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

[Daniel Patterson]

That looks great - thanks!

Adam Chlipala <adamc at csail.mit.edu> writes:
> I've pushed a small changeset with a new error message, inspired by your 
> suggestion.
>
> By the way, I expect the error message you reported originally was 
> especially confusing because you must have configured an empty URI for 
> your application's main page handler.  The "()" part of your message 
> would have said, e.g., "(main)" if some nonempty name were configured 
> for that handler.  My error message tweak is also aimed at explaining 
> this part of the message better.
>
> On 06/14/2013 01:27 PM, Daniel Patterson wrote:
>> 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...
>>>>        
>
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