Difference between revisions of "UrlDisp"
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(added a "real world" (tm) usage example)
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output $ show v
output $ show v
Revision as of 20:40, 4 February 2009
What is UrlDisp
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.
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.
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!"
-- 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.
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.
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