TypeAssert.hs

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeApplication, ExplicitNamespaces, AllowAmbiguousTypes, StandaloneDeriving #-}
import Data.Either

-- @ Control/TypeAssert.hs
{- module Control.TypeAssert where -}
class Assert t n y | n -> t, y -> t where
  assert :: t -> Either n y
  clearL :: n -> t
  clearR :: y -> t
  check :: t -> Bool
  {-# MINIMAL assert, clearL, clearR #-}
  check = isRight . assert @t @n @y

-- @ Control/TypeAssert/EvenOdd.hs
{- module Control.TypeAssert.EvenOdd (type Even, type Odd) where -}
newtype Odd n = Odd n
newtype Even n = Even n
deriving instance Show n => Show (Odd n)
deriving instance Show n => Show (Even n)

instance Integral n => Assert n (Odd n) (Even n) where
  assert n | even n = Right $ Even n
           | otherwise = Left $ Odd n
  clearL (Odd n) = n
  clearR (Even n) = n
  check = even