Generically call a function on Variant's payload?

[Nick Sabalausky (Abscissa)]

On 08/20/2018 04:34 PM, Jonathan M Davis wrote:
> 
>      foreach(T; TypesThatVariantHolds)

Yea, that's what I would've just done, but I wanted to support 
user-created types not already known to my library.

> You
> can't just call functions on completely unknown types, because the compiler
> wouldn't know what code to generate or what to link against.
> 

Right. D's templates have really spoiled me. Because of them, I've grown 
accustomed to invoking functions on unknown types ;)

But you're right, that's not going to work for runtime dispatch without 
enumerating each possible type. Even if I try (like I was thinking) to 
provide a templated function to convert any of the concrete types to an 
internal-only supertype, I'd still need something to runtime dispatch to 
the right template instance. Darn.

A lot of times, I really wish I could introspect across *all* linked 
modules, not just specific ones. Ex: "Hey compiler, give me a 
compile-time list of ALL types in this program with UDA @xyz". Then 
things like this could be built out of that. No need to worry about 
unknown types/symbols because they would all be known and a finite list 
could always be constructed.

Although, what would REALLY be nice is if, for every type, the compiler 
built a list of every member and how to access it (which...really it has 
to do anyway), but then uses that info to build a master runtime 
dispatch system. Types such as Variant already have runtime knowledge of 
what type is currently being held, so that info could be passed to the 
compiler-generated "master runtime dispatch" system along with what 
member to invoke/access.

Come to think of it...aren't I just describing standard run-of-the-mill 
runtime reflection? I mean, hasn't Java already been able to do that for 
ages? But then, in Java everything is already part of the class 
hierarchy anyway so maybe that's why it's possible there?