Lens study

gistfile1.hs

import Control.Category

import Prelude hiding (id, (.))

data Lens a b
  = Lens
    { get :: a -> b
    , set :: b -> a -> a
    }

modify :: Lens a b -> (b -> b) -> a -> a
modify l f a = set l (f (get l a)) a

instance Category Lens where
  id =
    Lens id const
  l . m =
    Lens (get l . get m) (modify m . set l)

_fst :: Lens (a, b) a
_fst = Lens fst (\a (_, b) -> (a, b))

_snd :: Lens (a, b) b
_snd = Lens snd (\b (a, _) -> (a, b))

_lines :: Lens String [String]
_lines = Lens lines (const . unlines)

main :: IO ()
main = do
  print $ set (_snd >>> _fst) 777 (1, (2, 3))
  -- print $ set (_snd >>> _fst) "hoge" (1, (2, 3)) -- compile error