UrlDisp
What is UrlDisp
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.
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.
Usage examples
Basics
A regular CGI action looks like this:
output "hello, world!"
This one replies to all requests with "hello, world!"
.
One can add a predicate to make things more interesting:
h |/ "hello" *> output "hello, world!"
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:
a <|> b <|> c
is equivalent to
((a <|> b) <|> c)
Anyway, code using UrlDisp shouldn't depend on this property.
To introduce an "and" in your rule, apply (|/)
successively, as in:
h |/ "foo" |/ "bar"
Generally, other combinators will correspond to "and" and bind stronger than "or". For example:
h |// "GET" |/ "foo" |/ "bar" |? ("cmd", "foo") *> output "hello" <|> endPath |? ("cmd, "bar") *> output "goodbye"
Will behave as follows:
- all GET requests to
/foo/bar
(and anything that follows) and parametercmd
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.
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:
{-# 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 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 ..."