gistfile1.hs

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

data Point = Point
  { _pointX :: Int
  , _pointY :: Int
  } deriving Show

data Size = Size 
  { _height :: Int
  , _width :: Int
  } deriving Show

data Object = Object
  { _name :: String
  , _position :: Point
  , _size :: Size
  } deriving Show

----
-- アクセッサをマクロで自動生成
-- アクセッサの型： forall f. Functor f => (a -> f b) -> s -> f t

makeLenses ''Point
makeLenses ''Size
makeLenses ''Object

----

sampleObject :: Object
sampleObject = Object
  { _name = "sample"
  , _position = Point
    { _pointX = 100
    , _pointY = 200
    }
  , _size = Size
    { _height = 300
    , _width = 400
    }
  }

-- 手続きは次の２種類の演算で構成される
-- return :: a -> m a
-- bind :: m a -> (a -> m b) -> m b

-- 状態(副作用)の計算を次の型で表せる
-- s -> (a, s)

sampleProc :: State Object ()
sampleProc = do 
  name .= "new name"
  position.pointX += 100
  size.width .= 500 

----

main :: IO ()
main = do
  putStrLn "処理前"
  print sampleObject
  putStrLn "処理後"
  print $ execState sampleProc sampleObject