m (grammar fix)
Latest revision as of 07:30, 26 May 2009
 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 (e.g., /foo/bar/ or /bar/baz/quix) is tokenized (turned into a list of "URL fragments", e.g. ["foo","bar"]) 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 the order given (<|> associates to the left). The matching algorithm is backtracking.
 2 Usage examples
 2.1 Basics
A regular CGI action looks like this:
This one replies to all requests with "hello, world!".
One can add a predicate to make things more interesting:
This one will greet people only if the URL starts with "/hello". It will give a 404 error page otherwise.
Such "if-then" clauses can be combined using "or" -- (<|>), which associates to the left, so
is equivalent to
Anyway, code using UrlDisp shouldn't depend on this property.
To introduce an "and" in your rule, apply (|/) successively, as in
Generally, other combinators will correspond to "and" and bind stronger than "or". For example:
<|> endPath |? ("cmd, "bar") *> output "goodbye"
Will behave as follows:
- all GET requests to /foo/bar (and anything that follows) and parameter "cmd" set to "foo" will output "hello"
- requests with empty path and parameter cmd set to "bar" will output "goodbye"
- other requests will trigger a 404 page
As you can see, the |/ combinator matches current token against its 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 the outside world. Let's take a look at how to extend UrlDisp to handle database access.
Wrapping UrlDisp around a ReaderT will do the trick.
import Control.Exception (bracket)
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 its 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 queryText )
output $ show v
-- you will have to provide this one
queryText = "select * from ..."