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Why can't a monad be decomposed?

I don't understand why I can't decompose a (let's say IO) monad. Like IO a -> a?

My question originated when using happstack and wanting to get the Text out of ServerPart (Maybe Text) which is returned by (optional $ lookText "domain"). Then I remembered reading that IO monad can't be escaped.

I've read about unsafePerformIO and the reasons why it is bad, but none of those reasons seem to answer my question.

Solution

  • Can Monads be escaped from?

    Yes. This is very easy with many Monads, such as Maybe, Either a, State s, Identity, and more. One of the most common Monads that is escaped from is the function Monad: (->) r. If it weren't possible to turn a function into a value, then Haskell wouldn't have much going for it.

    Can IO be escaped from?

    Unfortunately, yes. It would be a lot better for beginners if they didn't google around and see that they can technically escape from IO using unsafePerformIO, which as you might have guessed is not safe. It is not meant to be used in normal code, but is rather a backdoor into the runtime system for when you really need it. Primarily, it's used for implementing some lower level libraries like Vector, but also for interfacing with external shared libraries (DLLs). If you're not writing that kind of code, don't use unsafePerformIO. Otherwise, you will end up with code that becomes difficult to reason about and maintain because it bypasses the type system.

    How do we escape from other Monads?

    It varies from Monad to Monad, but most monad transformers have run-, eval- or exec- methods:

    > :m Control.Monad.State
> runState (modify (*10) >> get >>= return . show) 1
("10", 10)
> :type runState (modify (*10) >> get >>= return . show) 1
runState (modify (*10) >> get >>= return . show) 1 :: (String, Int)
> evalState (modify (*10) >> get >>= return . show) 1
"10"
> execState (modify (*10) >> get >>= return . show) 1
10

    The Maybe Monad has several ways to escape from it:

    > :m Data.Maybe
> maybe "nada" show (Just 2)
"2"
> maybe "nada" show Nothing
"nada"
> fromMaybe 1 (Just 10)
10
> fromMaybe 1 Nothing
1
> fromJust (Just 1)
1
> fromJust Nothing
*** Exception: Maybe.fromJust: Nothing

    So as you can see, not all of them are safe to use either.

    What does this have to do with Happstack?

    I don't know, I haven't used Happstack enough to know about it. However, a quick search led me to this example on their website, which looks pretty applicable to your situation.