spookylukey / ella

Ella

Ella is an attempt to create a simple, CGI based web framework in Haskell. It is mainly written to help me learn Haskell, and do so in the real, messy world of HTTP and HTML. I also didn't like the standard Haskell CGI module for various reasons, though, where possible, I reuse its code.

The code will evolve as my needs evolve, so I can't guarantee any API compatibility. I don't think that anyone else is using this code but me...

Features

I've been inspired by Django, but this framework is far less ambitious in scope. It does not include an ORM or anything like that. Some features of Ella, in no particular order:

• Explicit request and response objects that are passed around (like Django, and unlike the CGI monad).

• Strongly typed extensible dispatching system. This means that you can use a single type signature on a function to cause the URL parsing mechanism to match and parse only data which can be converted to the correct type e.g.:

  -- routes definition:
  
  routes = [
           -- more routes here ...
           "posts/" <+/> anyParam </+> "edit/"  //-> editPost $ [loginRequired]
           ]
  
  editPost :: Int -> Request -> IO Response
  

  The type signature on editPost means that "posts/123/edit/" will be parsed and will pass the integer 123 and the request object to editPost, but "posts/abc/edit/" will not match. anyParam is based on a type class Param, so you can extend it to match your own types.

  The major downside to this approach is that 'DRY' URL reversing is impossible. You have to create permalink functions that will necessarily duplicate information found in the 'routes' definition.

• 'View processors' - inspired by Django middleware, these allow pre and post processing for request handling, which can be installed globally, or on a function by function basis (as in the 'loginRequired' processor in the example above). View processors for signed cookies and CSRF protection are included.

• Proper support for character sets. At the moment I've only added defaults for UTF8, but this is already much better than the broken support that the CGI module has (it packs bytestrings directly into Strings without attempting to convert, which works for latin1 only). Output is ByteStrings.

Example code

A small but complete example, which I have actually used, is a simple web app for asking people to sign up to a mailing list via clicking on URLs an e-mail.

http://bitbucket.org/spookylukey/mailinglistconfirm/src/tip/src/ConfirmCgi.hs

A much bigger but incomplete example is my blog code:

http://bitbucket.org/spookylukey/haskellblog/

Some failings:

• Assumes use of the IO monad in lots of places - it would be probably be better to generalise to MonadIO or something.
• 'DRY' URL reversing impossible
• CGI only at the moment. It might be possible to adapt for FastCGI etc, I don't know.
• No decent form handling or generating code. I've experimented with defining widgets, and I've used them in real code, but the advantages over raw HTML are not compelling. I have looked at other form handling libraries and haven't found anything that satisfies my needs. 'Formlets' is neat, but far too inflexible and simplistic, especially with the generated HTML.