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Today's Topics:
1. Learning Monads with 'setjmp' and 'longjmp' like actions
(Michael Roth)
----------------------------------------------------------------------
Message: 1
Date: Sun, 24 Jul 2016 19:17:08 +0200
From: Michael Roth <l...@mroth.net>
To: "beginners@haskell.org" <beginners@haskell.org>
Subject: [Haskell-beginners] Learning Monads with 'setjmp' and
'longjmp' like actions
Message-ID: <5794f814.5010...@mroth.net>
Content-Type: text/plain; charset=utf-8; format=flowed
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
------------------------------
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