PureScript by Example - 6.11 Superclasses - Exercise 1-4 Solutions

Superclasses.purs

module Superclasses where

import Prelude
import Data.Array
import Data.Foldable
import Data.Maybe

-- Exercise 1
maxOf :: Partial => Array Int -> Int
maxOf = fromJust <<< maximum

-- Boilerplate for Exercises 2-3
class Monoid m <= Action m a where
   act :: m -> a -> a

newtype Multiply = Multiply Int

instance semigroupMultiply :: Semigroup Multiply where
  append (Multiply n) (Multiply m) = Multiply (n * m)

instance monoidMultiply :: Monoid Multiply where
  mempty = Multiply 1

-- Exercise 2
instance repeatAction :: Action Multiply String where
  act (Multiply 0) _ = ""
  act (Multiply n) s = s <> act (Multiply (n - 1)) s

-- Exercise 3
instance actionOnArrays :: Action m a => Action m (Array a) where
  act = map <<< act

-- Boilerplate for Exercise 4
newtype Self m = Self m

-- Exercise 4
instance monoidActsOnSelf :: Monoid m => Action m (Self m) where
  act m (Self m') = Self (m <> m')

-- For debugging purposes
instance showMultiply :: Show Multiply where
  show (Multiply n) = "Multiply (" <> show n <> ")"

instance showSelf :: Show a => Show (Self a) where
  show (Self x) = "Self (" <> show x <> ")"