[web-devel] Type-safe URL handling
michael at snoyman.com
Tue Mar 16 11:16:42 EDT 2010
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:
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
Chris mentioned earlier to me the idea of using quasi-quoting on the link
generation side, perhaps like:
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]]
[ [StaticPiece "user"]
, [StaticPiece "user", StaticPiece "find", IntPiece]
, [StaticPiece "user", StaticPiece "name", StringPiece]
Now if you write
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.
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
> 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.
> 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
> > 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 .
> > -chris
> > : http://src.seereason.com/~jeremy/SimpleSite1.html
> > _______________________________________________
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> > web-devel at haskell.org
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