UrlDisp

Latest revision as of 07:30, 26 May 2009

[edit] 1 What is UrlDisp

[edit] 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.

[edit] 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.

[edit] 2 Usage examples

[edit] 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:

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.

[edit] 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.