MonadPlus.purs

module Test.StrongCheck.Laws.Control.MonadPlus where

import Prelude

import Control.Alt ((<|>))
import Control.Monad.Eff.Console (log)
import Control.MonadPlus (class MonadPlus)

import Type.Proxy (Proxy2)

import Test.StrongCheck (SC, quickCheck')
import Test.StrongCheck.Arbitrary (class Arbitrary)
import Test.StrongCheck.Laws (A, B)

-- | - Distributivity: `(x <|> y) >>= f == (x >>= f) <|> (y >>= f)`
checkMonadPlus
  ∷ ∀ eff m
  . MonadPlus m ⇒ Arbitrary (m A) ⇒ Arbitrary (m B) ⇒ Eq (m B)
  ⇒ Proxy2 m
  → SC eff Unit
checkMonadPlus _ = do

  log "Checking 'Distributivity' law for MonadPlus"
  quickCheck' 1000 distributivity

  where

  distributivity ∷ m A → m A → (A → m B) → Boolean
  distributivity x y f = ((x <|> y) >>= f) == ((x >>= f) <|> (y >>= f))