small example that breaks GHC type safety by combining GADTs and TypeFamilies

gistfile1.txt

{-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving #-}

module Test where

newtype Age = Age Int
  deriving (Foo)

type family F a :: *
type instance F Int = Int
type instance F Age = Bool

class Foo a where
  foo :: a -> F a -> Double


instance Foo Int where
  foo _ = intToDouble
    where
      intToDouble :: Int -> Double
      intToDouble = fromIntegral

main :: IO ()
main = do
  putStrLn $ show $ foo (Age 1) True