LensMatchers.hs

{-# LANGUAGE TemplateHaskell #-}
module Rect (Rect, x, y, rect) where

import Control.Lens
import Data.Functor

data Rect = Rect { _x :: Int, _y :: Int }

makeLenses ''Rect

-- rect :: ...
rect f x y = Rect <$> (f x y)

-- example usage
square :: Int -> Rect
square x = create rect $ x x

abox :: Rect
abox = create rect $ 4 10

describe :: Rect -> String
describe = [match|
              rect 4 4 -> "square with sides of length 4"
              rect x x -> "a square"
              rect x y -> "a rectangle"
            |]