Am Donnerstag 11 März 2010 15:23:32 schrieb Yitzchak Gale:
TeXitoi wrote:
why is foldl used by Data.List for sum?
Daniel Fischer wrote:
Because Haskell is a non-strict language, and foldl' is strict -- someone might write a (legitimate) Num instance for a datatype such that foldl (+) 0 xs returns a good value, but foldl' (+) 0 xs gives ***Exception: Prelude.undefined for some lists xs.
It is possible to define such a Num instance, but it is extremely rare for anything like that to come up in practice.
Yes. And I'd expect foldr (+) 0 to be much more useful for lazy Num instances than foldl (+) 0. Nevertheless, one has to take that possibility into account.
However, with optimisations turned on... GHC knows that sum is actually strict
GHC does that when optimizations are turned on, but that behavior is not required by the Haskell standard. So there is no guarantee that any given compiler will produce usable output if you use foldl instead of foldl' for sum.
In GHCi sum is broken, because optimizations are not in effect there. You have to define your own version of sum using foldl' for every GHCi session (or put it in your .ghci file).
So it's a trade-off between a slight convenience in a bizarre corner case and general usability. I agree with Don that this is a bug in the Haskell 98 standard.
I'm not sure whether it's a wart or a bug, but I agree that it would be better to have the default sum strict (and provide lazysum for the cases where it's useful).
Regards, Yitz
Cheers, Daniel