[Haskell-cafe] PROPOSAL: Web application interface

Mark Lentczner markl at glyphic.com
Mon Jan 18 01:54:16 EST 2010

I like this project! Thanks for resurrecting it!

Some thoughts:

Methods in HTTP are extensible. The type RequestMethod should probably have a "catchall" constructor
	| Method B.ByteString

Other systems (the WAI proposal on the Wiki, Hack, etc...) have broken the path into two parts: scriptName and pathInfo. While I'm not particularly fond of those names, they do break the path into "traversed" and "non-traversed" portions of the URL. This is very useful for achieving "location independence" of one's code. While this API is trying to stay agnostic to the web framework, some degree of traversal is pretty universal, and I think it would benefit being in here.

The fields serverPort, serverName, and urlScheme are typically only used by an application to "reconstruct" URLs for inclusion in the response. This is a constant source of bugs in many web sites. It is also a problem in creating modular web frameworks, since the application can't be unaware of its context (unless the server interprets and re-writes HTML and other content on the fly - which isn't realistic.) Perhaps a better solution would be to pass a "URL generating" function in the Request and hide all this. Of course, web frameworks *could* use these data to dispatch on "virtual host" like configurations. Though, perhaps that is the provenance of the server side of the this API? I don't have a concrete proposal here, just a gut that the inclusion of these breaks some amount of encapsulation we'd like to achieve for the Applications.

The HTTP version information seems to have been dropped from Request. Alas, this is often needed when deciding what response headers to generate. I'm in favor of a simple data type for this:
	data HttpVersion = Http09 | Http10 | Http11

Using ByteString for all the non-body values I find awkward. Take headers, for example. The header names are going to come from a list of about 50 well known ones. It seems a shame that applications will be littered with expressions like:

	[(B.pack "Content-Type", B.pack "text/html;charset=UTF-8")]
Seems to me that it would be highly beneficial to include a module, say Network.WAI.Header, that defined these things:

	[(Hdr.contentType, Hdr.mimeTextHtmlUtf8)]

Further, since non-fixed headers will be built up out of many little String bits, I'd just as soon have the packing and unpacking be done by the server side of this API, and let the applications deal with Strings for these little snippets both in the Request and the Response.

For header names, in particular, it might be beneficial (and faster) to treat them like RequestMethod and make them a data type with nullary constructors for all 47 defined headers, and one ExtensionHeader String constructor.

Finally, note that HTTP/1.1 actually does well define the character encoding of these parts of the protocol. It is a bit hard to find in the spec, but the request line, status line and headers are all transmitted in ISO-8859-1, (with some restrictions), with characters outside the set encoded as per RFC 2047 (MIME Message Header extensions). Mind you, I believe that most web servers *don't* do the 2047 decoding, and only either a) pass the strings as ISO-8859-1 strings, or decode that to native Unicode strings.

	- Mark

Mark Lentczner
IRC: mtnviewmark

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