Thu, 27 May 1999 09:16:43 +0400 (MSD), S.D.Mechveliani <[EMAIL PROTECTED]> pisze:
> And sm is the implementation of sum shown in src/.../PrelList.lhs
ghc/lib/std/PrelList.lhs from ghc-4.02 sources:
>>>>>>>>
-- sum and product compute the sum or product of a finite list of numbers.
{-# SPECIALISE sum :: [Int] -> Int #-}
{-# SPECIALISE product :: [Int] -> Int #-}
sum, product :: (Num a) => [a] -> a
#ifdef USE_REPORT_PRELUDE
sum = foldl (+) 0
product = foldl (*) 1
#else
sum l = sum' l 0
where
sum' [] a = a
sum' (x:xs) a = sum' xs (a+x)
product l = prod l 1
where
prod [] a = a
prod (x:xs) a = prod xs (a*x)
#endif
<<<<<<<<
So `foldl (+) 0' is only for the report as far as I understand it.
The real implementation is different.
But I'm not sure why it makes a difference. Definition of foldl is
similar to sum', only with a different argument order:
>>>>>>>>
foldl :: (a -> b -> a) -> a -> [b] -> a
foldl _ z [] = z
foldl f z (x:xs) = foldl f (f z x) xs
<<<<<<<<
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