lenses.hs

{-# LANGUAGE TemplateHaskell #-}
module Main where

import Control.Lens.TH (makeLenses)
import Control.Lens (over, traverse, taking)

data Person = Person { _clothing :: [Clothing] } deriving Show

data Clothing 
    = Dress -- no pockets :(
    | Jacket { _jacketLeftPocket :: [Item], _jacketRightPocket :: [Item] }
    deriving Show

data Item
    = Pen
    | Chapstick
    deriving Show

makeLenses ''Person
makeLenses ''Clothing

addChapstickToLeftPocket :: Person -> Person
addChapstickToLeftPocket = 
    over (clothing . taking 1 (traverse . jacketLeftPocket)) (Chapstick :)

main :: IO ()
main = do
    
    let p = Person [Dress, Jacket [] [], Jacket [] []]
    print (addChapstickToLeftPocket p)
    
    -- Person {_clothing = [Dress,Jacket {_jacketLeftPocket = [Chapstick], _jacketRightPocket = []},Jacket {_jacketLeftPocket = [], _jacketRightPocket = []}]}