std.data.json formal review

[Andrei Alexandrescu via Digitalmars-d]

On 8/14/15 7:40 AM, Andrei Alexandrescu wrote:
> On 8/12/15 5:43 AM, Sönke Ludwig wrote:
>>> Anyway, I've just started to work on a generic variant of an enum based
>>> algebraic type that exploits as much static type information as
>>> possible. If that works out (compiler bugs?), it would be a great thing
>>> to have in Phobos, so maybe it's worth to delay the JSON module for that
>>> if necessary.
>>>
>>
>> First proof of concept:
>> https://gist.github.com/s-ludwig/7a8a60150f510239f071#file-taggedalgebraic-d-L148
>>
>>
>>
>> It probably still has issues with const/immutable and ref in some
>> places, but the basics seem to work as expected.
>
> struct TaggedAlgebraic(U) if (is(U == union)) { ... }
>
> Interesting. I think it would be best to rename it to TaggedUnion
> (instantly recognizable; also TaggedAlgebraic is an oxymoron as there's
> no untagged algebraic type). A good place for it is straight in
> std.variant.
>
> What are the relative advantages of using an integral over a pointer to
> function? In other words, what's a side by side comparison of
> TaggedAlgebraic!U and Algebraic!(types inside U)?
>
> Thanks,
>
> Andrei

Ping on this. My working hypothesis:

- If there's a way to make a tag smaller than one word, e.g. by using 
various packing tricks, then the integral tag has an advantage over the 
pointer tag.

- If there's some ordering among types (e.g. all types below 16 have 
some property etc), then the integral tag again has an advantage over 
the pointer tag.

- Other than that the pointer tag is superior to the integral tag at 
everything. Where it really wins is there is one unique tag for each 
type, present or future, so the universe of types representable is the 
total set. The pointer may be used for dispatching but also as a simple 
integral tag, so the pointer tag is a superset of the integral tag.

I've noticed many people are surprised by std.variant's use of a pointer 
instead of an integral for tagging. I'd like to either figure whether 
there's an advantage to integral tags, or if not settle for good a 
misconception.


Andrei