/*************************************************************************
*									 *
*	 YAP Prolog 							 *
*									 *
*	Yap Prolog was developed at NCCUP - Universidade do Porto	 *
*									 *
* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997	 *
*									 *
**************************************************************************
*									 *
* File:		grammar.pl						 *
* Last rev:								 *
* mods:									 *
* comments:	BNF grammar for Prolog					 *
*									 *
*************************************************************************/


/** 
@defgroup Grammars Grammar Rules
@ingroup YAPBuiltins
@{

Grammar rules in Prolog are both a convenient way to express definite
clause grammars and  an extension of the well known context-free grammars.

A grammar rule is of the form:

~~~~~
head --> body
~~~~~
where both \a head and \a body are sequences of one or more items
linked by the standard conjunction operator `,`.

<em>Items can be:</em>

+ 
a <em>non-terminal</em> symbol may be either a complex term or an atom.
+ 
a <em>terminal</em> symbol may be any Prolog symbol. Terminals are
written as Prolog lists.
+ 
an <em>empty body</em> is written as the empty list `[ ]`.
+ 
<em>extra conditions</em> may be inserted as Prolog procedure calls, by being
written inside curly brackets `{` and `}`.
+ 
the left side of a rule consists of a nonterminal and an optional list
of terminals.
+ 
alternatives may be stated in the right-hand side of the rule by using
the disjunction operator `;`.
+ 
the <em>cut</em> and <em>conditional</em> symbol (`->`) may be inserted in the 
right hand side of a grammar rule


Grammar related built-in predicates:

 
*/


:- system_module( '$_grammar', [!/2,
        (',')/4,
        (->)/4,
        ('.')/4,
        (;)/4,
        'C'/3,
        []/2,
        []/4,
        (\+)/3,
        phrase/2,
        phrase/3,
        {}/3,
        ('|')/4], ['$translate_rule'/2]).

:- use_system_module( '$_errors', ['$do_error'/2]).

% :- meta_predicate ^(?,0,?).
% ^(Xs, Goal, Xs) :- call(Goal).

% :- meta_predicate ^(?,1,?,?).
% ^(Xs0, Goal, Xs0, Xs) :- call(Goal, Xs).

/*
    Variables X in grammar rule bodies are translated as
    if phrase(X) had been written, where phrase/3 is obvious.
    Also, phrase/2-3 check their first argument.
*/

'$translate_rule'((LP-->RP), (NH:-B)) :-
	'$t_head'(LP, NH, NGs, S, SR, (LP-->RP)),
	 (var(NGs) ->
	     '$t_body'(RP, _, last, S, SR, B1)
	 ;
	     '$t_body'((RP,{NGs}), _, last, S, SR, B1)
	 ),
	'$t_tidy'(B1, B).


'$t_head'(V, _, _, _, _, G0) :- var(V), !,
	'$do_error'(instantiation_error,G0).
'$t_head'((H,List), NH, NGs, S, S1, G0) :- !,
	'$t_hgoal'(H, NH, S, SR, G0),
	'$t_hlist'(List, S1, SR, NGs, G0).
'$t_head'(H, NH, _, S, SR, G0) :-
	'$t_hgoal'(H, NH, S, SR, G0).
	
'$t_hgoal'(V, _, _, _, G0) :- var(V), !,
	'$do_error'(instantiation_error,G0).
'$t_hgoal'(M:H, M:NH, S, SR, G0) :- !,
	'$t_hgoal'(H, NH, S, SR, G0).
'$t_hgoal'(H, NH, S, SR, _) :-
	'$extend'([S,SR],H,NH).

'$t_hlist'(V, _, _, _, G0) :- var(V), !,
	'$do_error'(instantiation_error,G0).
'$t_hlist'([], _, _, true, _).
'$t_hlist'(String, S0, SR, SF, G0) :- string(String), !,
	string_codes( String, X ),
	'$t_hlist'( X, S0, SR, SF, G0).
'$t_hlist'([H], S0, SR, ('C'(SR,H,S0)), _) :- !.
'$t_hlist'([H|List], S0, SR, ('C'(SR,H,S1),G0), Goal) :- !,
	'$t_hlist'(List, S0, S1, G0, Goal).
'$t_hlist'(T, _, _, _, Goal) :-
	'$do_error'(type_error(list,T),Goal).


