contra.hs

-- Playing around with ContraFunctors
-- http://blog.ezyang.com/2010/07/flipping-arrows-in-coburger-king/

class ContraFunctor t where
    contramap :: (a -> b) -> t b -> t a

newtype ContraF a b = ContraF (b -> a)
instance ContraFunctor (ContraF a) where
    contramap g (ContraF f) = ContraF (f . g)

coreplicate :: a -> ContraF [a] Int
coreplicate x = ContraF (\n -> replicate n x)

apply :: ContraF a b -> b -> a
apply (ContraF f) = f


contraReplace :: b -> [a] -> [b]
contraReplace x = apply (contramap length (coreplicate x))

replace :: a -> [b] -> [a]
replace e xs = replicate (length xs) e