[Haskell-cafe] Fuse functions given to map and filter without RULES pragma

[Roman Beslik]

I've found a funny way to fuse functions given to map and filter without
RULES pragma:

toSelect = (,) Just
fromSelect = uncurry mapMaybe
transformFirst f (x, y) = (f x, y)
maybe f = transformFirst (\g x -> g x >>= f)
map f = maybe (Just . f)
filter f = maybe (\x -> if f x then Just x else Nothing)

test :: [Int]
test = fromSelect (map (*2) (filter (<10) (map (+3) (toSelect
[1,3,4,6,8]))))

Everything looks like obvious application of filter and map, but GHC inline
(*2), (<10), (+3) (with -O flag) :^).
--
Best regards, RB.