Monad Transformers Explained

Below is a short explanation of and motivating example for monad transformers. Comments and aggressive editing welcome.

Thanks to Ryan Ingram and Tommy M McGuire of Haskell-Cafe for their comments on an earlier draft.

Basically it's like making a double, triple, quadruple, ... monad by wrapping around existing monads that provide wanted functionality.

You have an innermost monad (usually Identity or IO but you can use any monad). You then wrap monad transformers around this monad to make bigger, better monads.

Concrete example: Suppose I was writing a server. Each client handling thread must be of type IO () (that's because forkIO :: IO () -> IO ThreadID)).

Suppose also that this server has some configuration that (in an imperative program) would be global because client handling threads all need to query it.

data Config = Config Foo Bar Baz

One way of doing this is to use currying and making all the client threads of type Config -> IO (). Not too nice because any functions they call have to be passed the Config parameter manually. The Reader monad solves this problem but we've already got one monad. We need to wrap IO in a ReaderT. The type constructor for ReaderT is ReaderT r m a, with r the shared environment to read from, m the inner monad and a the return type. Our client_func becomes:

client_func :: ReaderT Config IO ()

We can then use the ask, asks and local functions as if Reader was the only Monad:

(these examples are inside do blocks)

p <- asks port

(Assuming some function port :: Config -> Int or similar.)

To do stuff in an inner monad, there is the function lift. Because having big chains of lift $ lift $ lift $ foo can get annoying, the libraries in mtl (the Monad Transformer Library) play a trick using the type system. Each monad in the mtl is defined in terms of a type class. For example, Reader is actually an instance of MonadReader, as is ReaderT. Further, anything in the mtl that wraps a MonadReader is also set up to be a MonadReader, so the ask, asks and local functions will work without any (manual) lifting. Other mtl monads behave in a similar way.

To do stuff in IO the liftIO function is used:

(given h :: Handle, the client's handle)

liftIO $ hPutStrLn h "You win"
liftIO $ hFlush h

A type class called MonadIO is used to implement a similar trick as above. IO is an instance of MonadIO, and any mtl transformer that wraps a MonadIO instance also is an instance of MonadIO. This means that IO functions need only use liftIO and not a big chain of lifts. Note also that IO has no transformer and must therefore always be the innermost monad.

This is all well and good, but the client_func now has type ReaderT Config IO () and forkIO needs a function of type IO (). The escape function for Reader is runReader :: Reader r a -> r -> a and similarly for ReaderT the escape function is runReaderT :: ReaderT r m a -> r -> m a:

(Given some c :: Config that's been assembled from config files or the like)

forkIO (runReaderT client_func c)

Will do the trick.

Monad transformers are like onions. At first, they make you cry but then you learn to appreciate them. Like onions, they're also made of layers. Each layer is the functionality of a new monad, you lift monadic functions to get into the inner monads and you have transformerised functions to unwrap each layer. They're also like a present in that regard: in this example we unwrapped the outer wrapping paper to get to the present: an object of type IO (), which lets us make Haskell do something.