今度こそ多相型に対応 https://github.com/tokiwoousaka/reasonable-lens

gistfile1.hs

{-# LANGUAGE TemplateHaskell #-}
module Main where
import Control.Lens
import Control.Monad.State

data Hoge = Hoge
  { _Foo :: Int 
  , _Bar :: String
  } deriving (Show, Read, Eq, Ord)

data Piyo = Piyo
  { _baz :: Hoge
  , _qux :: Hoge
  , foobar :: String
  } deriving (Show, Read, Eq, Ord)

data Tuple a b = Tuple { _first :: a, _second :: b } deriving (Show, Read, Eq, Ord)

data Fuga a b = Fuga
  { _tuple1 :: Tuple a Int
  , _tuple2 :: Tuple b Int
  , _quux :: String
  } deriving (Show, Read, Eq, Ord)

hoge = Hoge
  { _Foo = 100
  , _Bar = "HogeHoge"
  }

piyo = Piyo
  { _baz = hoge
  , _qux = hoge
  , foobar = "PiyoPiyo"
  }

fuga :: Fuga String Bool
fuga = Fuga 
  { _tuple1 = Tuple "Hoge" 10
  , _tuple2 = Tuple True 20
  , _quux = "Sushi"
  }

makeLenses ''Hoge
makeLenses ''Piyo
makeLenses ''Tuple
makeLenses ''Fuga

main :: IO ()
main = do
  print piyo
  print $ piyo^.baz
  print $ piyo^.baz.bar
  putStrLn "-----"
  let piyo2 = piyo&qux.foo.~999
  print piyo2
  print $ piyo2^.qux
  putStrLn "-----"
  print fuga
  print $ fuga^.tuple1
  print $ fuga^.tuple2.first
  putStrLn "-----"
  let fuga2 = fuga&tuple1.first.~[1,2,3]
  let fuga3 = fuga&tuple2.second.~999
  let fuga4 = fuga&tuple1.~Tuple ["a", "b", "c"] 777
  print fuga2
  print fuga3
  print fuga4