inside, from lens

gistfile1.hs

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens
import Data.Tree

data Person = Person {
        _name :: String,
        _years :: Int    
    } deriving Show        

$(makeLenses ''Person)

main = do
    let sillyfun b | b = Person "Wanda" 36 
                   | otherwise = Person "Rodrigo" 34
        newfun = sillyfun^.inside years
        newfun' = sillyfun & inside years .~ (\b -> if b then 45 else 55)
    putStrLn . show $ newfun True
    putStrLn . show $ newfun' True