Stateにたいして色んな処理が書けるようになったよー https://github.com/tokiwoousaka/reasonable-lens

gistfile1.hs

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

type StateIO s a = StateT s IO a

data Hoge a = Hoge
  { _foo :: a
  , _bar :: String
  } deriving (Show, Read, Eq, Ord)

data Piyo a b = Piyo
  { _buz :: a
  , _qux :: Hoge b
  } deriving (Show, Read, Eq, Ord)

makeLenses ''Hoge
makeLenses ''Piyo

piyo :: Piyo Double [Int]
piyo = Piyo
  { _buz = 0
  , _qux = Hoge
    { _foo = []
    , _bar = ""
    }
  }

main :: IO ()
main = evalStateT prog piyo

prog :: StateIO (Piyo Double [Int]) ()
prog = do
  liftIO $ putStrLn "----------------------------"
  traceState
  buz .= 999
  traceState
  qux.foo .= [1,2,3]
  traceState
  qux .= Hoge [999] "Hello, Lens World!"
  traceState
  liftIO $ putStrLn "----------------------------"
  qux.foo %= (99:)
  traceState
  buz += 1
  traceState
  buz -= 900
  traceState
  buz *= 5
  traceState
  buz //= 10
  traceState
  liftIO $ putStrLn "----------------------------"
  v_buz <- use buz
  v_qux <- use qux
  v_quxfoo <- use $ qux.foo
  liftIO $ print v_buz
  liftIO $ print v_qux
  liftIO $ print v_quxfoo

traceState :: StateIO (Piyo Double [Int]) ()
traceState = do
  state <- get
  liftIO . print $ state