To my
>> ghc-4.02 treats sum strangely.
>> In the below program, sum xs needs small, constant size stack,
>> sm xs needs stack proportional to length xs.
>> And sm is the implementation of sum shown in src/.../PrelList.lhs
>> I apply ghc -c -O.
>> [..]
Simon Peyton-Jones <[EMAIL PROTECTED]> replies
> foldl looks like this:
>
> foldl k z [] = z
> foldl k z (x:xs) = foldl k (z `k` x) xs
>
> To work in constant space, foldl needs to evaluate its second argument
> before the call, rather than building a thunk that is later forced
> (which in turn builds stack). But in general, foldl is not strict in z.
> [..]
> Alternatively, you can write
>
> foldl_strict k z [] = z
>
> [..]
Thank you. I see.
But it remains a puzzle: ghc-4.02-i386-unknown-linux
performs `sum' of Prelude as if it was NOT compiled from the sources
of distribution.
The source - src/.../PrelList.lhs says
sum = foldl (+) 0
{-# SPECIALISE sum :: [Int] -> Int #-}
Testing `sum' in ghc-4.02-i386-unknown-linux
shows that it needs *small stack* for sum [1..n] :: Int.
But when the user defines `sm' exactly as above `sum', then
sm [1..n] :: Int needs large stack!
I am sorry, if i confuse something, but could somebody repeat this
experiment, explain it?
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
