Prism adventures

gistfile1.txt

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Control.Lens
import System.IO

-- Note that data constructors can't begin with underscore _
data Foo = A Int Int | B Char Char
makePrisms ''Foo

main :: IO ()
main = do
    putStrLn . show $ firstOf _A $ B 'a' 'c'
    putStrLn . show $ firstOf _A $ A 3 5
    putStrLn . show $ preview _A $ B 'a' 'c'
    putStrLn . show $ preview _A $ A 3 5