{-# LANGUAGE DataKinds #-}

module Two where

import Text.Read (readPrec, Lexeme(Ident), lexP)

data Two (b :: Bool) where
  Zero :: Two False
  One  :: Two True

instance Read (Two False) where
  readPrec = do
    Ident "Zero" <- lexP
    pure Zero

instance Read (Two True) where
  readPrec = do
    Ident "One" <- lexP
    pure One

noConstraint :: String -> Two b
noConstraint = read