http://conscientiousprogrammer.com/blog/2014/08/07/understanding-cloure-transducers-through-types/

transducer.hs

-- Understanding Clojure transducers through types
-- http://conscientiousprogrammer.com/blog/2014/08/07/understanding-cloure-transducers-through-types/
{-# LANGUAGE RankNTypes #-}

type Reducer a r = r -> a -> r
--type Transducer a b = Reducer a r -> Reducer b r
type Transducer a b = forall r . Reducer a r -> Reducer b r

--class Foldable f where
--  fold :: Transducer a (f a)

class Conjable f where
  empty :: f a
  conj :: Reducer a (f a)

--instance Foldable [] where
--  fold = foldl

instance Conjable [] where
  empty = []
  conj xs x = xs ++ [x]

--mapping :: (a -> b) -> (r -> b -> r) -> (r -> a -> r)
mapping :: (a -> b) -> Transducer b a
mapping f xf r a = xf r (f a)

filtering :: (a -> Bool) -> Transducer a a
filtering p xf r a = if p a then xf r a
                            else r

flatmapping :: Foldable f => (a -> f b) -> Transducer b a
flatmapping f xf r a = foldl xf r (f a)


--conj xs x = xs ++ [x]
--xlist :: ((r -> a -> r) -> (r -> b -> r)) -> [b] -> [a]
xlist:: (Foldable f, Conjable f) => Transducer a b -> f b -> f a
xlist xf = foldl (xf conj) empty

xmap :: (Foldable f, Conjable f) => (a -> b) -> f a -> f b
--xmap :: (a -> b) -> [a] -> [b]
xmap f = xlist $ mapping f

xfilter :: (Foldable f, Conjable f) => (a -> Bool) -> f a -> f a
xfilter p = xlist $ filtering p

xform :: Transducer Int Int
xform = mapping (+ 1) . filtering even . flatmapping (\x -> [0..x])

munge :: (Foldable f, Conjable f) => f Int -> f Int
munge = xlist xform