simple_lens.hs

#!/usr/bin/env stack
{- stack
  script
  --resolver lts-10.4
  --package text
  --package protolude
  --package text
  --package lens
-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TemplateHaskell #-}

module Main where
import Protolude
import qualified Data.Text as Txt
import           Control.Lens
import           Control.Lens.TH

data Parent = Parent { _pName :: Text
                     , _pChild :: Child
                     } deriving (Show)
            
data Child = Child { _cName :: Text
                   } deriving (Show)

makeLenses ''Parent
makeLenses ''Child

main :: IO ()
main = do
  let parent = Parent { _pName = "parent1"
                      , _pChild = Child { _cName = "child1" }
                      }
  
  putText "-------------------------------------"
  putText "^."
  putText $ parent ^. pName
  putText $ parent ^. pChild ^. cName
  putText "-------------------------------------"
  putText ""
  
  
  putText "-------------------------------------"
  putText "^. equiv"
  putText $ _pName parent
  putText $ _cName . _pChild $ parent
  putText "-------------------------------------"
  putText ""
  
  
  putText "-------------------------------------"
  putText ".~"
  print $ parent & pName .~ "newName"
  print $ parent & (pChild . cName) .~ "new childName"
  putText "-------------------------------------"
  putText ""


  putText "-------------------------------------"
  putText ".~ equiv"
  print $ parent { _pName = "newName" }
  print $ parent { _pChild = (_pChild parent) { _cName = "new childName" } }
  putText "-------------------------------------"
  putText ""



  putText "-------------------------------------"
  putText "%~"
  print $ parent & pName %~ Txt.toUpper
  print $ parent & (pChild . cName) %~ Txt.toUpper
  putText "-------------------------------------"
  putText ""



  putText "-------------------------------------"
  putText "%~ equiv"
  print $ parent { _pName = Txt.toUpper . _pName $ parent }
  print $ parent { _pChild = (_pChild parent) { _cName = Txt.toUpper . _cName . _pChild $ parent  } }
  putText "-------------------------------------"
  putText ""