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)  2010-2012, University of 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(block_directive,
   36	  [ (block)/1,			% +Heads
   37	    op(1150, fx, (block))
   38	  ]).   39:- use_module(library(prolog_wrap), [wrap_predicate/4]).   40
   41/** <module> Block: declare suspending predicates
   42
   43This module provides SICStus Prolog-compatible
   44=|:- block BlockSpec, ...|= declarations for delaying predicate calls if
   45certain arguments are unbound.
   46
   47@see	https://sicstus.sics.se/sicstus/docs/3.12.11/html/sicstus/Block-Declarations.html
   48*/
   49
   50:- op(1150, fx, user:(block)).   51
   52:- multifile
   53	user:term_expansion/2,
   54	block_declaration/2.		% Head, Module
   55
   56head(Var, _) :-
   57	var(Var), !, fail.
   58head((H:-_B), Head) :- !,
   59	head(H, Head).
   60head(H, Head) :-
   61	(   H = _:_
   62	->  Head = H
   63	;   prolog_load_context(module, M),
   64	    Head = M:H
   65	).
   66
   67
   68%%	block(+Heads).
   69%
   70%	Declare predicates to suspend on certain modes. The argument is,
   71%	like  meta_predicate/1,  a   comma-separated    list   of  modes
   72%	(_BlockSpecs_). Calls to the predicate is  suspended if at least
   73%	one of the conditions  implies  by   a  blockspec  evaluated  to
   74%	=true=. A blockspec  evaluated  to   =true=  iff  all  arguments
   75%	specified as `-' are unbound.
   76%
   77%	Multiple BlockSpecs for a single predicate  can appear in one or
   78%	more :- block declarations. The   predicate  is suspended untill
   79%	all mode patterns that apply to it are satisfied.
   80%
   81%	The implementation is realised by creating a wrapper that checks
   82%	the block conditions and either calls the original predicate
   83%	immediately (if none of the block conditions were true) or uses
   84%	attributed variables to delay re-evaluating the block condition
   85%	until any of the arguments in question are bound.
   86%
   87%	@compat SICStus Prolog
   88
   89block(Spec) :-
   90	throw(error(context_error(nodirective, block(Spec)), _)).
   91
   92expand_block_declaration(Spec, Clauses) :-
   93	prolog_load_context(module, Module),
   94	phrase(expand_specs(Spec, Module), Clauses).
   95
   96expand_specs(Var, _) -->
   97	{ var(Var), !,
   98	  instantiation_error(Var)
   99	}.
  100expand_specs(M:Spec, _) --> !,
  101	expand_specs(Spec, M).
  102expand_specs((A,B), Module) --> !,
  103	expand_specs(A, Module),
  104	expand_specs(B, Module).
  105expand_specs(Head, Module) -->
  106	{ valid_head(Head),
  107	  check_dynamic(Module:Head),
  108	  functor(Head, Name, Arity),
  109	  functor(GenHead, Name, Arity),
  110	  Clause = '$block_pred'(Head)
  111	},
  112	(   { current_predicate(Module:'$block_pred'/1) }
  113	->  []
  114	;   [ (:- discontiguous('$block_pred'/1)),
  115	      (:- public('$block_pred'/1))
  116	    ]
  117	),
  118	(   { prolog_load_context(module, Module) }
  119	->  [ Clause ]
  120	;   [ Module:Clause ]
  121	),
  122	[ block_directive:block_declaration(GenHead, Module) ].
  123
  124valid_head(Head) :-
  125	callable(Head),
  126	forall(arg(_, Head, A), block_arg(A)).
  127
  128check_dynamic(Head) :-
  129	(   predicate_property(Head, dynamic)
  130	;   predicate_property(Head, foreign)
  131	),
  132	permission_error(block, predicate, Head).
  133check_dynamic(_).
  134
  135block_arg(A) :-
  136	var(A), !,
  137	instantiation_error(A).
  138block_arg(-) :- !.
  139block_arg(+) :- !.
  140block_arg(?) :- !.
  141block_arg(A) :-
  142	domain_error(block_argument, A).
