Instances.hs

-- A.hs

module A where

class Foo a where
  foo :: a -> Bool

data X = X Int

-- B.hs

module B where

import A

instance Foo X where
  foo (X _) = True

-- C.hs

{-# LANGUAGE TypeSynonymInstances #-}
module C where

import A

type Z = X

instance Foo Z where
  foo _ = False


-- Main.hs
import A
import B

-- or import C
-- dont import both B and C

main :: IO ()
main = do
  putStrLn $ show $ foo (X 5) -- True if B is imported; False if C is imported