Dar Akio, I just noticed that even with your approach, the code for foldl-as-foldr is not automatically beautiful. Consider this: I modified the eft function to do to some heavy work at each step (or at least to look like that): myEft :: Int -> Int -> [Int] myEft = \from to -> buildW (myEftFB from to) {-# INLINE myEft #-} expensive :: Int -> Int expensive = (1+) {-# NOINLINE expensive #-} myEftFB :: Int -> Int -> (Wrap f r) -> (Int -> r -> r) -> r -> r myEftFB from to (Wrap wrap unwrap) cons nil = wrap go from nil where go = unwrap $ \i rest -> if i <= to then cons i $ wrap go (expensive i) rest else rest {-# INLINE[0] myEftFB #-} Then I wanted to see if "sum [f..t]" using this code is good: sumUpTo :: Int -> Int -> Int sumUpTo f t = WW.foldl' (+) 0 (myEft f t) And this is the core I get for the inner loop: letrec { $wa :: GHC.Prim.Int# -> GHC.Types.Int -> GHC.Types.Int [LclId, Arity=1, Str=DmdType L] $wa = \ (ww2 :: GHC.Prim.Int#) -> case GHC.Prim.<=# ww2 ww1 of _ { GHC.Types.False -> GHC.Base.id @ GHC.Types.Int; GHC.Types.True -> let { e [Dmd=Just D(L)] :: GHC.Types.Int [LclId, Str=DmdType] e = F.expensive (GHC.Types.I# ww2) } in \ (acc :: GHC.Types.Int) -> case acc of _ { GHC.Types.I# x -> case e of _ { GHC.Types.I# ww3 -> $wa ww3 (GHC.Types.I# (GHC.Prim.+# x ww2)) } } }; } in $wa ww F.sumUpTo1 (GHC 7.6.3, -O). See how it is still building up partial applications. So I am a bit confused now: I thought the (or one) motivation for your proposal is to produce good code in these cases. Or am I using your code wrongly? Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org