How to use “IO String” as an HTTP response in Happ

问题:

I'm retrieving data from a database using HDBC, then trying to send this data to a web client using Happstack.

myFunc :: Integer -> IO String
myFunc = ... fetch from db here ...

handlers :: ServerPart Response
handlers =
    do decodeBody (defaultBodyPolicy "/tmp/" 0 1000 1000)
       msum [ 
                dir "getData" $ ok $ toResponse $ myFunc $ toInteger 1
            ]

mainFunc = simpleHTTP nullConf handlers

When I build the above code I get this error:

No instance for (ToMessage (IO String)) arising from a use of `toResponse'

What did I try ?

1. I tried to convert the IO String to String (using liftIO for example).
2. I tried to find any similar questions here.
3. I tried to find a similar example in the Happstack Crash Course.
4. I googled all related keywords in all different combinations.

Thanks in advance.

回答1:

You have to design your handlers around the fact that fetching from a database is a magical action that may not give you what you expect. (For example, your database may crash.) This is why its result is served as an IO, which is a particular case of a monad.

A monad is a jar with a very narrow neck, so narrow even that, once you put something in there, you cannot unput it. (Unless it happens to also be a comonad, but that's a whole another story and not the case with IO nor with ServerPart.) So, you would never convert an IO String to a String. Not that you can't, but your program would become incorrect.

Your case is kind of tricky as you have two monads at play there: IO and ServerPart. Fortunately, ServerPart builds upon IO, it is " larger " and can, in a sense, absorb IO: we can put some IO into a ServerPart and it will be a ServerPart still, so we may then give it to simpleHTTP. In happstack, this conversion may be done via require function, but there is a more general solution as well, involving monad transformers and lift.

 

Let's take a look at the solution with require first. Its type (simplified to our case) is:

IO (Maybe a) -> (a -> ServerPart r) -> ServerPart r

— So, it takes an IO jar with some argument and makes it suitable for a function that lives in the ServerPart jar. We just have to adjust types a bit and create one lambda abstraction:

myFunc :: Integer -> IO (Maybe String)
myFunc _ = return . Just $ "A thing of beauty is a joy forever."

handlers :: ServerPart Response
handlers = require (myFunc 1) $ \x ->
    do decodeBody (defaultBodyPolicy "/tmp/" 0 1000 1000)
       msum [
                dir "getData" $ ok $ toResponse x
            ]

mainFunc = simpleHTTP nullConf handlers

As you see, we have to make 2 modifications:

• Adjust myFunc so that it returns Maybe, as necessitated by require. This is a better design because myFunc may now fail in two ways:

  • As a Maybe, it may return Nothing, which means 404 or the like. This is rather common a situation.
  • As an IO, it may error out, which means the database crashed. Now is the time to alert the DevOps team.

• Adjust handlers so that myFunc is external to them. One may say more specifically: abstract myFunc from handlers. This is why this syntax is called a lambda abstraction.

 

require is the way to deal with monads in happstack specifically. Generally though, this is just a case of transforming monads into larger ones, which is done via lift. The type of lift (again, simplified), is:

IO String -> ServerPart String

So, we can just lift the myFunc 1 :: IO String value to the right monad and then compose with >>=, as usual:

myFunc :: Integer -> IO String
myFunc _ = return $ "Its loveliness increases,.."

handlers :: ServerPart Response
handlers = lift (myFunc 1) >>= \x ->
    do decodeBody (defaultBodyPolicy "/tmp/" 0 1000 1000)
       msum [
                dir "getData" $ ok $ toResponse x
            ]

mainFunc = simpleHTTP nullConf handlers

As simple as that. I used the same lambda abstraction trick again, but you may as well use do-notation:

myFunc :: Integer -> IO String
myFunc _ = return $ "...it will never pass into nothingness."

handlers :: ServerPart Response
handlers = do
    x <- lift (myFunc 1)
    decodeBody (defaultBodyPolicy "/tmp/" 0 1000 1000)
    msum [
            dir "getData" $ ok $ toResponse x
         ]

mainFunc = simpleHTTP nullConf handlers

 

P.S. Returning to the story of large and small jars: you can put IO into ServerPart precisely because ServerPart is also an IO monad — it is an instance of the MonadIO class. That means that anything you can do in IO you can also do in ServerPart, and, besides general lift, there is a specialized liftIO function that you can use everywhere I used lift. You are likely to meet many other monads out there that are instances of MonadIO as it's a handy way of structuring code in large applications.

In your particular case, I would stick with the require way nevertheless because I think it's how the designers of happstack meant it to be done. I'm not particularly knowledgeable about happstack though, so I may be wrong.

 

That's it. Happy hacking!