  143
  144%%	block_wrapper_clauses(+Module, +Head, -Clauses) is det.
  145%
  146%	Build a list of clauses that define a block wrapper around
  147%	predicate Head in Module. If a wrapper for this predicate has
  148%	already been defined, Clauses is an empty list.
  149
  150block_wrapper_clauses(Module, Head, Clauses) :-
  151	functor(Head, Name, Arity),
  152	atom_concat('$block_helper$', Name, HelperName),
  153	functor(HelperHead, HelperName, Arity),
  154	(   current_predicate(_, Module:HelperHead)
  155	->  Clauses = []
  156	;   findall(Wrapper, block_wrapper_clause(Module, Name, HelperHead, Wrapper), Clauses)
  157	).
  158
  159%%	block_wrapper_clause(+Module, +Name, +HelperHead, -Clause) is nondet.
  160%
  161%	Generate the clauses for the wrapper. The blockspecs are
  162%	translated into a helper predicate, where each clause checks one
  163%	block condition. If a block condition is true, attributes are
  164%	added to all arguments marked as `-`, so that once any of them
  165%	are bound, the predicate is called again and the block
  166%	conditions are re-evaluated. If no block condition was true,
  167%	the helper predicate fails.
  168%
  169%	Finally, an initialization clause is generated that sets up the
  170%	actual wrapper. This wrapper first calls the helper predicate
  171%	to check all block conditions and delay the call if necessary.
  172%	If the helper predicate fails (i. e. no block condition was
  173%	true), the wrapped predicate is called immediately.
  174%
  175%	The wrapper must be set up in an initialization clause and not
  176%	as part of the term expansion, because wrap_predicate/4 wrappers
  177%	are not retained in saved states, which would cause block
  178%	declarations to break when loading a saved state.
  179
  180block_wrapper_clause(Module, Name, HelperHead, (HelperHead :- GenBody)) :-
  181	HelperHead =.. [_|HelperArgs],
  182	length(HelperArgs, Arity),
  183	functor(BlockHead, Name, Arity),
  184	Module:'$block_pred'(BlockHead),
  185	BlockHead =.. [_|BlockArgs],
  186	find_args_to_block_on(BlockArgs, HelperArgs, ToBlockOn),
  187	args_to_var_conditions(ToBlockOn, GenBody, GenBody1),
  188	GenBody1 = (!, GenBody2),
  189	MainHead =.. [Name|HelperArgs],
  190	args_to_suspend_calls(ToBlockOn, _IsAlreadyUnblocked, Module:MainHead, GenBody2, true).
  191block_wrapper_clause(Module, Name, HelperHead, (:- initialization WrapCall)) :-
  192	HelperHead =.. [_|HelperArgs],
  193	ToWrapHead =.. [Name|HelperArgs],
  194	atom_concat('$block_wrapper$', Name, WrapperName),
  195	WrapCall = prolog_wrap:wrap_predicate(Module:ToWrapHead, WrapperName, Wrapped, (HelperHead -> true ; Wrapped)).
  196
  197%%	find_args_to_block_on(+BlockArgs, +HeadArgs, -ArgsToBlockOn) is semidet.
  198%
  199%	Collect into ArgsToBlockOn all arguments from HeadArgs for which
  200%	the corresponding argument in BlockArgs is `-`, indicating that
  201%	the argument is part of the block condition.
  202
  203find_args_to_block_on([], [], []) :- !.
  204find_args_to_block_on([-|MoreBlockArgs], [Arg|MoreHeadArgs], [Arg|MoreToBlockOn]) :-
  205	!,
  206	find_args_to_block_on(MoreBlockArgs, MoreHeadArgs, MoreToBlockOn).
  207find_args_to_block_on([_|MoreBlockArgs], [_|MoreHeadArgs], ToBlockOn) :-
  208	find_args_to_block_on(MoreBlockArgs, MoreHeadArgs, ToBlockOn).
  209
  210%%	args_to_var_conditions(+ArgsToBlockOn, -Conditions, ?ConditionsTail) is semidet.
