问题
Below I have a Definite Clause Grammar that should accept the string aabccc, however when I tested the grammar, the only string I was able to get accepted was abc.
I'm pretty sure I've gotten rid of left-hand recursion, so I'm not sure what's going wrong.
s --> x, y, z.
x --> [a], x.
x --> [a].
y --> [b], y.
y --> [b].
z --> [c], z.
z --> [c].
Also, would I be able to define the above grammar as...
s --> x, y, z.
x --> [a], x; [a].
y --> [b], y; [b].
z --> [c], z; [c].
回答1:
Both version of your grammar work as expected:
?- phrase(s, [a,a,b,b,c,c,c], R).
R = [] .
?- phrase(s, [a,a,b,b,c,c,c]).
true .
Maybe the issue was that you're trying to call it not with e.g. a [a,a,b,b,c,c,c] list of tokens but with a aabccc atom? If that's the case, you can use the standard atom_chars/2 built-in predicate to convert an atom into a list of characters:
?- atom_chars(aabccc, Tokens).
Tokens = [a, a, b, c, c, c].
来源：https://stackoverflow.com/questions/26871678/definite-clause-grammar-prolog