[web-devel] Type-safe URL handling

Jeremy Shaw jeremy at n-heptane.com
Mon Mar 29 14:37:12 EDT 2010

On Mon, Mar 29, 2010 at 12:16 PM, Michael Snoyman <michael at snoyman.com>wrote:

> The reason I'm unexcited is that I never would have dreamed of defining my
> routes that way. I don't feel like drawing out this point too much, because
> you clearly *would* define your routes that way. However, just to draw the
> distinction in how I would do things differently, I'll use an example of
> mine that you quoted earlier:
> instance Yesod PB where
>     resources = [$mkResources|
> /:
>     GET: indexHandler
> /entries/$entryId:
>     GET: entry
> /entries/$entryId/$filename:
>     GET: media
> /feed:
>     GET: feed
> If I were to convert this to a datatype, it would be:
> data PBRoutes = Home | Entry String | File String String | Feed

You do still have nested data-types here. Namely the String. In this case it
is trivial to handle by hand, but it does pose a problem for things like TH
and Regular. That is why I had PathInfo in the original code in the first
place.. I couldn't figure out how to write the TH code with out it.

I simply wouldn't nest a datatype inside any of the constructors. I
> understand that you want to do this in some circumstances, but I would
> simply "duplicate" the parsing code for the Entry and File constructors,
> since I find that parsing code trivial. In particular:
> parsePB ["entries", eid] = Entry eid
> parsePB ["entries", eid, filename] = File eid filename
> I don't see a need for providing a sophisticated parser.

If you are going to duplicate the code instead of calling fromPathSegments,
then you don't really need PathInfo at all, right? The current code is
designed so that you are not forced to use PathInfo.

We have:

data Site = Site { ...
                           , parsePathSegments  :: [String] -> Either String

And you can do:

Site { parsePathSegments = parsePB }

The only real reason to have PathInfo is to build composable parsers as far
as I can tell. So, I guess maybe you are suggesting that PathInfo should be
a separate package? I don't see a big win here since we will depend on
parsec 2 anyway, and since web-routes-wai would need to depend on it anyway
to provide the wai related functions that do use PathInfo..

I did add a new parser combinator though:

patternParse :: ([String] -> Either String a) -> URLParser a

so you can do:

fromPathSegments = patternParse parsePB

patternParse consumes all the remaining segments and passes them to parsePB.

> So, I've thought about the syntax for this, and I have this idea in mind.
> $(createRoutes MyRoutes [$parseRoutes|
> /:
>   name: Home
>   methods: [GET]
> /user/#userid:
>   name: User
>   methods: [GET, PUT, DELETE]
> /static:
>   name: Static
>   subsite: StaticRoutes
>   dispatch: staticRoutes
> |])
> This would generate a datatype:
> data MyRoutes = Home | User Integer | Static StaticRoutes

So your idea is to generate the data-type from the routes, rather than try
to map the routes onto an existing datatype?

Your approach sounds easier. The advantage of the latter is that you could
change the look of the url with out having to go change all your code that
uses the URL type.. Not sure how doable the latter is though.

Handler functions would be getHome, getUser, putUser, deleteUser. Static
> would be a pluggable subsite; I'd have to play around with the syntax of
> that a bit. Also, this will allow *any* type of application, not just wai (I
> want this to be as general as possible).

right. I see no reason for it to be wai specific.

Speaking of wai, there is a bug in wai-extra in SimpleServer. It does not
put a space between the status code and the status message

~/n-heptane/projects/haskell/web-routes $ curl -v
* About to connect() to localhost port 3000 (#0)
*   Trying ::1... Connection refused
*   Trying connected
* Connected to localhost ( port 3000 (#0)
> GET /MyHomeoeu HTTP/1.1
> User-Agent: curl/7.19.5 (i486-pc-linux-gnu) libcurl/7.19.5 OpenSSL/0.9.8g
zlib/ libidn/1.10 libssh2/0.18
> Host: localhost:3000
> Accept: */*
< HTTP/1.1 404Not Found
* no chunk, no close, no size. Assume close to signal end

Note the second to last line.

- jeremy
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