%
% Two extra variables:
% ToFill tells whether we need to explictly close the chain of
% variables.
% Last tells whether we are the ones who should close that chain.
%
'$t_body'(Var, filled_in, _, S, S1, phrase(Var,S,S1)) :-
	var(Var),
	!.
'$t_body'(!, to_fill, last, S, S1, (!, S1 = S)) :- !.
'$t_body'(!, _, _, S, S, !) :- !.
'$t_body'([], to_fill, last, S, S1, S1=S) :- !.
'$t_body'([], _, _, S, S, true) :- !.
'$t_body'(X, FilledIn, Last, S, SR, OS) :- string(X), !,
	string_codes( X, Codes),
	'$t_body'(Codes, FilledIn, Last, S, SR, OS).
'$t_body'([X], filled_in, _, S, SR, 'C'(S,X,SR)) :- !.
'$t_body'([X|R], filled_in, Last, S, SR, ('C'(S,X,SR1),RB)) :- !,
	'$t_body'(R, filled_in, Last, SR1, SR, RB).
'$t_body'({T}, to_fill, last, S, S1, (T, S1=S)) :- !.
'$t_body'({T}, _, _, S, S, T) :- !.
'$t_body'((T,R), ToFill, Last, S, SR, (Tt,Rt)) :- !,
	'$t_body'(T, ToFill, not_last, S, SR1, Tt),
	'$t_body'(R, ToFill, Last, SR1, SR, Rt).
'$t_body'((T->R), ToFill, Last, S, SR, (Tt->Rt)) :- !,
	'$t_body'(T, ToFill, not_last, S, SR1, Tt),
	'$t_body'(R, ToFill, Last, SR1, SR, Rt).
'$t_body'(\+T, ToFill, _, S, SR, (Tt->fail ; S=SR)) :- !,
	'$t_body'(T, ToFill, not_last, S, _, Tt).
'$t_body'((T;R), _ToFill, _, S, SR, (Tt;Rt)) :- !,
	'$t_body'(T, _, last, S, SR, Tt),
	'$t_body'(R, _, last, S, SR, Rt).
'$t_body'((T|R), _ToFill, _, S, SR, (Tt;Rt)) :- !,
       '$t_body'(T, _, last, S, SR, Tt),
       '$t_body'(R, _, last, S, SR, Rt).
'$t_body'(M:G, ToFill, Last, S, SR, M:NG) :- !,
	'$t_body'(G, ToFill, Last, S, SR, NG).
'$t_body'(T, filled_in, _, S, SR, Tt) :-
	'$extend'([S,SR], T, Tt).



'$extend'(More, OldT, NewT) :-
	OldT =.. OldL,
	lists:append(OldL, More, NewL),
	NewT =.. NewL.


'$t_tidy'(P,P) :- var(P), !.
'$t_tidy'((P1;P2), (Q1;Q2)) :- !,
	'$t_tidy'(P1, Q1),
	'$t_tidy'(P2, Q2).
'$t_tidy'((P1->P2), (Q1->Q2)) :- !,
	'$t_tidy'(P1, Q1),
	'$t_tidy'(P2, Q2).
'$t_tidy'(((P1,P2),P3), Q) :-
	'$t_tidy'((P1,(P2,P3)), Q).
'$t_tidy'((true,P1), Q1) :- !,
	'$t_tidy'(P1, Q1).
'$t_tidy'((P1,true), Q1) :- !,
	'$t_tidy'(P1, Q1).
'$t_tidy'((P1,P2), (Q1,Q2)) :- !,
	'$t_tidy'(P1, Q1),
	'$t_tidy'(P2, Q2).
'$t_tidy'(A, A).

/** @pred  `C`( _S1_, _T_, _S2_) 


This predicate is used by the grammar rules compiler and is defined as
`C`([H|T],H,T)`.




 */
'C'([X|S],X,S).


/** @pred  phrase(+ _P_, _L_)

This predicate succeeds when  _L_ is a phrase of type  _P_. The
same as `phrase(P,L,[])`.

Both this predicate and the previous are used as a convenient way to
start execution of grammar rules.

 
*/
phrase(PhraseDef, WordList) :-
	phrase(PhraseDef, WordList, []).

/** @pred  phrase(+ _P_, _L_, _R_) 


This predicate succeeds when the difference list ` _L_- _R_`
is a phrase of type  _P_.

 
*/
phrase(P, S0, S) :-
	call(P, S0, S).

!(S, S).

[](S, S).

[](H, T, S0, S) :- lists:append([H|T], S, S0).

'.'(H,T, S0, S) :-
	lists:append([H|T], S, S0).

{}(Goal, S0, S) :-
	Goal,
	S0 = S.

','(A,B, S0, S) :-
	 '$t_body'((A,B), _, last, S0, S, Goal),
	 '$execute'(Goal).

;(A,B, S0, S) :-
	 '$t_body'((A;B), _, last, S0, S, Goal),
	 '$execute'(Goal).

'|'(A,B, S0, S) :-
	 '$t_body'((A|B), _, last, S0, S, Goal),
	 '$execute'(Goal).

->(A,B, S0, S) :-
	 '$t_body'((A->B), _, last, S0, S, Goal),
	 '$execute'(Goal).

\+(A, S0, S) :-
	 '$t_body'(\+ A, _, last, S0, S, Goal),
	 '$execute'(Goal).

/**
@}
*/