play with haskell lenses

Lenses.hs

{-# LANGUAGE TemplateHaskell, FlexibleContexts #-}

import Data.Lenses
import Data.Lenses.Template

-- data structures to work on
--  for fields ending with an underscore, special lens accessor functions
--  can be created with "deriveLenses"
data Address = Address { street_ :: String
                       , houseNumber_ :: Int
                       , city_ :: String
                       , postcode_ :: String
                       } deriving Show

data Person = Person { name_ :: String
                     , age_ :: Int
                     , address_ :: Address
                     } deriving Show

-- template haskell magic to create accessor functions
$( deriveLenses ''Address )
$( deriveLenses ''Person )

-- test data
myhome = Address "Foostreet" 42 "Heerbrugg" "12345"
myself = Person "Rico" 32 myhome

-- first naive aproach
otherStreet = let a = myself `fetch` address
                  a' = a `update` street $ "blub"
              in myself `update` address $ a'

-- better when combining lenses
otherStreet' = myself `update` (address . street) $ "blub"

main :: IO ()
main = do 
    print $ myself `fetch` (address . street)
    print $ otherStreet' `fetch` (address . street)