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 <[email protected]> > On 13 October 2011 20:53, Albert Y. C. Lai <[email protected]> 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 > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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