lens-composite-getter-example.hs

#!/usr/bin/env stack
-- stack --resolver lts-10.8 --install-ghc exec ghci --package lens

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens (Getter, makeLenses, view)

data Foo = Foo
  { _fooInt    :: Int
  , _fooString :: String
  , _fooDouble :: Double
  }

data Bar = Bar
  { _barInt    :: Int
  , _barString :: String
  } deriving (Show)

makeLenses ''Foo
makeLenses ''Bar

-- type Getter s a = forall f. (Contravariant f, Functor f) => (a -> f a) -> s -> f s
-- barFromFoo :: forall f. (Contravariant f, Functor f) => (Bar -> f Bar) -> Foo -> f Foo
barFromFoo :: Getter Foo Bar
barFromFoo f foo = fmap (const foo) (f bar) where
  bar        = Bar intPart stringPart
  intPart    = view fooInt foo
  stringPart = view fooString foo

main :: IO ()
main = print . view barFromFoo $ foo where
  foo = Foo 42 "abcd" 33.3