  211%
  212%	Convert a list of arguments into a conjunction of var/1 checks
  213%	that succeeds if all arguments are unbound variables.
  214%
  215%	This effectively generates an unrolled version of
  216%	`maplist(var, ArgsToBlockOn), ConditionsTail`.
  217
  218args_to_var_conditions([], Tail, Tail) :- !.
  219args_to_var_conditions([Arg|MoreArgs], Conditions, Tail) :-
  220	Conditions = (var(Arg), MoreConditions),
  221	args_to_var_conditions(MoreArgs, MoreConditions, Tail).
  222
  223%%	args_to_suspend_calls(+ArgsToBlockOn, -IsAlreadyUnblocked, +BlockedGoal, -SuspendCalls, +Tail) is semidet.
  224%
  225%	Build a sequence of calls that delays BlockedGoal until any
  226%	variable in ArgsToBlockOn is bound. IsAlreadyUnblocked should be
  227%	an unbound fresh variable - it is passed directly to unblock/2,
  228%	which will bind the variable so that the same blocked goal is
  229%	not called again by another unblock/2 call from the same group.
  230
  231args_to_suspend_calls([], _, _, Tail, Tail) :- !.
  232args_to_suspend_calls([Arg|MoreArgs], IsAlreadyUnblocked, BlockedGoal, SuspendCalls, Tail) :-
  233	SuspendCalls = ('$suspend'(Arg, block_directive, block_directive:unblock(IsAlreadyUnblocked, BlockedGoal)), MoreSuspendCalls),
  234	args_to_suspend_calls(MoreArgs, IsAlreadyUnblocked, BlockedGoal, MoreSuspendCalls, Tail).
  235
  236
  237attr_unify_hook(call(ThisGoals), NewVar) :-
  238	var(NewVar),
  239	!,
  240	(   get_attr(NewVar, block_directive, call(OtherGoals))
  241	->  put_attr(NewVar, block_directive, call((ThisGoals, OtherGoals)))
  242	;   put_attr(NewVar, block_directive, call(ThisGoals))
  243	).
  244attr_unify_hook(call(Goals), _) :- Goals.
  245
  246:- public unblock/2.  247unblock(IsAlreadyUnblocked, _) :- IsAlreadyUnblocked == (-), !.
  248unblock(-, BlockedGoal) :- BlockedGoal.
  249
  250attribute_goals(Var) -->
  251	{get_attr(Var, block_directive, call(Goals))},
  252	!,
  253	render_block_goals(Goals).
  254
  255render_block_goals((Left, Right)) -->
  256	render_block_goals(Left),
  257	render_block_goals(Right).
  258render_block_goals(block_directive:unblock(IsAlreadyUnblocked, BlockedGoal)) -->
  259	(   {IsAlreadyUnblocked == (-)}
  260	->  []
  261	;   [BlockedGoal]
  262	).
  263
  264
  265%%	rename_clause(+Clause, +Prefix, -Renamed) is det.
  266%
  267%	Rename a clause by prefixing its old name wit h Prefix.
  268
  269rename_clause((Head :- Body), Prefix, (NewHead :- Body)) :- !,
  270        rename_clause(Head, Prefix, NewHead).
  271rename_clause(M:Head, Prefix, M:NewHead) :-
  272	rename_clause(Head, Prefix, NewHead).
  273rename_clause(Head, Prefix, NewHead) :-
  274        Head =.. [Name|Args],
  275        atom_concat(Prefix, Name, WrapName),
  276        NewHead =.. [WrapName|Args].
  277
  278
  279		 /*******************************
  280		 *	  EXPANSION HOOKS	*
  281		 *******************************/
  282
  283system:term_expansion((:- block(Spec)), Clauses) :-
  284	expand_block_declaration(Spec, Clauses).
  285system:term_expansion(Term, Clauses) :-
  286	head(Term, Module:Head),
  287	block_declaration(Head, Module),
  288	block_wrapper_clauses(Module, Head, WrapperClauses),
  289	append(WrapperClauses, [Term], Clauses)