An "introduction" to how to make *really* lazy lenses

LazyPatterns.hs

module LazyPatterns where

import Control.Lens

{-# ANN module "HLint: ignore Use camelCase" #-}


data T = C { _x :: Int, _y :: Char } deriving (Show, Eq)

-- |
--
-- >>> undefined & strictX .~ 7
-- *** Exception: Prelude.undefined
strictX :: Lens' T Int
strictX f (C p y) = f p <&> \p' -> C p' y

-- |
--
-- >>> undefined & lazyX .~ 7
-- C {_x = 7, _y = *** Exception: Prelude.undefined
lazyX :: Lens' T Int
lazyX f ~(C p y) = f p <&> \p' -> C p' y

-- |
--
-- >>> undefined & lazyX .~ 7 & lazyY .~ 'f'
-- C {_x = 7, _y = 'f'}
lazyY :: Lens' T Char
lazyY f ~(C x p) = f p <&> \p' -> C x p'