RelativeValidity.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

-- | Relative validity
module Data.RelativeValidity
  ( RelativeValidity (..),
    isInvalidFor,
  )
where

-- | A class of types that have additional invariants defined upon them
-- that aren't enforced by the type system
--
-- If there is a @Validity a@ instance as well, then @a `isValidFor` b@
-- should imply @isValid a@ for any @b@.
--
-- If there is a @Validity b@ instance as well, then @a `isValidFor` b@
-- should imply @isValid b@ for any @a@.
class RelativeValidity a b where
  isValidFor :: a -> b -> Bool

isInvalidFor :: RelativeValidity a b => a -> b -> Bool
isInvalidFor a b = not $ isValidFor a b