gistfile1.hs

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens

data Hoge = A Int
          | B Bool
          | C Char
          | D
          | E String deriving Show

makePrisms ''Hoge

{-
*Main> has _A (A 3)
True
*Main> has _A (B False)
False
*Main> (E "hoge") ^? _A
Nothing
*Main> (E "hoge") ^? _E
Just "hoge"
-}