Example of how to get intermediate results from a Fold

foldl.hs

import qualified Control.Foldl as Foldl

average :: Foldl.Fold Double Double
average = (/) <$> Foldl.sum <*> Foldl.genericLength

-- This could have been done more easily with the Foldl.scan function, but
-- I wanted to demonstrate how to access the intermediate states manually.
averages :: [Double] -> [Double]
averages xs
    | Foldl.Fold step s0 obs <- average 
    = obs <$> scanl step s0 xs

main = print $ averages [1,2,3,4,5]
-- [NaN,1.0,1.5,2.0,2.5,3.0]