{-# LANGUAGE FlexibleContexts, KindSignatures, TypeFamilies #-}
import Data.Proxy

class IsFun a where
    isFunProxy :: (Proxy a) -> Bool

data Fun

data NotFun

instance IsFun Fun where
    isFunProxy _ = True

instance IsFun NotFun where
    isFunProxy _ = False

type family IsFun' (a :: *) where
    IsFun' (a -> b) = Fun
    IsFun'  a       = NotFun

proxy :: a -> Proxy (IsFun' a)
proxy _ = Proxy

isFun :: (IsFun (IsFun' a)) => a -> Bool
isFun a = isFunProxy $ proxy a

f1 :: Int -> Int
f1 = id

f2 :: Int -> Int -> Int
f2 = const

v :: Int
v = 0

main = do
    print $ isFun f1
    print $ isFun f2
    print $ isFun v