24 Days of Hackage: web-routes-boomerang

As you start building larger web applications, it quickly becomes important how you deal with the site’s routing. Managing simple strings rapidly becomes limiting and a maintenance nightmare; we’re so used to type safety, it’s easy to forget how cumbersome it is to pass strings around. Furthermore, we need to ensure that the routes that we accept in are the same links that were placing on web pages - there should always be a one-to-one correspondence. What we’d really like is type-safe routing with minimal fuss, and that’s what today’s post is about.

Jeremy Shaw’s web-routes library abstracts the pattern of routing, but today we’ll focus specifically on web-routes-boomerang, which is a library for building type safe routing with minimal fuss.

web-routes-boomerang builds on web-routes by using boomerang, which is a reversible parsing library. Usually, when we work with parsers like parsec the operation moves in one direction - we can parse text into Haskell types, but we don’t get a corresponding pretty-printer. boomerang is different here. By creating a parser, we also create a pretty-printer - the same definition now works in both directions. Furthermore, the output of the pretty-printer can be fed back into the parser, so we have a true isomorphism between text and Haskell values. This is ideal when we work with web sites, as we can parse requests into Haskell types representing paths on the website, and then later encode these paths back into URLs. This pushes string handling to the boundaries of application, and this isolation gives us less scope for errors.

To start using web-routes-boomerang, we begin by defining our site map. We’ll use the canonical web programming example of a blog.

type PostId = Integer

data Sitemap
  = Index
  | Post PostId
  | Tagged [Text]

Our blog is simple - we have a landing page, a page for an individual blog post, and a route that shows all posts that intersect with a set of tags. Ideally, we’d like to have routes like the following:

To get going with boomerang, we need to use a minimal amount of Template Haskell.

makeBoomerangs ''Sitemap

This brings into scope individual boomerangs for each constructor - specifically, rIndex, rPost and rTagged. We can now build the router for our website. To do this, we combine distinct routes of our site using the Monoid instance, and combine parts of the route using the Category instance. Let’s take it slow, and begin with the index page:

siteRouter = mconcat
  [ rIndex

The Index path parses the empty string, and is printed as the empty string. The next route needs to parse an integer for the PostId. To do so, we’ll begin by parsing the string post, then expect a /, and then parse an integer:

  , rPost . "post" </> integer

As you can see, boomarang-based site routes have a really nice DSL that concisely captures the sites routes. Finally, we need to parse the tags route. This one begins as before, but we’ll use the rListSep combinator to parse tags separated by +:

  , rTagged . "tags" </> (satisfyStr (not . Text.null) `rListSep` "+")
  ]

Lovely stuff! All that remains now is to put it all together into something we can use. For this, we can use the boomerangSiteRouteT helper, which takes a handler function and a router, and gives back a Site. However, first of all we need a handler function.

Our handler will be given a value of type Sitemap, which we can pattern match on to learn the page a user requested. Then, we produce a value under the RouteT monad - a monad which gives us access to the Sitemap in order to format links on a page. Usually, you would layer this on top of a monad provided by a web server, but today we’ll just use IO and demonstrate things in the console. For brevity, I’ll demonstrate just one route here, but you can see more routes in today’s associated code.

handler :: Sitemap -> RouteT Sitemap IO ()
handler route = case route of
  Index -> do
    posts <- liftIO getPosts
    liftIO $ putStrLn "Posts:"
    forM_ posts $ \post -> do
      postUrl <- showURL (Post (postId post))
      liftIO $ putStrLn $
        Text.unpack (postTitle post) ++ " - " ++ Text.unpack postUrl

Finally, we combine this into a Site:

site :: Site Sitemap (IO ())
site = boomerangSiteRouteT handler siteRouter

We can now prod this in GHCI. For example, if we request /, which is an empty list of path parts, we’re presented with an index listing:

> either error id $ runSite "" site []
Posts:
24 Days of Hackage - /post/42
10 Reasons Why P = NP - /post/91

If we received a request for /post/42, we can split this apart into the request for [ "post", "42" ], which shows details about post #42:

> either error id $ runSite "" site [ "post", "42" ]
You are reading "24 Days of Hackage"

And just to wrap it all up, if you request a route that can’t be parsed, runSite will return Left String to indicate parsing fails:

> either error id $ runSite "" site [ "admin", "h4x" ]
*** Exception: parse error at path segment 1, character 0: unexpected "admin";
               expecting "post" or "tags" while parsing ["admin","h4x"]

web-routes-boomerang is a fantastic library when it comes to web programming. It doesn’t require a lot more investment to get some tremendous results, and it easily sets strong foundations for building much more complex web applications in the future. Not only that, it shows off some really cool techniques in Haskell programming - which makes it all the more fun to work with!


You can contact me via email at ollie@ocharles.org.uk or tweet to me @acid2. I share almost all of my work at GitHub. This post is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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