Re: [PATCH] Fix `split-at-predicate' in `scm/lily-library.scm'.

[Neil Puttock]

2009/11/10 Thomas <address@hidden>:
> Don't make the assumption that if `(PRED previous_element element)'
> is false, `(PRED element previous_element)' must be true.
>
> Suppose we want to split `(1 2 3 5 7)' into two lists such that
> the first list contains the initial consecutive integers `(1 2 3)'
> and the second contains the remaining elements `(5 7)'.  We expect
> `(split-at-predicate (lambda (x y) (= (- y x) 1)) '(1 2 3 5 7))'
> to return `((1 2 3) . (5 7))', but in fact it returns `((1 2 3 5 7))'
> because of this erroneous assumption.
> ---
>  scm/lily-library.scm |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
>
> diff --git a/scm/lily-library.scm b/scm/lily-library.scm
> index adb2bdf..1624288 100644
> --- a/scm/lily-library.scm
> +++ b/scm/lily-library.scm
> @@ -343,7 +343,9 @@ found."
>   Example: (split-at-predicate < '(1 2 3 2 1)) ==> ((1 2 3) . (2 1))"
>   (if (null? lst)
>       (list lst)
> -      (let ((i (list-index pred (cdr lst) lst)))
> +      (let ((i (list-index (lambda (x y) (not (pred x y)))
> +                          lst
> +                          (cdr lst))))
>         (if i
>             (cons (take lst (1+ i)) (drop lst (1+ i)))
>             (list lst)))))
> --
> 1.6.0.4

Thanks for fixing this regression, Thomas.  I guess we might have
noticed it earlier if the example in the docstring hadn't been
changed. :)

Regards,
Neil