foldl' strictness

$ ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> :m + Data.List
Prelude Data.List> let f (acc, len) x = (acc + x, len + 1)
Prelude Data.List> let f' (acc, len) x = let acc' = acc + x; len' = len + 1 in acc' `seq` len' `seq` (acc', len')
Prelude Data.List> :set +s
Prelude Data.List> foldl' f (0, 0) [1..1000000]
(500000500000,1000000)
(0.95 secs, 355,322,552 bytes)
Prelude Data.List> foldl' f' (0, 0) [1..1000000]
(500000500000,1000000)
(0.32 secs, 184,080,976 bytes)