Simulating first-class patterns by combining prisms into `Getting First`

Getting.hs

{-# LANGUAGE TemplateHaskell #-}
module Lens where

import Control.Lens
import Data.Monoid (First, (<>))

data FooBar =
     Foo (Either Int Bool)
   | Bar (Maybe Bool)

makePrisms ''FooBar

right :: Prism' FooBar Bool
right =
  _Foo . _Right

just :: Prism' FooBar Bool
just =
  _Bar . _Just

pats :: Getting (First Bool) FooBar Bool
pats =
  right <> just

bool :: FooBar -> Maybe Bool
bool =
  preview pats

{--
[1 of 1] Compiling Lens             ( /tmp/Lens.hs, interpreted )
Ok, modules loaded: Lens.
λ> bool (Foo (Right True))
Just True
λ> bool (Bar (Just True))
Just True
--}