Find out if a value matches a certain constructor.

matchConstructor.hs

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
import Data.Data
import Unsafe.Coerce

type family Last (a :: *) :: * where
  Last (b -> c) = Last c
  Last d = d

full :: (Typeable a, Data (Last a)) => a -> Last a
full x = case typeRepArgs (typeOf x) of
           []  -> error "Impossible"
           xs  -> apps (length xs - 1) x
  where apps 0 t = unsafeCoerce t
        apps n t = apps (n - 1) ((unsafeCoerce t) undefined)

valueIsConstructedBy :: (Typeable a, Data (Last a)) => a -> Last a -> Bool
valueIsConstructedBy x y = toConstr (full x) == toConstr y

main :: IO ()
main = do
  print $ filter (valueIsConstructedBy Just) ([Just 4, Nothing, Just 5] :: [Maybe Int]) 
  print $ filter (valueIsConstructedBy (:)) ([[1,2],[],[3,4],[],[5]] :: [[Int]])