Composing applicative functors.

gistfile1.hs

import Control.Monad.Trans.List
import Control.Monad.Trans.Writer
import Control.Applicative
import Data.Functor.Compose
import Data.Monoid

tell' :: Monoid a => a -> Compose [] (Writer a) ()
tell' = Compose . pure . tell

list :: Monoid a => [x] -> Compose [] (Writer a) x
list = Compose . map pure

foo :: Compose [] (Writer String) Int
foo = tell' "a" *>
      list [3,4] <**>
      Compose [pure succ,pure pred] <*
      Compose [tell "c", tell "d"]

fim :: [Writer String Int]
fim = getCompose $ foo

fum :: [(Int,String)]
fum = map runWriter fim

main = putStrLn . show $ fum

-- *Main> [2,3] <* [(),()]
-- [2,2,3,3]
-- Cool!