On Fri, Mar 02, 2007 at 05:58:56AM -0800, Malcolm Wallace wrote:
Thu Mar 1 08:54:37 PST 2007 Malcolm.Wallace@cs.york.ac.uk * Prelude sum and product should really use foldl' not foldl.
Not according to the Haskell 98 report: http://haskell.org/onlinereport/standard-prelude.html#$vsum The difference is shown up by: ------------------------------------------------- import Data.List data Foo = Zero | A | B | C deriving (Eq, Show) instance Num Foo where _ + Zero = error "Adding Zero" _ + A = error "Adding A" _ + B = error "Adding B" _ + C = C fromInteger 0 = Zero xs = [A, B, C] f = foldl (+) 0 xs f' = foldl' (+) 0 xs s = sum xs ------------------------------------------------- ghc (6.6 and HEAD) agrees with the report: *Main> f C *Main> f' *** Exception: Adding A *Main> s C *Main> while hugs (September 2006) disagrees: Main> f C Main> f' Program error: Adding A Main> s Program error: Adding A Main> I guess a decision should be made for Haskell', probably either going with foldl' or introducing sum' and product'. Thanks Ian