[web-devel] Type-safe URL handling

Tue Mar 16 11:24:20 EDT 2010

Creating a number of datatypes (each for every "component") is interesting because of two things:

* You can only produce valid URLs (you get that for free)
* You can provide an individual component as a library (e.g. a hackage package)

I guess that all these approaches can be made equivalent, it's mostly a matter of style and preference. What I like about my approach is that it's very light on Template Haskell: the only TH comes from the regular library and is well-tested.

-chris

On 16 mrt 2010, at 16:16, Michael Snoyman wrote:

> These approaches will definitely work, but I'm worried that creating a whole set of datatypes to represent URLs is overkill. In Yesod (I'm sure many of you have seen) I use quasi-quoting for defining the resources, like such:
> 
> [$mkResources|
> /user:
>     GET: userList
> /user/find/#userid
>     GET: userFind
> /user/name/$username
>     GET: userName
> |]
> 
> And so on and so forth. I don't think defining UserRoute adds much, besides making the job of the library writer a little bit easier by pushing the work off to the user. I think the six lines above succinctly:
> 
> * define the valid routes
> * define the data types for arguments
> * define the appropriate mapping to handler functions for each request method
> 
> Chris mentioned earlier to me the idea of using quasi-quoting on the link generation side, perhaps like:
> 
> [$link|/user/find/6]
> 
> I think the only piece of the puzzle missing to combine these two together is to have mkResources output something along the lines of:
> 
> data RoutePiece = StaticPiece String | IntPiece | StringPiece
> _validRoutes :: [[RoutePiece]]
> _validRoutes =
>   [ [StaticPiece "user"]
>   , [StaticPiece "user", StaticPiece "find", IntPiece]
>   , [StaticPiece "user", StaticPiece "name", StringPiece]
>   ]
> 
> Now if you write
> 
> [$link|/user/find/michael]
> 
> link can look up in _validRoutes that there is no matching route and complain at compile time.
> 
> Advantages: less typing by the user.
> Disadvantages: we'll have to restrict the data types allowed, but in practice I think people will usually want only strings and ints anyway. Also, this approach is more complex.
> 
> Michael
> 
> On Tue, Mar 16, 2010 at 7:04 AM, Chris Eidhof <chris at eidhof.nl> wrote:
> I have the feeling it adds a lot of complexity. I agree with you that, if you want modularity, your components should only provide relative URLs and need to be parameterized over how to build an absolute URL. I didn't think of that problem, and using a custom monad transformer is definitely a solution.
> 
> However, I'm always hesitant to build up stacks of monad transformers, it adds a lot of complexity. I would rather use something like typeclass, but I'm not sure yet how to do that.
> 
> -chris
> 
> On 16 mrt 2010, at 14:29, Jeremy Shaw wrote:
> 
> > Hello,
> >
> > It looks nearly identical, but without the URLT monad transformer.
> >
> > Instead of ToURL I have the class:
> >
> > class AsURL a where
> >     toURLS :: a -> ShowS
> >     fromURLC :: Consumer String (Failing a)
> >
> > With is basically the same. Except toURLS returns a ShowS instead of [String]. fromURLC consumes a list of [String]. These functions are wrapped up to provide:
> >
> > toURL :: (AsURL a) => a -> String
> > fromURL :: (AsURL a) => String -> Failing a
> >
> > I do not have generics based url printing/parsing, but there is no reason it could not be added. I do have template haskell based code though.
> >
> > http://src.seereason.com/urlt/URLT/TH.hs
> >
> > The thing you don't have is the URLT monad transformer:
> >
> > http://src.seereason.com/urlt/URLT/Base.hs
> >
> > Here is why you want it. Imagine you write an image gallery library:
> >
> > data ImageURL = Upload | ViewImage Int
> >
> > when you call toURL, you are going to get urls like, /Upload, /ViewImage/1, etc.
> >
> > Now let's say I try to use your library in my application. So at first I try:
> >
> > data MyApp = Upload | FooBar
> >
> > But when a URL comes in, how do I know if I should decode it as MyApp or ImageURL? Do I try both and see which one succeeds? Except we both have a constructor Upload, so both will succeed. There is no way to tell with Upload the path "/Upload" is referring to.
> >
> > So now I try:
> >
> > data MyApp = Upload | FooBar | Images ImageURL
> >
> > now I know that all incoming urls are decoded as MyApp. But there is still a problem. In my code I could write:
> >
> >  toUrl (Images (ViewImage 1))
> >
> > but in your library code, you don't know anything about the Images constructor. So you just call,
> >
> > toURL (ViewImage 1)
> >
> > which generates /ViewImage/1 instead of the required /Images/ViewImage/1.
> >
> > What I need is someway to tell your library code what prefix to add at the beginning. That is exactly what the URLT monad does. It just holds a function that adds a prefix to the URL.
> >
> > so in your library you have:
> >
> > image :: ImageURL -> URLT ImageURL m ()
> > image Upload =
> >      do ...
> >           u <- showURL (ViewImage n)
> >           ...
> > image (ViewImage num) = ...
> >
> > Instead of calling toURL, it calls showURL, which adds the context to the URL and then calls toURL on it.
> >
> > And in my code I have:
> >
> > myApp :: MyAPP -> URLT MyApp m ()
> > mpApp Upload = ...
> > myApp FooBar = ...
> > myApp (Images subURL) = nestURL Images $ images subURL
> >
> > the 'nextURL Images' adds the Images context to the URLT environment. It can be used to nest multiple levels if needed:
> >
> >  nestURL A $ nestURL B $ nestURL Images $ showURL (ViewImage 1)
> >
> > would get turned into something like:
> >
> >  "/A/B/Images/ViewImage/1"
> >
> > What do you think?
> >
> > - jeremy
> >
> >
> > On Tue, Mar 16, 2010 at 3:52 AM, Chris Eidhof <chris at eidhof.nl> wrote:
> > Hey everyone,
> >
> > I just wrote down some of my ideas about type-safe URL handling on github, it's at http://gist.github.com/333769
> >
> > I think it's similar to what Jeremy is doing with his urlt package [1].
> >
> > -chris
> >
> > [1]: http://src.seereason.com/~jeremy/SimpleSite1.html
> >
> > _______________________________________________
> > web-devel mailing list
> > web-devel at haskell.org
> > http://www.haskell.org/mailman/listinfo/web-devel
> >
> 
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