Cale Gibbard wrote:
On 01/11/05, Sebastian Sylvan <[EMAIL PROTECTED]> wrote:
On 11/1/05, Scherrer, Chad <[EMAIL PROTECTED]> wrote:
I was wondering... In my experience, it's worked much better to use
sum' = foldl' (+) 0
than the built-in "sum" function, which leaks memory like crazy for
large input lists. I'm guessing the built-in definition is
sum = foldr (+) 0
But as far as I know, (+) is always strict, so foldl' seems much more
natural to me. Is there a case where the build-in definition is
preferable?
The library definition in ghc actually uses foldl.
It's conceivable that you may not want (+) to be non-strict for
certain data types.
The question then becomes, is there a case where you want _sum_ to be
non-strict?
I'd argue that the likely answer is no, given that (+) is an operation
in an Abelian group and not a monoid. Regardless of whether (+) is
strict, when you compute a sum you will always have to consume the
entire list.
(+) is not an operation in an Abelian group. (+) is a function with
type a->a->a for some particular a. Haskell makes no mention about (+)
having any other properties.
Furthermore, ghc has a WRONG definition of sum. You cannot change a
foldr into a foldl, unless the operator is associative, and (+) does
not have to be.
-- Lennart
PS. I think additional properties of class methods would be great,
but since Haskell cannot enforce them we should not rely on them.
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