====== Parser abc ======
{{tag>parser}}
===== Description =====
A Prolog program parsing the language a*b*c

**Source**:  The Art of Prolog
===== Download =====
Program source code: {{parser_abc.pl}}
===== Listing =====
<code prolog>

:- op(40,xfx,\).

        s(As\Xs) :-  a(As\Bs), b(Bs\Cs), c(Cs\Xs).

        a(Xs\Ys) :-  connect([a],Xs\Xs1), a(Xs1\Ys).
        a(Xs\Ys) :-  connect([ ],Xs\Ys).

        b(Xs\Ys) :-  connect([b],Xs\Xs1), b(Xs1\Ys).
        b(Xs\Ys) :-  connect([ ],Xs\Ys).

        c(Xs\Ys) :-  connect([c],Xs\Xs1), c(Xs1\Ys).
        c(Xs\Ys) :-  connect([ ],Xs\Ys).

        connect([ ],Xs\Xs).
        connect([W|Ws],[W|Xs]\Ys) :-  connect(Ws,Xs\Ys).

%     Program 18.8: A Prolog program parsing the language a*b*c
</code>
===== Comments =====