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UrlDisp

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Contents

1 What is UrlDisp

1.1 Problem statement

URLs are everywhere on the web. Most of them, however, are hard to remember, because they are meaningless for humans. This is wrong: URLs are a part of user interface, and therefore should be kept simple, meaningful and memorizeable.

1.2 Solution

UrlDisp provides (Fast)CGI programs a minimalistic domain-specific parser for URLs.

Hierarchical part of the URL is tokenized and matched against rules defined using UrlDisp combinators. Every rule consists of, basically, a predicate and a CGI action. Once a predicate is satisfied, an action is performed; otherwise, alternatives are tried in order. The matching algorithm is backtracking.

2 Usage examples

2.1 Basics

A regular CGI action looks like this:

output "hello, world!"

Adding a predicate:

-- if URL matches /hello, then output "hello, world!"
h |/ "hello" *> output "hello, world!"

More examples:

-- if URL contains /hello, output "woot, it works!", otherwise check for
-- /foo
(h |/ "hello" *> output "woot, it works!") <|> (h |/ "foo" *> output "foo")

As you can see, the |/ combinator matches current token against it's right operand. h is a special predicate that matches anything, it is used to begin a string of combinators.

One can also match against

  • URL parameters,
  • HTTP methods,
  • and also convert token into a variable which is an instance of Read

There's also an API which is believed to be more human-readable.

2.2 Extending UrlDisp

The examples given above are not very interesting since one wants to interact with outside world. Let's see how to extend UrlDisp to handle database access.

Wrapping UrlDisp around a ReaderT will do the trick.

{-# LANGUAGE FlexibleInstances, ScopedTypeVariables #-}
import Network.UrlDisp
import Database.HDBC
import Database.HDBC.ODBC
import Control.Exception (bracket)
import Network.CGI
import Network.CGI.Monad

instance MonadCGI (ReaderT Connection (CGIT IO)) where
    cgiAddHeader n v = lift $ cgiAddHeader n v
    cgiGet = lift . cgiGet

-- once a request to "/db/" is sent,
-- execute an SQL query and show it's results
main :: IO ()
main = bracket (connectODBC connStr) disconnect
    (\c -> runCGI $ (flip runReaderT) c $ evalUrlDisp $
        ((h |/ "db" *> m) <|> output "not found"))

m :: UrlDisp (ReaderT Connection (CGIT IO)) CGIResult
m = do
    v <- lift ask >>= \c -> liftIO (quickQuery' c "select * from ..." [])
    output $ show v