[Ur] Confusing error message when using effectful javascript ffi function in onload handler

[Daniel Patterson]

I still think the message is misleading. Because seeing that message
(and not having source information to track down what was causing it),
I removed _all_ links from the application - ie, there were no calls
to "url", and no "<a>" tags. So it isn't that I'm using a link when I
should be using a form - it is that a page that is accessible via a
url is causing effects.

If the error was more along the lines of "A handler accessible via GET
requests could cause side effects - try accessing it only via forms,
or removing it from the list of exported handlers", I would have had a
lot easier time figuring out what was wrong. Of course, being able to
specify what handler (ie, the handler at 101:0, or even better,
sourceloc and name) would be even better.

On Fri, Jun 14, 2013 at 12:53 PM, Adam Chlipala <adamc at csail.mit.edu> wrote:
> I agree that it would be nice to get the proper source location information
> propagated into that error message.  However, I don't think the message
> itself is so misleading.  Presumably your 'onload' handler is on a page that
> can be reached via a GET request to a link.  The HTTP standard or some
> related advice specifies that GET requests should not have side effects, and
> Ur/Web is detecting that your [Js.init] function might actually cause a side
> effect upon loading of the link.  From a user's perspective, there is little
> practical difference between "fetching URL causes a side effect" and
> "fetching URL returns a page whose JavaScript immediately causes a side
> effect."
>
> I hope the Ur/Web manual makes clear that 'benignEffectful' is for functions
> with only page-local side effects, while 'effectful' is for functions that
> may cause persistent side effects.  Does your [Js.init] really belong in the
> second category?  If so, then the compiler seems to me to be making a
> reasonable inference that you aren't following the rules about GET requests
> not causing persistent side effects.
>
>
> On 06/13/2013 08:14 PM, Daniel Patterson wrote:
>>
>> I have an ffi function declared like:
>>
>> ffi js
>> jsFunc Js.init=init
>> effectful Js.init
>>
>> I have it in my body onload handler as:
>>
>> Js.init ...
>>
>> And the error that it produces is:
>>
>> :0:0: (to 0:0) A link () could cause side effects; try implementing it
>> with a form instead
>> make: *** [app] Error 1
>>
>>
>> Which is confusing because:
>>
>> a. it has no source location information, so I have no idea where the
>> error is occuring and
>>
>> b. it is not caused by a link.
>>
>> The problem goes away if I either don't say that it is effectful, or
>> say it is benignEffectful, but I'm not sure why this is - because it
>> seems like onload handlers should be able to cause effects...
>>
>
>
> _______________________________________________
> Ur mailing list
> Ur at impredicative.com
> http://www.impredicative.com/cgi-bin/mailman/listinfo/ur