PrismDisjunction.hs

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE ImpredicativeTypes #-}
{-# LANGUAGE RankNTypes #-}

import Control.Lens
import Control.Applicative

data Expr
  = Lit Int
  | Add Expr Expr
  | Sub Expr Expr
  deriving (Show)

makePrisms ''Expr

replaceLits :: Expr -> Expr
replaceLits (Lit n) = Lit 0
replaceLits e =
  e & ((_Add `failing` _Sub).both %~ replaceLits)


data Expr'
  = Lit' Int
  | Add' Expr' Expr'
  | Sub' Expr' Expr'
  | If' Expr' Expr' Expr'
  deriving (Show)

makePrisms ''Expr'

replaceLeftmostLit :: Expr' -> Expr'
replaceLeftmostLit (Lit' n) = Lit' 0
replaceLeftmostLit e =
  e & ((_Add'._1 `failing` _Sub'._1 `failing` _If'._1) %~ replaceLeftmostLit)