicate can appear in one or % more :- block declarations. The predicate is suspended untill % all mode patterns that apply to it are satisfied. % % The implementation is realised by creating a wrapper that checks % the block conditions and either calls the original predicate % immediately (if none of the block conditions were true) or uses % attributed variables to delay re-evaluating the block condition % until any of the arguments in question are bound. % % @compat SICStus Prolog block(Spec) :- throw(error(context_error(nodirective, block(Spec)), _)). expand_block_declaration(Spec, Clauses) :- prolog_load_context(module, Module), phrase(expand_specs(Spec, Module), Clauses). expand_specs(Var, _) --> { var(Var), !, instantiation_error(Var) }. expand_specs(M:Spec, _) --> !, expand_specs(Spec, M). expand_specs((A,B), Module) --> !, expand_specs(A, Module), expand_specs(B, Module). expand_specs(Head, Module) --> { valid_head(Head), check_dynamic(Module:Head), functor(Head, Name, Arity), functor(GenHead, Name, Arity), Clause = '$block_pred'(Head) }, ( { current_predicate(Module:'$block_pred'/1) } -> [] ; [ (:- discontiguous('$block_pred'/1)), (:- public('$block_pred'/1)) ] ), ( { prolog_load_context(module, Module) } -> [ Clause ] ; [ Module:Clause ] ), [ block_directive:block_declaration(GenHead, Module) ]. valid_head(Head) :- callable(Head), forall(arg(_, Head, A), block_arg(A)). check_dynamic(Head) :- ( predicate_property(Head, dynamic) ; predicate_property(Head, foreign) ), permission_error(block, predicate, Head). check_dynamic(_). block_arg(A) :- var(A), !, instantiation_error(A). block_arg(-) :- !. block_arg(+) :- !. block_arg(?) :- !. block_arg(A) :- domain_error(block_argument, A). %% block_wrapper_clauses(+Module, +Head, -Clauses) is det. % % Build a list of clauses that define a block wrapper around % predicate Head in Module. If a wrapper for this predicate has % already been defined, Clauses is an empty list. block_wrapper_clauses(Module, Head, Clauses) :- functor(Head, Name, Arity), atom_concat('$block_helper$', Name, HelperName), functor(HelperHead, HelperName, Arity), ( current_predicate(_, Module:HelperHead) -> Clauses = [] ; findall(Wrapper, block_wrapper_clause(Module, Name, HelperHead, Wrapper), Clauses) ). %% block_wrapper_clause(+Module, +Name, +HelperHead, -Clause) is nondet. % % Generate the clauses for the wrapper. The blockspecs are % translated into a helper predicate, where each clause checks one % block condition. If a block condition is true, attributes are % added to all arguments marked as `-`, so that once any of them % are bound, the predicate is called again and the block % conditions are re-evaluated. If no block condition was true, % the helper predicate fails. % % Finally, an initialization clause is generated that sets up the % actual wrapper. This wrapper first calls the helper predicate % to check all block conditions and delay the call if necessary. % If the helper predicate fails (i. e. no block condition was % true), the wrapped predicate is called immediately. % % The wrapper must be set up in an initialization clause and not % as part of the term expansion, because wrap_predicate/4 wrappers % are not retained in saved states, which would cause block % declarations to break when loading a saved state. block_wrapper_clause(Module, Name, HelperHead, (HelperHead :- GenBody)) :- HelperHead =.. [_|HelperArgs], length(HelperArgs, Arity), functor(BlockHead, Name, Arity), Module:'$block_pred'(BlockHead), BlockHead =.. [_|BlockArgs], find_args_to_block_on(BlockArgs, HelperArgs, ToBlockOn), args_to_var_conditions(ToBlockOn, GenBody, GenBody1), GenBody1 = (!, GenBody2), MainHead =.. [Name|HelperArgs], args_to_suspend_calls(ToBlockOn, _IsAlreadyUnblocked, Module:MainHead, GenBody2, true). block_wrapper_clause(Module, Name, HelperHead, (:- initialization WrapCall)) :- HelperHead =.. [_|HelperArgs], ToWrapHead =.. [Name|HelperArgs], atom_concat('$block_wrapper$', Name, WrapperName), WrapCall = prolog_wrap:wrap_predicate(Module:ToWrapHead, WrapperName, Wrapped, (HelperHead -> true ; Wrapped)). %% find_args_to_block_on(+BlockArgs, +HeadArgs, -ArgsToBlockOn) is semidet. % % Collect into ArgsToBlockOn all arguments from HeadArgs for which % the corresponding argument in BlockArgs is `-`, indicating that % the argument is part of the block condition. find_args_to_block_on([], [], []) :- !. find_args_to_block_on([-|MoreBlockArgs], [Arg|MoreHeadArgs], [Arg|MoreToBlockOn]) :- !, find_args_to_block_on(MoreBlockArgs, MoreHeadArgs, MoreToBlockOn). find_args_to_block_on([_|MoreBlockArgs], [_|MoreHeadArgs], ToBlockOn) :- find_args_to_block_on(MoreBlockArgs, MoreHeadArgs, ToBlockOn). %% args_to_var_conditions(+ArgsToBlockOn, -Conditions, ?ConditionsTail) is semidet. % % Convert a list of arguments into a conjunction of var/1 checks % that succeeds if all arguments are unbound variables. % % This effectively generates an unrolled version of % `maplist(var, ArgsToBlockOn), ConditionsTail`. args_to_var_conditions([], Tail, Tail) :- !. args_to_var_conditions([Arg|MoreArgs], Conditions, Tail) :- Conditions = (var(Arg), MoreConditions), args_to_var_conditions(MoreArgs, MoreConditions, Tail). %% args_to_suspend_calls(+ArgsToBlockOn, -IsAlreadyUnblocked, +BlockedGoal, -SuspendCalls, +Tail) is semidet. % % Build a sequence of calls that delays BlockedGoal until any % variable in ArgsToBlockOn is bound. IsAlreadyUnblocked should be % an unbound fresh variable - it is passed directly to unblock/2, % which will bind the variable so that the same blocked goal is % not called again by another unblock/2 call from the same group. args_to_suspend_calls([], _, _, Tail, Tail) :- !. args_to_suspend_calls([Arg|MoreArgs], IsAlreadyUnblocked, BlockedGoal, SuspendCalls, Tail) :- SuspendCalls = ('$suspend'(Arg, block_directive, block_directive:unblock(IsAlreadyUnblocked, BlockedGoal)), MoreSuspendCalls), args_to_suspend_calls(MoreArgs, IsAlreadyUnblocked, BlockedGoal, MoreSuspendCalls, Tail). attr_unify_hook(call(ThisGoals), NewVar) :- var(NewVar), !, ( get_attr(NewVar, block_directive, call(OtherGoals)) -> put_attr(NewVar, block_directive, call((ThisGoals, OtherGoals))) ; put_attr(NewVar, block_directive, call(ThisGoals)) ). attr_unify_hook(call(Goals), _) :- Goals. :- public unblock/2. unblock(IsAlreadyUnblocked, _) :- IsAlreadyUnblocked == (-), !. unblock(-, BlockedGoal) :- BlockedGoal. attribute_goals(Var) --> {get_attr(Var, block_directive, call(Goals))}, !, render_block_goals(Goals). render_block_goals((Left, Right)) --> render_block_goals(Left), render_block_goals(Right). render_block_goals(block_directive:unblock(IsAlreadyUnblocked, BlockedGoal)) --> ( {IsAlreadyUnblocked == (-)} -> [] ; [BlockedGoal] ). %% rename_clause(+Clause, +Prefix, -Renamed) is det. % % Rename a clause by prefixing its old name wit h Prefix. rename_clause((Head :- Body), Prefix, (NewHead :- Body)) :- !, rename_clause(Head, Prefix, NewHead). rename_clause(M:Head, Prefix, M:NewHead) :- rename_clause(Head, Prefix, NewHead). rename_clause(Head, Prefix, NewHead) :- Head =.. [Name|Args], atom_concat(Prefix, Name, WrapName), NewHead =.. [WrapName|Args]. /******************************* * EXPANSION HOOKS * *******************************/ system:term_expansion((:- block(Spec)), Clauses) :- expand_block_declaration(Spec, Clauses). system:term_expansion(Term, Clauses) :- head(Term, Module:Head), block_declaration(Head, Module), block_wrapper_clauses(Module, Head, WrapperClauses), append(WrapperClauses, [Term], Clauses).