Main.purs

module Main where

import Prelude
import Data.List
import Data.Newtype
import Data.Monoid (mempty)

-- From purescript-indexed-monad
class IxMonad m where
  ipure ∷ ∀ a x. a → m x x a
  ibind ∷ ∀ a b x y z. m x y a → (a → m y z b) → m x z b

infixl 1 ibind as :>>=

data Stopped
data Running

newtype Machine i o a = Machine a

instance ixMonadMachine :: IxMonad Machine where
  ipure = Machine
  ibind (Machine x) f = Machine <<< runMachine $ f x

runMachine :: forall s t a. Machine s t a -> a
runMachine (Machine x) = x

start :: Machine Stopped Running Unit
start = Machine unit

stop :: Machine Running Stopped Unit
stop = Machine unit

example1 :: Machine Running Running Unit
example1 = ipure unit
  :>>= \_ -> stop
  :>>= \_ -> start

-- Uncomment for error!
-- example2 :: Machine Running Running Unit
-- example2 = do
--     pure unit
--     stop
--     start
--   where
--     bind = (:>>=)
--     pure = ipure