Re: [Haskell-cafe] curious about sum

Daniel Fischer wrote:

Am Samstag 13 Juni 2009 17:00:36 schrieb Jochem Berndsen:

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2009/6/13 Jochem Berndsen :

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Keith Sheppard wrote:

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Is there any reason that sum isn't strict? I can't think of any case where that is a good thing.

Prelude> sum [0 .. 1000000] *** Exception: stack overflow It is useful if the (+) is nonstrict; although I cannot think of any useful mathematical structure where (+) would be nonstrict. I remember needing a non-strict sum at least once, but I do not remember the exact application. But imagine having a (very) long list of numbers and you want to do A if the sum exceeds a small number, otherwise B.

if sum [0..100000] > 10 then A else B

However, this idea didn't work, because of strictness. You can only do such things if you know that all entries of your list are nonnegative. That requires a custom solution anyway (not to mention

Deniz Dogan wrote: the fact that to determine whether x > 10 or not, we need to explicitly compute x).

Well, if you have lazy Peano numbers of any kind, you know that all entries are non- negative and you needn't evaluate x fully to determine whether it's > 10. Isn't that the point why one would use lazy numbers at all?

Yes. (That's what I meant with 'custom solution', using Peano numbers instead of Ints or Integers.) Regards, -- Jochem Berndsen | jochem@functor.nl GPG: 0xE6FABFAB