OverloadedRecordFieldsのサンプル

OverloadedRecordFieldsExample.hs

{-# LANGUAGE DataKinds             #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE NamedFieldPuns        #-}
{-# LANGUAGE OverloadedLabels      #-}
{-# LANGUAGE TypeFamilies          #-}

module OverloadedRecordFieldsExample where

import           GHC.OverloadedLabels

data Point2 =
    Point2
    { x :: Double
    , y :: Double
    }
    deriving (Eq, Ord, Show, Read)

instance IsLabel "x" (Point2 -> Double) where
    fromLabel _ Point2 { x } = x

instance IsLabel "y" (Point2 -> Double) where
    fromLabel _ Point2 { y } = y

data Point3 =
    Point3
    { x :: Double
    , y :: Double
    , z :: Double
    }
    deriving (Eq, Ord, Show, Read)

instance IsLabel "x" (Point3 -> Double) where
    fromLabel _ Point3 { x } = x

instance IsLabel "y" (Point3 -> Double) where
    fromLabel _ Point3 { y } = y

instance IsLabel "z" (Point3 -> Double) where
    fromLabel _ Point3 { z } = z

class Normalize a where
    normalize :: a -> a

instance Normalize Point2 where
    normalize Point2{ x, y } = let base = sqrt $ x ^ (2 :: Int) + y ^ (2 :: Int)
        in Point2 { x = x / base, y = y / base }

instance Normalize Point3 where
    normalize Point3{ x, y, z } = let base = sqrt $ x ^ (2 :: Int) + y ^ (2 :: Int) + z ^ (2 :: Int)
        in Point3 { x = x / base, y = y / base, z = z / base }

point2X :: Point2 -> Double
point2X = #x

point2Y :: Point2 -> Double
point2Y = #y