Create an Algebraic type at compile time and more

[uknys]

On Friday, 15 June 2018 at 07:27:22 UTC, Simen Kjærås wrote:
> On Thursday, 14 June 2018 at 19:15:38 UTC, uknys wrote:
>> [...]
>
> 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

Yeah I saw that Algebraic doesn't work at compile time, then I 
thought of using an Interface with one function (execute()) and 
making Hello and Toto as classes implementing even if that's 
somewhat slower.
Then I could use pragma(msg, getSymbolsByUDA!(mixin(__MODULE__), 
X)); and make an array sorted by opcode of this interface. Am I 
right about this ?