jeff.polakow:
Hello,
For example, the natural and naive way to write Andrew's "mean" function doesn't involve tuples at all: simply tail recurse with two accumulator parameters, and compute the mean at the end. GHC's strictness analyser does the right thing with this, so there's no need for seq, $!, or the like. It's about 3 lines of code.
Is this the code you mean?
meanNat = go 0 0 where go s n [] = s / n go s n (x:xs) = go (s+x) (n+1) xs If so, bang patterns are still required bang patterns in ghc-6.8.2 to run in constant memory:
meanNat = go 0 0 where go s n [] = s / n go !s !n (x:xs) = go (s+x) (n+1) xs
Is there some other way to write it so that ghc will essentially insert the bangs for me?
Yes, give a type annotation, constraining 'n' to Int. meanNat :: [Double] -> Double meanNat = go 0 0 where go :: Double -> Int -> [Double] -> Double go s n [] = s / fromIntegral n go s n (x:xs) = go (s+x) (n+1) xs And you get this loop: M.$wgo :: Double# -> Int# -> [Double] -> Double# M.$wgo = \ (ww_smN :: Double#) (ww1_smR :: Int#) (w_smT :: [Double]) -> case w_smT of wild_B1 { [] -> /## ww_smN (int2Double# ww1_smR); : x_a9k xs_a9l -> case x_a9k of wild1_am7 { D# y_am9 -> M.$wgo (+## ww_smN y_am9) (+# ww1_smR 1) xs_a9l } } Without the annotation you get: M.$wgo :: Double# -> Integer -> [Double] -> Double GHC sees through the strictness of I#. -- Don