On Wed, 04 Feb 2004 23:00:50 +0000
Graham Klyne <[EMAIL PROTECTED]> wrote:
> I'm getting a "C stack overflow" error from Hugs when processing a
> moderately large dataset, but not doing anything that looks unreasonably
> complex or involved.
<snip>
>
> I think the function graphLabels1 is set to call itself recursively to a
> depth of a little over 1000. Can this kind of recursion blow the Hugs stack?
"foldl f x xs" generally requires #xs (the number of elements of xs) stacks to be
evaluated.
Note that ls(the second argument of graphLabels1) is not evaluated until
> graphLabels1 [] ls = ls
.
> foldl (flip addSetElem) ls (arcLabels t)
ls is also defined by a foldl and its ls is also ...
So, let gs = [g_1, g_2, ..., g_N],
you need at least \sum_{n = 1}^N #g_n stacks. Is'n it large?
The simplest and safest solution is to change foldl to foldl', but it produces
the unnecessary evaluation of `elem`s in addSetElem's.
I think that
> import DeepSeq
> {- or
> f $!! x = force x `seq` f x
> where force [] = ()
> force (x:xs) = x `seq` force xs
> -}
> ...
> graphLabels1 (t:gs) ls = graphLabels1 gs $!! foldl (flip addSetElem) ls (arcLabels t
is better in your case.
Sorry for my poor English and hope it helps,
Koji Nakahara
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