Is it function??

IsFun.hs

{-# LANGUAGE DataKinds, TypeOperators, KindSignatures, TypeFamilies #-}
{-# LANGUAGE UndecidableInstances, FlexibleInstances, FlexibleContexts #-}

module IsFun (
    Proxy
  , isFun
  , cntArgs
) where

import Data.Proxy
import GHC.TypeLits

type family CntArgs (a :: *) :: Nat where
    CntArgs (a -> b) = 1 + CntArgs b
    CntArgs  a       = 0

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

cntArgs :: KnownNat (CntArgs a) => a -> Integer
cntArgs x = natVal $ proxy' x

isFun :: KnownNat (CntArgs a) => a -> Bool
isFun x = cntArgs x > 0 

f1 :: Int -> Int 
f1 = id

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

v :: Int 
v = 0 

main = do
    print $ isFun f1 -- a -> a の a が曖昧で数えられない
    print $ isFun f2
    print $ isFun v