[web-devel] lazy RESTful Content

Greg Weber greg at gregweber.info
Thu Jan 6 22:50:30 CET 2011

Thanks for the detailed response. My application essentially has permalink
routes that end up rendering the same html as a non-permalink route, but the
permalink route does indicate some existing state. Right now it is coded so
the permalink route will grab the extra state over a separate JSON request.
The problem being that the original html request also gathered the JSON. I
suppose my current setup is not truly RESTful and the html response should
render the state instead of using an extra JSON call, although I do find
this a convenient technique. One way or another I should be able to change
things to avoid the extra IO.

On Thu, Jan 6, 2011 at 1:20 PM, Michael Snoyman <michael at snoyman.com> wrote:

> On Thu, Jan 6, 2011 at 5:29 AM, Greg Weber <greg at gregweber.info> wrote:
> > lazy and REST seem to go together :)
> > Perhaps this will expose my ignorance of laziness/strictness and isn't at
> > all Yesod specific. I have a handler that can serve html or json. When an
> > html request comes in it still executes the json code. I assume this is
> > because the json code uses liftIO. What is the strategy for avoiding
> > executing the json code?
> Let me start with the theoretical response: if you have both an HTML
> and JSON representation for a resource, they should be giving
> *exactly* the same data, just in a different format. Therefore, there
> should just be a pure function applied to some data to generate the
> JSON version, and thus no extra processing should be performed (due to
> laziness) when giving an HTML response. Therefore, your problem
> doesn't exist ;).
> Of course, in theory, theory and practice are the same, but in
> practice they're not. Sometimes it *is* necessary to provide slightly
> different data, and producing that data will require some IO action.
> There are a few approaches:
> * What you have right now: run actions for both the HTML and JSON
> versions, and let Yesod pick the correct result afterward. This is
> slightly inefficient, but easy to program.
> * Provide two different resources, one providing only HTML, the other
> only JSON. Depending on how different your representations are, this
> might make sense.
> * If the actions can be run inside the IO monad and don't require a
> full Handler monad, there is some tricky stuff you can do involving
> enumerators. This is not often necessary.
> In theory this is an area where Yesod could provide a nicer interface
> for users. In practice, the problem doesn't come up very often, and
> therefore I don't have enough experience to know what a good interface
> would be. I'd like to hear (even in private) a more detailed
> explanation of what you're trying to accomplish here.
> Michael
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