Stefan O'Rear wrote:
This code causes GHC to incorrectly fail - the case *is* reachable. (I invented this technique in an attempt to directly access the internal System FC newtype coercion; it promised until a few minutes ago to solve all the EnumMap performance concerns.)
stefan@stefans:/tmp$ cat A.lhs
{-# OPTIONS_GHC -fglasgow-exts #-}
data IsIntT x where IsIntT :: IsIntT Int
class IsIntC a where isInt :: IsIntT a instance IsIntC Int where isInt = IsIntT
newtype Foo = Foo Int deriving(IsIntC)
x :: IsIntT Foo -> Int x IsIntT = (Foo 2) + 2
IsIntT Foo is a concrete type. IsIntT has concrete type IsInt Int. These types cannot possibly match. This may be what you want:
{-# OPTIONS_GHC -fglasgow-exts #-} data IsIntT :: * -> * where IsIntT :: IsIntT Int
class IsIntC a where isInt :: IsIntT a instance IsIntC Int where isInt = IsIntT
newtype Foo = Foo Int deriving(IsIntC)
x :: IsIntT free -> Int x IsIntT = 2
y = x (isInt :: IsIntT Foo)
Note that (Foo 2) + 2 is an attempt to add a Foo and an Int, which cannot possibly compile. So I replaced it with just a 2.