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]).

   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	).

 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.

Compatibility

- SICStus Prolog

   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).

 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.

  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	).

 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.

  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)).

 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.

  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).

 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.

  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).

 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.

  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	).

 rename_clause(+Clause, +Prefix, -Renamed) is det

Rename a clause by prefixing its old name wit h Prefix.

  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)