module Main where

import Prelude
import Effect (Effect)
import Effect.Console (log)
import Math (pi)

-- Here's a basic interface for geometric shapes.
class Geometry a where
  area :: a -> Number
  perim :: a -> Number

-- For our example we'll implement this interface on
-- `Rect` and `Circle` types.
data Rect = Rect { width :: Number, height :: Number }
data Circle = Circle { radius :: Number }

-- To implement an interface in PureScript, we need to create
-- an instance of the typeclass for our types.
-- Here we implement `Geometry` for `Rect`.
instance geometryRect :: Geometry Rect where
  area (Rect r) = r.width * r.height
  perim (Rect r) = 2.0 * r.width + 2.0 * r.height

-- The implementation for `Circle`.
instance geometryCircle :: Geometry Circle where
  area (Circle c) = pi * c.radius * c.radius
  perim (Circle c) = 2.0 * pi * c.radius

-- If a function takes a typeclass constraint, then we can call
-- methods that are in the named typeclass. Here's a
-- generic `measure` function taking advantage of this
-- to work on any `Geometry`.
measure :: forall a. Geometry a => a -> Effect Unit
measure g = do
  log $ show g
  log $ show $ area g
  log $ show $ perim g

main :: Effect Unit
main = do
  let r = Rect { width: 3.0, height: 4.0 }
  let c = Circle { radius: 5.0 }

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