Subtyping of an enum

[XavierAP]

On Monday, 15 April 2019 at 08:39:24 UTC, Anton Fediushin wrote:
>
> Hello! I am currently trying to add a custom `toString` method

Several remarks... First of all, strings can be compared 
(alphabetically) as well as integers, e.g.
assert("foo" > "bar")
Perhaps not your use case, but worth noting.

You have defined your sub-typing the opposite way that you wanted 
it to work: every `Enum` is an `internal`, but the other way 
around an `internal` may not work as an `Enum`. Your `fun` would 
in principle work if it were defined with an `internal` but 
passed an `Enum`... Of course you have defined `internal` as 
nested private so no... But then how did you want anything to 
work if no one outside Enum knows the super-type?

You obviously need to re-think your problem and your design :)

> Obvious solution is to wrap an enum in a structure and utilize 
> 'alias this' for subtyping like this:

Actually the obvious solution (not sure if it otherwise works for 
you) would be to take advantage of D's Uniform Function Call 
Syntax [1] and define toString as a global function that can be 
called as a method:

enum Fubar { foo, bar }

string toString(Fubar fb)
{
	return "It works.";
}

void main()
{
	import std.stdio;
	writeln(Fubar.foo.toString);
}

_________
[1] 
https://tour.dlang.org/tour/en/gems/uniform-function-call-syntax-ufcs