{-# LANGUAGE InstanceSigs #-}

module Geometry where

import Text.Printf (printf)

-- Here's a basic typeclass for geometric shapes.
class Shape a where
    area :: a -> Double
    perim :: a -> Double

-- For our example we'll implement this typeclass on
-- Rectangle and Circle types.
data Rectangle = Rectangle Double Double
data Circle = Circle Double

-- To implement a typeclass in Haskell, we use the 'instance' keyword.
-- Here we implement Shape for Rectangle.
instance Shape Rectangle where
    area :: Rectangle -> Double
    area (Rectangle width height) = width * height

    perim :: Rectangle -> Double
    perim (Rectangle width height) = 2 * width + 2 * height

-- The implementation for Circle.
instance Shape Circle where
    area :: Circle -> Double
    area (Circle radius) = pi * radius * radius

    perim :: Circle -> Double
    perim (Circle radius) = 2 * pi * radius

-- If a function takes a Shape constraint, then we can call
-- methods that are in the Shape typeclass. Here's a
-- generic 'measure' function taking advantage of this
-- to work on any Shape.
measure :: Shape a => a -> IO ()
measure shape = do
    print shape
    printf "Area: %.2f\n" (area shape)
    printf "Perimeter: %.2f\n" (perim shape)

-- We need to define how to print our shapes
instance Show Rectangle where
    show (Rectangle w h) = printf "Rectangle %.2f %.2f" w h

instance Show Circle where
    show (Circle r) = printf "Circle %.2f" r

main :: IO ()
main = do
    let r = Rectangle 3 4
    let c = Circle 5

    -- The Rectangle and Circle types both
    -- implement the Shape typeclass so we can use
    -- instances of these types as arguments to measure.
    measure r
    measure c