Jeff Polakow wrote:
[...] This can be easily fixed by defining a suitable strict sum:
sum' = foldl' (+) 0
and now sum' has constant space. We could try to redefine mean using sum':
mean1 xs = sum' xs / fromIntegral (length xs)
but this still gobbles up memory. The reason is that xs is used twice and cannot be discarded as it is generated. As an experiment I tried using "pointfree" to see if it would do something similar.
$ pointfree "\xs -> foldl' (+) 0 xs / fromIntegral (length xs)" ap ((/) . foldl' (+) 0) (fromIntegral . length)
But when I try this in GHCi 6.8.2 I get:
Prelude Data.List Control.Monad> let mean2 = ap ((/) . foldl' (+) 0) (fromIntegral . length)
<interactive>:1:12: No instance for (Monad ((->) [b])) arising from a use of `ap' at <interactive>:1:12-58 Possible fix: add an instance declaration for (Monad ((->) [b])) In the expression: ap ((/) . foldl' (+) 0) (fromIntegral . length) In the definition of `mean2': mean2 = ap ((/) . foldl' (+) 0) (fromIntegral . length)
Any ideas? Would the auto-generated pointfree version be any better if it could be made to work? Paul.