How to take out a value out of a monad in Haskell?

问题

Is there any way to take "things" out of a monad?

I am developing a game, and I am now trying to understand about databases. I found happstack really nice, but I can't get the thing.

For example, I have this function (hope you are familiar with happstack)

getAllThings :: MonadIO m => m [Thing]
getAllThings = do
            elems <- query GetThings
            return elems

So I get m [Things], but I can't use this in my model! For instance

doSomeThingWithThings :: [Thing] -> Something

I googled this and I found nothing.

回答1:

You are not supposed to exit IO monad this way (except unsafePerformIO function), but you can still use your function inside it:

process :: MonadIO m => m ()
process = do
          elems <- getAllThings
          let smth = doSomeThingWithThings elems
          -- ...

回答2:

After elems <- query GetThings the elems is [Thing] so <- inside do is about getting things out of monad (called bind operation). The last statement return put things inside a monad. So either you can call you other function after getting elems and before return or where ever you are calling getAllThings you can use extract the value using <- from the monad and pass it to your function

来源：https://stackoverflow.com/questions/7314789/how-to-take-out-a-value-out-of-a-monad-in-haskell