What is MonadBaseControl for?

问题:

I'm digging deeper into Yesod's monads, and have encountered MonadBaseControl. I took a look at the hackage doc, and got lost. Could someone tell me the problem it is trying to solve?

回答1:

It comes from the package monad-control, and is one of a pair of type classes (the other one being MonadTransControl) that enhance MonadBase (resp. MonadTrans) by supporting an alternative liftBase (resp. lift) operation for monads that implement it. This enhanced version no longer takes a simple action in the absolute base monad (resp. immediate base monad), but instead takes a function that gets the base monad's (resp. monad transformer's) whole state at that point as its only parameter and returns the aforementioned action.

As the package documentation states, this enhancement, along with the rest of the contents of these type classes, allow you to lift functions like catch, alloca, and forkIO from the absolute base monad (resp. immediate base monad), which is not possible with the simpler scheme present in MonadBase (resp. MonadTrans) because the latter pair do not allow you to lift the arguments of a function, just the results, while the approach taken by monad-control allows both.

As a result, the set of monads (resp. monad transformers) that can be used with MonadBaseControl (resp. MonadTransControl) is a strict subset of the set of monads that can be used with MonadBase (resp. MonadTrans), but the former groups are much more powerful than the latter for the same reason.

回答2:

Michael Snoyman actually wrote a small tutorial on monad-control: http://www.yesodweb.com/book/monad-control

The gist of that article might be the following:

Imagine you have this piece of code:

withMyFile :: (Handle -> IO a) -> IO a
withMyFile = withFile "test.txt" WriteMode

You can apply withMyFile to any function of the type Handle -> IO a and get a nice IO a value. However, what if you have a function of the type Handle -> ErrorT MyError IO a and want to get a value of type ErrorT MyError IO a? Well, basically, you will have to modify withMyFile in order to incorporate a lot of wrapping/unwrapping. MonadBaseControl allows you to somewhat 'lift' functions like withMyFile to certain monad transfromers which allows unwrapping ("running"). Thus, resulting code looks like this:

useMyFileError :: (Handle -> ErrorT MyError IO ()) -> ErrorT MyError IO ()
useMyFileError func = control $ \run -> withMyFile $ run . func