Hacking with HAppS – what type, handler?

(These posts are prettified versions of my notes that I made whilst stumbling around HAppS for the first time. I hope they’ll be of use to future HAppS explorers!)

Note 1 .. In which we figure out what ServerPartT and WebT really are.

When we start HAppS running with ..

   simpleHTTP config list_of_handlers :: IO ()

… what type do the handlers have? Let’s explore some possible universes!

In a simple world, a handler could have type

    Request -> Maybe Response

This allows handlers to be selective about which Requests they handle, which is useful. But it also means that handlers must be pure functions so there’d be no way of maintaining any ‘application state’ between requests. Not very useful!

Let’s try to remedy that. If the handlers had type:

   Request -> State S Response

.. then we’d be able to retain and modify some application state (of type S) between handler invocations. But that’s all we’d be able to do. We still couldn’t write logs to disk, read file contents or talk to databases.

To allow handlers to do that, handlers would need to have type ..

  Request -> IO Response

(As an aside, note that whilst simpleHTTP has type “IO ()” it’s up to simpleHTTP whether or not it exposes the full power of the IO monad to the handlers. However, as we’ve just seen, not doing so would pretty much cripple the handlers)

So is this really what HAppS gives you? Let’s see. In the real world HAppS handlers have type “ServerPartT IO a” which is defined to be …

newtype ServerPartT m a = ServerPartT { unServerPartT :: Request -> WebT m a }

In other words, if you have a value of type “Request -> WebT m a”, you can tag it with the ServerPartT constructor to say “this ain’t just any function of that type, it’s part of a web server, dogammit!”.

But, apart from the tag, our handlers really just have type “Request -> WebT m a”. So what’s a WebT?

newtype WebT m a = WebT { unWebT :: m (Result a) }

So, ignoring the tag, it looks kinda like our handlers can produce any monadic computation that, when run, yields a “Result a” (the “Result a” type lets us say “I handled this request, here’s my answer” or “I didn’t handle this request”)

Hmm, there’s something not right there. If that was the case, it would be possible to use, for example, both “Result -> IO (Result a)” and “Result -> State S (Result a)” as handlers. But how would simpleHTTP ‘run’ such monadic computations? To run a State computation you use “evalState”. To run an IO computation, you have to return it from main. There’s no single way to run all possible monads.

Did I miss something? Yes, sort of – I just blindly assumed ‘m’ was a monad because of the way it was used. But there’s no “Monad m =>” constraints anywhere. And, in fact, if we go back to the start ..

simpleHTTP :: ToMessage a => Conf -> [ServerPartT IO a] -> IO ()

.. we can see that, right from the top, simpleHTTP only deals with ServerPartT’s and WebT’s where the ‘m’ type parameter is IO.

So, after all that, the handlers in HAppS handlers can be anything of type: Request -> IO (Response a). The ‘a’ type needs to be something that HApps can figure out a content-type and be able to make an HTTP message from (the ToMessage type class). The “IO (Response a)” part gets dressed up as a “WebT” (think: computation that produces the response). And the whole thing things gets dressed up as a “ServerPartT” (think: request handler).

What does each bit do? The IO part gives you free reign to do side-effecting computations, which get sequentially executed by the runtime. The Response part allows you to say ‘I handled this’ or ‘didn’t handle it’ and the ‘a’ part can be your XML or HTML data type (so long as its an instance of ToMessage).

So that’s it.

Man, that was a lot of complexity for such a simple end result. I don’t understand why there’s so much generality in the types of ServerT and WebT – I’d love to know!