/************************************************************************* * * * 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 builtins @{ 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 `,`. Items can be: + a non-terminal symbol may be either a complex term or an atom. + a terminal symbol may be any Prolog symbol. Terminals are written as Prolog lists. + an empty body is written as the empty list `[ ]`. + extra conditions 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 cut and conditional symbol (`->`) may be inserted in the right hand side of a grammar rule Grammar related built-in predicates: */ :- module( '$_grammar', [!/2, (',')/4, (->)/4, ('.')/4, (;)/4, 'C'/3, []/2, []/4, (\+)/3, phrase/2, phrase/3, {}/3, ('|')/4]). :- use_system_module( '$_errors', ['$do_error'/2]). % :- meta_predicate ^(?,0,?). % ^(Xs, Goal, Xs) :- call(Goal). % :- meta_predicate ^(?,1,?,?). % ^(Xs0, Goal, Xs0, Xs) :- call(Goal, Xs). /* Variables X in grammar rule bodies are translated as if phrase(X) had been written, where phrase/3 is obvious. Also, phrase/2-3 check their first argument. */ prolog:'$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)`. */ prolog:'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. */ prolog: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_. */ prolog:phrase(P, S0, S) :- call(P, S0, S). prolog:!(S, S). prolog:[](S, S). prolog:[](H, T, S0, S) :- lists:append([H|T], S, S0). prolog:'.'(H,T, S0, S) :- lists:append([H|T], S, S0). prolog:{}(Goal, S0, S) :- Goal, S0 = S. prolog:','(A,B, S0, S) :- t_body((A,B), _, last, S0, S, Goal), '$execute'(Goal). prolog:';'(A,B, S0, S) :- t_body((A;B), _, last, S0, S, Goal), '$execute'(Goal). prolog:('|'(A,B, S0, S)) :- t_body((A|B), _, last, S0, S, Goal), '$execute'(Goal). prolog:'->'(A,B, S0, S) :- t_body((A->B), _, last, S0, S, Goal), '$execute'(Goal). prolog:'\\+'(A, S0, S) :- t_body(\+ A, _, last, S0, S, Goal), '$execute'(Goal). /** @} */