Type literal usage

Question

I'm trying to understand the usage of haskell type literals. In particular, I thought I'd write a function that shows me the type literal for a custom type

newtype Fixed (p :: Nat) a = Fixed a

instance (KnownNat p) => Show (Fixed p a) where
    show _ = show $ natVal (Proxy::Proxy p)

However, ghc (7.8) can't deduce KnownNat n0, which means I'm not constraining things as I think I should be. Can anyone suggest what is wrong?

Answer 1

You need -XScopedTypeVariables for GHC to recognize that the p of your Proxy p is the same as the p of your type signature.

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
import Data.Proxy
import GHC.TypeLits

newtype Fixed (p :: Nat) a = Fixed a

instance (KnownNat p) => Show (Fixed p a) where
        show _ = show $ natVal (Proxy::Proxy p)