optics-not-syntax.hs

{-# LANGUAGE LambdaCase #-}

import Control.Lens

data Blah =
  Blah1 Int
  | Blah2 String
  | Blah3 [()]
  deriving (Eq, Show)

_Blah1or2 ::
  Prism' Blah (Either Int String)
_Blah1or2 =
  prism'
    (either Blah1 Blah2)
    (\case
        Blah1 n -> Just (Left n)
        Blah2 s -> Just (Right s)
    )