blah.hs

{-# LANGUAGE LambdaCase #-}

import Control.Lens

data Blah a =
  Blah1 a
  | Blah2 a
  | BlahS String
  deriving (Eq, Show)

isBlah1orS :: Blah a -> Bool
isBlah1orS b = not (isn't _Blah1 b || isn't _BlahS b)

_Blah1 :: Prism' (Blah a) a
_Blah1 =
  prism'
    Blah1
    (\case
      Blah1 a -> Just a
      _ -> Nothing
    )

_Blah2 :: Prism' (Blah a) a
_Blah2 =
  prism'
    Blah2
    (\case
      Blah2 a -> Just a
      _ -> Nothing
    )

_BlahS :: Prism' (Blah a) String
_BlahS =
  prism'
    BlahS
    (\case
      BlahS s -> Just s
      _ -> Nothing
    )