[Haskell-beginners] Learning Monads with 'setjmp' and 'longjmp' like actions

24 Jul 2016

      Hi,

I'm really trying hard to understand monads but I guess my brain is 
trapped in the imperative world.

Below what I have done so far. But now I would like to implement 
something like this:

do
    foo
    (label, jumped) <- setjmp
    bar
    longjmp label

But I'm stuck. How can I implement 'setjmp' and 'longjmp' with my monad?
I'm not able to grasp how can I design 'setjmp' and somehow create a 
label to use in 'longjmp'.
Maybe my monad structure is not appropriate?

Code follows:
---------------------------------------------------------------

data World = SomeWorld
     deriving (Eq, Show)

data ProgramResult a =
     Result      { world :: World, result :: a }
   | Continue    { world :: World, continue :: Program a }
   | Stopped     { world :: World }

newtype Program a = Program { runProgram :: World -> ProgramResult a }

runComplete :: Program a -> World -> (World, a)
runComplete prg world0 =
     case runProgram prg world0 of
          (Result world1 x1)     -> (world1, x1)
          (Continue world1 prg1) -> runComplete prg1 world1

nop :: Program ()
nop = return ()

yield :: Program ()
yield = Program $ \world -> Continue world nop

stop :: Program ()
stop = Program $ \world -> Stopped world

instance Functor Program where
     fmap = liftM

instance Applicative Program where
     pure  = return
     (<*>) = ap

instance Monad Program where
     return x = Program $ \world0 -> Result world0 x

     ma >>= fb = Program $ \world0 ->
         case runProgram ma world0 of
              (Result world1 x1)         -> runProgram (fb x1) world1
              (Continue world1 prg1)     -> Continue world1 (prg1 >>= fb)
              (Stopped world1)           -> Stopped world1

[Haskell-beginners] Learning Monads with 'setjmp' and 'longjmp' like actions

Michael Roth