Type to value reflection

TypeSum.hs

{-# LANGUAGE DataKinds           #-}
{-# LANGUAGE ScopedTypeVariables #-}

import           Data.Proxy
import           Data.Type.Bool

-- Types
data One
data Add a b

-- Type Classes
class Eval x where
  eval :: Proxy x -> Int

instance Eval One where
  eval _ = 1

instance forall a b. (Eval a, Eval b) =>
         Eval (Add a b) where
  eval _ = eval (Proxy :: Proxy a) + eval (Proxy :: Proxy b)

-- Example
type Two = Add One One

two :: Int
two = eval (Proxy :: Proxy Two)