handle error in prolog

Question

/**************************
Assignment Statment Module
**************************/
assgnStmnt(Clist) :-
        Clist = [H|T], id(H), conc(Exp, [';'|AnotherStmnt], T), Exp = [Op|Rem], Op = '=', expr(Rem), assgnStmnt(AnotherStmnt),!
        ;
        Clist = [], !.

This code is to parse assignment statement in complier. But i need to output to user where's the syntax error in his code.

like : if he entered : x = x + 1, I want to output that expected ';' not found.

How can i do that ???

Answer 1

Technically depends from your grammar, but if we assume you already know where error productions will go, you could code this way:

assgnStmnt(Clist) :-
    Clist = [H|T],
    id(H),
    conc(Exp, [StmntSep|AnotherStmnt], T),
    Exp = [Op|Rem],
    Op = '=',
    expr(Rem),
    expected(StmntSep, ';'), % the right position depends on grammar
    assgnStmnt(AnotherStmnt),
    !
    ; Clist = [].

expected(Token, Token) :- !.
expected(Wrong, Token) :-
    format('expected ~w, found ~w~n', [Token, Wrong]),
    fail. % fail here depends on error recovery strategy

test:

?- assgnStmnt([x,=,x+1,';',x,=,x+1,';']).
true.

?- assgnStmnt([x,=,x+1,';',x,=,x+1]).
expected ;, found x
expected ;, found =
expected ;, found x+1
false.