问题
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.
回答1:
Can Monads be escaped from?
Yes. This is very easy with many Monads, such as Maybe, Either a, State, 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.
回答2:
Let me answer your question with another question: why do you think you can get things out of a monad?
data Dud a = Dud
instance Functor Dud where
fmap _ _ = Dud
instance Monad Dud where
return _ = Dud
Dud >>= _ = Dud
More directly, Monad gives you the ability to create and combine types. Many types exist of this form which may not allow you to actually extract anything at all.
A more meaningful, direct example is the list monad.
returnList :: a -> [a]
returnList a = [a]
bindList :: [a] -> (a -> [b]) -> [b]
bindList as f = concat (map f as)
Lists obviously may contain things, but they also may fail to. There is no function
[a] -> a
which doesn't throw an error when passed [].
Typically, the reason why you want to "get something out" of a monad is that you have an operation like
f :: a -> b
and a monadic value like
m :: [a]
and you'd like to pull that a out of the monad and apply it to your function. As stated above, there's no reason to believe that you can do that.
Instead, you do the opposite—you bring your function into the type!
map f :: [a] -> [b]
来源：https://stackoverflow.com/questions/23901801/why-cant-a-monad-be-decomposed