/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@uva.nl
    WWW:           http://www.swi-prolog.org
    Copyright (C): 2010, University of Amsterdam

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

    As a special exception, if you link this library with other files,
    compiled with a Free Software compiler, to produce an executable, this
    library does not by itself cause the resulting executable to be covered
    by the GNU General Public License. This exception does not however
    invalidate any other reasons why the executable file might be covered by
    the GNU General Public License.
*/

:- module(block_directive,
	  [ (block)/1,			% +Heads
	    op(1150, fx, (block))
	  ]).

/** <module> Block: declare suspending predicates

This  module  realises  SICStus  Prolog   =|:-  block  BlockSpec,  ...|=
declarations  using  a   wrapper   predicate    that   calls   the  real
implementation through a coroutining primitive   (typically  when/2, but
freeze/2 for simple cases).

@tbd	This emulation is barely tested.
*/

:- 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 uses
%	when/2 to realize suspension of the renamed predicate.
%
%	@compat SICStus Prolog
%	@compat If the predicate is blocked on multiple conditions, it
%		will not unblock before _all_ conditions are satisfied.
%		SICStus unblocks when one arbitrary condition is
%		satisfied.
%	@bug	It is not possible to block on a dynamic predicate
%		because we cannot wrap assert/1.  Likewise, we cannot
%		block foreign predicates, although it would be easier
%		to support this.

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)) ]
	),
	(   { 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).

%%	wrap_block(+Head, +Term, -Clauses) is det.
%
%	Create a wrapper. The first clause deal   with the case where we
%	already created the wrapper. The second  creates the wrapper and
%	the first clause.

wrap_block(Pred, Term, Clause) :-
	current_predicate(_, Pred), !,
	rename_clause(Term, 'block ', Clause).
wrap_block(Pred, Term, [Wrapper,FirstClause]) :-
	block_declarations(Pred, Modes),
	Pred = _:Head,
	functor(Head, Name, Arity),
	length(Args, Arity),
	GenHead =.. [Name|Args],
	atom_concat('block ', Name, WrappedName),
	WrappedHead =.. [WrappedName|Args],
	when_cond(Modes, Args, Cond),
	simplify_coroute(when(Cond, WrappedHead), Coroute),
	Wrapper = (GenHead :- Coroute),
	rename_clause(Term, 'block ', FirstClause).

block_declarations(M:P, Modes) :-
	functor(P, Name, Arity),
	functor(H, Name, Arity),
	findall(H, M:'$block_pred'(H), Modes).

when_cond([Head], Args, Cond) :- !,
	one_cond(Args, Head, Cond).
when_cond([H|T], Args, (C1,C2)) :-
	one_cond(Args, H, C1),
	when_cond(T, Args, C2).

one_cond(Vars, Spec, Cond) :-
	cond_vars(Vars, 1, Spec, CondVars),
	nonvar_or(CondVars, Cond).

cond_vars([], _, _, []).
cond_vars([H|T0], I, Spec, L) :-
	(   arg(I, Spec, -)
	->  L = [H|T]
	;   L = T
	),
	I2 is I + 1,
	cond_vars(T0, I2, Spec, T).

nonvar_or([V], nonvar(V)).
nonvar_or([V|T], (nonvar(V);C)) :-
	nonvar_or(T, C).

simplify_coroute(when(nonvar(X), C), freeze(X, C)).
simplify_coroute(Coroute, Coroute).


%%	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, Wrapper) :-
	head(Term, Module:Head),
	block_declaration(Head, Module),
	wrap_block(Module:Head, Term, Wrapper).

