Create an Algebraic type at compile time and more

[Simen Kjærås]

On Thursday, 14 June 2018 at 19:15:38 UTC, uknys wrote:
> Hello,
>
> I wanted to know if such code was possible :
>
> alias Operation = Algebraic!(/* All type that implements X UDA 
> */)
>
> struct X
> {
>     int opcode;
>     Operation h;
> }
>
> @X(0x01, Hello(3))
> @X(0x02, Hello(4))
> struct Hello
> {
>     int Hello;
> }
>
> @X(0x03, Toto(5))
> @X(0x05, Toto(6))
> struct Toto
> {
>    int A;
> }
>
>
> It has two problems, Operation not being defined while I use it 
> in the UDA, and how to get back all the types that implements 
> this UDA to give it to Algebraic.
> I don't know if it's a good idea or not, but might as well try 
> the dark side of meta programming.
>
> Thanks a lot :)

First off - Algebraic doesn't work at compile-time[0]:

// Error: memcpy cannot be interpreted at compile time, because 
it has no available source code
enum a = Algebraic!(int, string)("Hello!");


Now, as for getting the list of types with an X UDA:

import std.traits : getSymbolsByUDA;
pragma(msg, getSymbolsByUDA!(mixin(__MODULE__), X));

However, as you sort-of point out, there's a problem of 
dependencies - if we assign the result of getSymbolsByUDA to a 
symbol (be that a type or an alias, or part of a type, as in 
Algebraic!(getSymbolsByUDA)), then getSymbolsByUDA will need to 
consider that symbol in its search.

In order to do that, we need to know more about the type. 
Theoretically, the compiler might be able to know which UDAs a 
symbol has without having to figure out all the other details, 
but such is not currently the case.


To get around these issues, maybe you could make Hello and Toto 
classes instead of structs?

--
   Simen

[0]: https://issues.dlang.org/show_bug.cgi?id=11864