prisms.hs

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE TemplateHaskell #-}

import Control.Lens

data These a b =
  This a
  | That b
  | Both a b
  deriving (Eq, Show)

makePrisms ''These

is :: APrism s t a b -> s -> Bool
is p = not . isn't p

-- can this be written "more neatly", without the duplication...
isThisOrThat :: These a b -> Bool
isThisOrThat t = is _This t || is _That t

-- like this, except in this case, the type has changed
isThisOrThat' :: These a a -> Bool
isThisOrThat' t = any (`is` t) [_This, _That]