Issue with recursion not hitting base case

Question

I'm working on the following DCG:

zero(1) --> 
    [0],
    !.
zero(N) -->
    {
        N < 1
    },
    [],
    !.
zero(N) -->
    zero(1),
    {
        M is N - 1
    },
    zero(M).

It works properly for positive test cases, eg

?- phrase(zero(5), A).
A = [0, 0, 0, 0, 0].

?- phrase(zero(2), [0,0]).
true.

But when I ran negative cases (typically something like phrase(zero(5), [0,0]).), it goes into oblivian. Curiously, during tracing, it seems whenever it goes to the zero(1) line in the third clause during recursion, it doesn't go to the basecase (first clause), instead jumps to the second one and fails because N = 1. Any guesses?

Did you try using `trace`? Also, try to rethink your DCG logic. The cuts indicate that there could be a better approach. — lurker, Dec 19 '14 at 22:05

score 1 · Accepted Answer · answered Dec 19 '14 at 22:09

I think your problem is over-specified, and you don't want the cuts. In particular, you don't need the "negative" case:

zero(N) -->
    {
        N < 1
    },
    [],
    !.

This will SUCCEED on what you consider to be a failure case.

Instead, try something a little simpler:

zero(1) --> [0].
zero(N) --> { N > 1, M is N - 1 }, [0], zero(M).

This defines only the positive cases. All other cases fail. So:

| ?- phrase(zero(5), A).

A = [0,0,0,0,0]

yes
| ?- phrase(zero(2), [0,0]).

yes
| ?- phrase(zero(5), [0,0]).

no
| ?-

Issue with recursion not hitting base case

1 Answers1