Main.hs

module Lib where
import  Control.Lens

data Foo = Foo { a :: Int, b :: String, c :: Char } deriving Show
data Bar = Bar { x :: Int, y :: String } deriving Show

constructBar :: Lens Foo Foo Bar Bar
constructBar = lens getFoo2Bar setFooFromBar

getFoo2Bar :: Foo -> Bar
getFoo2Bar (Foo a b _) = Bar a b

setFooFromBar :: Foo -> Bar -> Foo
setFooFromBar (Foo _ _ c) (Bar a b) = Foo a b c

test :: IO ()
test = do
  let foo1 = Foo 100 "Hello" 'a'
  let bar1 = Bar 200 "Lens"
  print foo1
  print bar1
  print $ foo1^.constructBar
  print $ foo1&constructBar.~bar1