{-# LANGUAGE TypeApplications, AllowAmbiguousTypes #-}

{- old-style, but without extensions #-}
import Data.Proxy

{-

data Proxy p = Proxy

-}

class Foo1 f where
  dofoo1 :: Proxy f -> IO ()

instance Foo1 Int where
  dofoo1 _ = print 0

instance Foo1 Double where
  dofoo1 _ = print 0.0

bar1 :: IO ()
bar1 = do
  dofoo1 (Proxy :: Proxy Int)
  dofoo1 (Proxy :: Proxy Double)


{- with -XTypeApplications, -XAllowAmbiguousTypes -}

class Foo2 f where
  dofoo2 :: IO ()

instance Foo2 Int where
  dofoo2 = print 1

instance Foo2 Double where
  dofoo2 = print 1.0

bar2 :: IO ()
bar2 = do
  dofoo2 @Int
  dofoo2 @Double


main :: IO ()
main = do
  bar1
  bar2