[lldb-dev] Renaming lldb_private::Error

Wed May 10 18:28:56 PDT 2017

Yes, this is just the rename.

Regarding the naming, if you call it ErrorAnd, or WithError, or anything
that includes the word error, you are implying that something actually went
wrong.  I don't think that's the intended use case, or at least not what I
have in mind (and from previous conversations on the list, I don't think
what Jim had in mind either).

If we're going to say that something does not need to be handled, I don't
know if we should be calling it an error at all.  By definition, we should
assert that errors must be handled, so the converse is that if it doesn't
need to be handled, it's not an error.

But if it does need to be handled (and as such is called an error), then
I'm not sure if it makes sense to say there's also a value.  So ErrorOr, or
Expected seems to convey that meaning in the only way possible.  If you
don't get the thing you're expected to get, you need to handle the error.

But it seemed like what we were talking was more of a way to provide
diagnostic information about a long process that you could return alongside
a result.  And if you don't get one, you don't necessarily care.  So it's
like one step down in the expectation chain from Expected.  Possible<T>
maybe?

I would expect an interface similar to Optional<T>, but with a way to get
error *like* diagnostic information or messages that the user could ignore
if they wanted to.

On Wed, May 10, 2017 at 6:09 PM Lang Hames <lhames at gmail.com> wrote:

> Cool. This is just the rename portion, right?
>
> Sorry I didn't respond to your last message too.
>
> I suppose, but I'm not sure ErrorAnd captures the intended meaning very
>> well.  In any case, that's not super important at this stage since this
>> isn't on the immediate horizon.
>
>
> Do you just mean that ErrorAnd isn't an especially nice name? I wasn't
> entirely sure what make_status<T>(...) was supposed to do so I assumed it
> was to create a pair of an Error and a T. If that's the case,
> make_with_error<T>(T, Error) (and WithError<T>) might be better names?
>
> Cheers,
> Lang.
>
>
> On Tue, May 9, 2017 at 8:58 PM, Zachary Turner <zturner at google.com> wrote:
>
>> I'm probably going to be looking at submitting this this week, more
>> likely sooner rather than later.  If I can get it all working hopefully
>> even tomorrow.
>>
>> On Mon, May 1, 2017 at 5:49 PM Zachary Turner <zturner at google.com> wrote:
>>
>>> I suppose, but I'm not sure ErrorAnd captures the intended meaning very
>>> well.  In any case, that's not super important at this stage since this
>>> isn't on the immediate horizon.
>>>
>>> On Mon, May 1, 2017 at 5:43 PM Lang Hames <lhames at gmail.com> wrote:
>>>
>>>> Hi Zachary,
>>>>
>>>> ... Then instead of Expected<T> you could have WithDiagnostics<T> that
>>>>> enforces the proper semantics.
>>>>
>>>>
>>>> You mean something like an ErrorAnd<T>? Chris Bieneman floated that
>>>> idea for some libObject code but we haven't got around to implementing it.
>>>> If it were generically useful we could do something like that.
>>>>
>>>> Cheers,
>>>> Lang.
>>>>
>>>>
>>>> On Mon, May 1, 2017 at 5:36 PM, Zachary Turner <zturner at google.com>
>>>> wrote:
>>>>
>>>>> Is there any chance of introducing something like make_status<T>()
>>>>> into llvm/Error.h, that constructs the llvm::Error in such a way that it
>>>>> still interoperates nicely with everything else?  Then instead of
>>>>> Expected<T> you could have WithDiagnostics<T> that enforces the proper
>>>>> semantics.
>>>>>
>>>>> On Mon, May 1, 2017 at 5:33 PM Zachary Turner <zturner at google.com>
>>>>> wrote:
>>>>>
>>>>>> On Mon, May 1, 2017 at 5:27 PM Jim Ingham <jingham at apple.com> wrote:
>>>>>>
>>>>>>>
>>>>>>> > On May 1, 2017, at 4:52 PM, Zachary Turner <zturner at google.com>
>>>>>>> wrote:
>>>>>>> >
>>>>>>> > Yea, grouping the error and the result together is one of the most
>>>>>>> compelling features of it.  It's called Expected<T>, so where we would
>>>>>>> currently write something like:
>>>>>>> >
>>>>>>> > int getNumberOfSymbols(Error &Err) {}
>>>>>>> >
>>>>>>> > or
>>>>>>> >
>>>>>>> > Error getNumberOfSymbols(int &Count) {}
>>>>>>> >
>>>>>>> > You would now write:
>>>>>>> >
>>>>>>> > Expected<int> getNumberOfSymbols() {
>>>>>>> >    if (foo) return 1;
>>>>>>> >    else return make_error<DWARFError>("No symbols!");
>>>>>>> > }
>>>>>>> >
>>>>>>> > and on the caller side you write:
>>>>>>> >
>>>>>>> > Error processAllSymbols() {
>>>>>>> >   if (auto Syms = getNumberOfSymbols()) {
>>>>>>> >     outs() << "There are " << *Syms << " symbols!";
>>>>>>> >   } else {
>>>>>>> >     return Syms.takeError();
>>>>>>> >     // alternatively, you could write:
>>>>>>> >     // consumeError(Syms.takeError());
>>>>>>> >     // return Error::success();
>>>>>>> >   }
>>>>>>> > }
>>>>>>> >
>>>>>>>
>>>>>>> Interesting.
>>>>>>>
>>>>>>> This pattern doesn't quite work for fetching symbols - maybe that
>>>>>>> really is more suitable as a Status than an Error.  After all, number of
>>>>>>> symbols == 0 is not necessarily an error, there just might not have been
>>>>>>> any symbols (e.g. a fully stripped main); and I'm going to work on whatever
>>>>>>> symbols I get back, since there's nothing I can do about the ones that
>>>>>>> didn't make it.  I just want to propagate the error so the user knows that
>>>>>>> there was a problem.
>>>>>>>
>>>>>>> Jim
>>>>>>>
>>>>>>
>>>>>> Sure, that was just a made up example.  You could imagine that being
>>>>>> some private function deep in the implementation details of a symbol
>>>>>> parser, where you've got some header that's supposed to be N bytes, and
>>>>>> getNumberOfSymbols() seeks to offset 42 and reads a 4 byte value and
>>>>>> returns it, but the function sees that there's only 40 bytes in the header,
>>>>>> so it's not that there's no symbols, it's that something is seriously
>>>>>> messed up.
>>>>>>
>>>>>> In that case you could return an error such as this.
>>>>>>
>>>>>> Of course, the person who called this function can either propagate
>>>>>> it, deal with it some other way and mask it, or whatever.  Mostly I was
>>>>>> just trying to show what the syntax looked like for grouping return values
>>>>>> with errors.
>>>>>>
>>>>>
>>>>
>
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