Use ".then(null, Cu.reportError);" with Promise.jsm

[Mark Hammond]

On 12/08/2013 11:28 PM, David Rajchenbach-Teller wrote:
> On 8/10/13 5:04 AM, Mark Hammond wrote:
>> On 9/08/2013 8:32 PM, David Rajchenbach-Teller wrote:
>>> Furthermore, I *strongly* suggest that we r- any promise chain that does
>>> not report errors, unless there is a comment explaining why we do not.
>>
>> This concept of a "promise chain" escapes me.  I understand that
>> conceptually, promise code should look like:
>>
>>    something.then(step1)
>>             .then(step2)
>>             .then(step3)
>>             .then(null, Cu.reportError)
>>
>> One nice promise chain and the final .then() looks reasonable.  But
>> every time I try and use promises, they actually end up looking like:
>>
>>    something.then(result => {
>>      step1.then(result => {
>>        step2.then(result => {
>>          step3.then(result => {
>>          });
>>        });
>>      });
>>    })
>>
>> in that case we have 4 "promise chains".  Thus, we need 4 extra
>> .then(null, Cu.reportError) calls, making the code much larger and
>> harder to follow.
>
> That's actually a single chain that promises to return the result of
>       step3.then(result => { ... })
> so, with respect to error reporting, fine with me.

But isn't it true that exceptions in any of those bodies would remain 
unreported?  ie, if the inner was changed to:

           step3.then(result => {
             throw "oh no";
           });

wouldn't it need to be:

           step3.then(result => {
             throw "oh no";
           }).then(null, Cu.reportError);

to see that exception?  And thus each of the .then() calls above needs 
the trailing .then()?  If so, I'm not sure how the original version 
could be "fine" in this regard?

[If nothing else though, this thread shows that it's extremely hard to 
write code around this that is "obviously correct"]

Cheers,

Mark