We've had a big debate over
mean xs = foldl' (+) 0 xs / fromIntegral (length xs)
For anyone who didn't follow it, the problem is that "mean" needs to traverse its argument twice, so the entire list has to be held in memory. So if "xs = [1..1000000000]" then "mean xs" uses all your memory, but "foldl' (+) 0 xs" and "length xs" by themselves do not. The solution is for the programmer to rewrite "mean" to accumulate a pair containing the running total and count together, then do the division. This makes me wonder: could there be a compiler optimisation rule for this, collapsing two iterations over a list into one. I've never tried writing GHC rules, but something like:
f (foldl' g x) (foldl' h x) = (uncurry f) (foldl' (\i (gt,ht) -> (g i gt, h i ht)))
The first problem that occurs to me is the number of permutations of fold and map functions. Paul.