[PATCH] D28569: Remove all variants of DWARFDie::getAttributeValueAs...() that had parameters that specified default values.

[Adrian Prantl via llvm-commits]

> On Jan 11, 2017, at 12:07 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Wed, Jan 11, 2017 at 12:00 PM Adrian Prantl <aprantl at apple.com> wrote:
>> 
>> > On Jan 11, 2017, at 11:33 AM, David Blaikie <dblaikie at gmail.com> wrote:
>> >
>> >
>> >
>> > On Wed, Jan 11, 2017 at 11:08 AM Greg Clayton <clayborg at gmail.com> wrote:
>> >>
>> >>> On Jan 11, 2017, at 10:36 AM, David Blaikie <dblaikie at gmail.com> wrote:
>> >>>
>> >>> Would it make sense to push the value retrieval down into DWARFFormValue to separate getting the value from a DIE from getting the value into some particular type - they seem pretty orthogonal & it's odd that DWARFDie handles the type handling side of that rather than an attribute/form abstraction.
>> >>>
>> >>> Perhaps it'd be a bit abusive, but we could even overload op[](dwarf::Attribute) on DWARFDie for easy map-like attribute lookup. Maybe 'find' would be a better name, though.
>> >>>
>> >>> I'm OK omitting the 'attribute' part of the name entirely if everyone's OK with overloading in this context & if the callsites tend to be pretty self documenting. (mostly if you're searching for a particular attribute you've probably got the attribute name in a literal at the callsite anyway, right?)
>> >>>
>> >>> So code like this, perhaps:
>> >>>
>> >>>   x = Die.find(DW_AT_name).asString();
>> >>>
>> >>> or perhaps:
>> >>>
>> >>>   x = asString(Die.find(DW_AT_name));
>> >>>
>> >>> (so that find/findAttr/[] whatever we call it can continue to return Optional<DWARFFormValue> which asString can pass along as NOne if it's passed None (or passed a non-string value), and otherwise decompose into a string, etc)
>> >>>
>> >>
>> >> We basically have find already in:
>> >>
>> >> Optional<DWARFFormValue> DWARFDie::getAttributeValue(dwarf::Attribute);
>> >>
>> >> We can easily rename it.
>> >>
>> >> If find returns an optional then your first code example would need to be:
>> >>
>> >> x = Die.find(DW_AT_name).getValueOr(DWARFFormValue()).asString();
>> >>
>> >> The second form could be made to work but it doesn’t look as natural IMHO.
>> >>
>> >> Because of the first is awkward and the second seems less clear than just asking the DIE for the value I do think there is value in having the calls on DWARFDie. Let me know what you think.
>> >
>> > I'd still probably go towards the second - for orthogonality of features. Means a DWARFFormValue can be passed around and its value retrieved later, etc.
>> >
>> > But would be good to hear from Adrian too.
>> 
>> No matter which version we end up using I would expect any heavy users of this to re-wrap it in something more specialized again so
>> 
>>   x = asString(Die.find(DW_AT_name).getValueOr(DWARFFormValue()));
>> 
>> becomes
>> 
>>   x = getAttrAsOr<StringRef>(Die, DW_AT_name, "")
> 
> Not sure I follow. I would figure:
> 
>   x = asString(Die.find(DW_AT_name)).getValueOr("");
> 

Ok. That is different from how I understood the proposal, since asString here takes and returns an Optional. This also answers my other question, since we then can do getValueOr to unwrap the optional result of asString.


> to be pretty readable/not be worth having wrappers, but I can see what you mean.

It's not awesome :-)
The order of execution/reading is: 
  2            1                 3  
  asString(Die.find(DW_AT_name)).getValueOr("");
Sure it's readable, but it takes a second look to correctly parse it mentally.

> I'm not sure if there would be a nice way to keep the orthogonality (so that asString/asAddress/etc didn't all need to support/reimplement default handling) while making it much more terse. It seems like there are 3 different things happening there.

Would it make sense to provide both variants?
I think it would be fine to compose the terse variant out of the generic orthogonal building blocks. 

> 
> But, yes, if it comes up often enough to be annoyingly verbose - sacrificing the orthogonality (by creating/paying in the form of duplication) for convenience by writing asFoo functions that handle the defaulting - asString(Die.find(DW_AT_name), "") - same as we had before, except now the conversion is orthogonal to the lookup so adding "bool recurse"/"find_recursively" doesn't explode the API surface area along another axis.
>  
>> to reclaim some readability again. From that point of view I don't think the decision of where asString lives is very important. I am wondering how to pass any default value other than an empty value into getValueOr(DWARFFormValue Default)? Is it easy to construct an on-the-fly DWARFFormValue for e.g., a string?
> 
> I don't think we'd want people to construct DWARFFormValues to represent values that weren't present in the DWARF as a main/common means of handling situations like this. That seems to be the wrong layer - I think at the point you want an appropriately typed value, then you collapse the optionality and values pace into that type if you want to, until then it remains Optional.
>  

Yes, I think I was misguided by the 
  asString(Die.find(DW_AT_name).getValueOr(DWARFFormValue()))
example.

-- adrian
>> 
>> >
>> >
>> > Another option that Adrian might have thoughts on, would be to remove the Optional wrapper around the DWARFFormValue from getAttributeValue. It's not something I'd be totally happy with - Optional's nice and explicit about the fact that there might not be a value, but if DWARFFormValue already has a "not present" state in it, we could arguably use that state to indicate that.
>> 
>> I think the answer to this depends on my previous question.
>> -- adrian
>