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).
That would be really inconsistent with the way the rest of the Haskell language and libraries works. Note that foldl' has a ' to indicate that it's not the same as foldl exactly. I would propose that sum' exist as well as sum, and that sum be lazy. Part of what's interesting about Haskell is the default non-strict evaluation of the language, and even if it's hard for newcomers to get used to it, they're better off having a consistent space to work in rather than all these exceptions to make algorithms like "sum" work more efficiently when the problem was one of documentation and education about the language. Of course if you want a strict functional language there's plenty of those out there. Dave
Regards, Yitz
Cheers, Daniel
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