Matching on variable path segments

`path`

Often times a path segment will contain a variable value we want to extract and use, such as a number or a string. We can use the path combinator to do that.

#ifdef HsColour > path :: (FromReqURI a, MonadPlus m, ServerMonad m) => (a -> m b) -> m b #endif

You may find that type to be a little hard to follow because it is pretty abstract looking. Fortunately, we can look at it in an easier way. A ServerPart is a valid instance of, ServerMonad m, so we can just replace the m with ServerPart. You can do this anywhere you see type signatures with (ServerMonad m) => in them. In this case, the final result would look like:

#ifdef HsColour > path :: (FromReqURI a) => (a -> ServerPart b) -> ServerPart b #endif

path will attempt to extract and decode a path segment, and if it succeeds, it will pass the decode value to the nested server part.

Let's start with the most basic example, extracting a String value. We will extend the Hello World server so that we can say hello to anyone.

> module Main where > > import Control.Monad (msum) > import Happstack.Server (nullConf, simpleHTTP, ok, dir, path) > > main :: IO () > main = simpleHTTP nullConf $ msum [ dir "hello" $ path $ \s -> ok $ "Hello, " ++ s > ]

[Source code for the app is here.]

Now, if we start the app and point our browser at: http://localhost:8000/hello/World we get the "Hello, World".

if we point it at http://localhost:8000/hello/Haskell, we get "Hello, Haskell".