Re: [web-devel] Type-safe URL handling

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 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

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