Re: GADTs and functional dependencies

[Chris Kuklewicz]

You cannot create a normal function "fun".  You can make a type class function

fun :: Class a b => GADT a -> b

data GADT a where
     GADT :: GADT ()
     GADT2 :: GADT String

-- fun1 :: GADT () -> () -- infers type
fun1 g = case g of
           (GADT :: GADT ()) -> ()

-- fun2 :: GADT String -> Bool -- infers type
fun2 g = case g of
           (GADT2 :: GADT String) -> True

-- "fun" cannot type check.  The type of 'g' cannot be both "GADT ()" and "GADT 
String"
-- This is because "fun" is not a member of type class.

{- fun g = case g of

           (GADT :: GADT ()) -> ()
           (GADT2 :: GADT String) -> True
-}

class Class a b | a -> b where
  fun :: GADT a -> b

instance Class () () where
  fun GADT = ()

instance Class String Bool where
  fun GADT2 = True

main = print $ (fun GADT, fun GADT2) == ( (), True )

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