Re: trying to flatten a list

Anders Lindén wrote:
>
> whats the difference between
> not(A) :- A,!,fail;true.
> and
> not(A) :- !,A,fail;true.

With

not(A) :- A, !, fail ; true.

a goal not(P) will call P. If P succeeds, the cut
commits to this arm of the disjunction, and then
the clause fails.

If P fails, Prolog backtracks into the second arm
of the disjunction which succeeds.

With

not(A) :- !, A, fail ; true.

a goal not(P) starts by committing to the first
disjunct. Then either P succeeds and we go on
to fail, or P fails and so not(P) fails. That
is, this definition of not/1 will always fail!

-- Ralph

PS What is it with Prolog programmers and bad code
layout? Adding a bit of whitespace doesn't cost
that much extra effort...

.

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