Re: [Haskell-cafe] Lists concatenation being O(n)

Wow, I don't get core haskell, but I get you point. It's indeed odd foldl' doesn't use foldr (and sum doesn't use foldl' instead of foldl as (+) is strict (*)) if foldr permits loop fusion. (*) Anyway, is there a place where foldl is preferable over foldl' ? Never happened to me, I always use right-folding if I want lazy evaluation, to benefit from guarded recursion. 2011/10/14 Bas van Dijk

On 13 October 2011 20:53, Albert Y. C. Lai wrote:

...

The number of new cons cells created in due course is Θ(length xs).

I was actually surprised by this because I expected: length(xs++ys) to fuse into one efficient loop which doesn't create cons cells at all.

Unfortunately, I was mistaken since length is defined recursively.

length :: [a] -> Int length l = len l 0# where len :: [a] -> Int# -> Int len [] a# = I# a# len (_:xs) a# = len xs (a# +# 1#)

However, if we would define it as:

length = foldl' (l _ -> l+1) 0

And implemented foldl' using foldr as described here:

http://www.haskell.org/pipermail/libraries/2011-October/016895.html

then fuse = length(xs++ys) where for example xs = replicate 1000000 1 and ys = replicate 5000 (1::Int) would compile to the following totally fused core:

fuse :: Int fuse = case $wxs 1000000 0 of ww_srS { __DEFAULT -> I# ww_srS }

$wxs :: Int# -> Int# -> Int# $wxs = \ (w_srL :: Int#) (ww_srO :: Int#) -> case <=# w_srL 1 of _ { False -> $wxs (-# w_srL 1) (+# ww_srO 1); True -> $wxs1_rs8 5000 (+# ww_srO 1) }

$wxs1_rs8 :: Int# -> Int# -> Int# $wxs1_rs8 = \ (w_srA :: Int#) (ww_srD :: Int#) -> case <=# w_srA 1 of _ { False -> $wxs1_rs8 (-# w_srA 1) (+# ww_srD 1); True -> +# ww_srD 1 }

Bas

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