[p0nce]

Like others said, this can be done as a library type in D: http://p0nce.github.io/d-idioms/#Recursive-Sum-Type-with-mat...

edit: not that it proves anything, just that "it can eventually be done".

[tomjakubowski]

Can you name the variants to distinguish two variants which "carry" the same types? It wouldn't be as useful if you can't express something like:

    enum ConnState {
        Disconnected,
        Connecting,
        Connected(net::TcpSocket),
        Transferring(net::TcpSocket),
     }

[jeremiep]

I would probably do it like this in D (disclaimer: I haven't tested this code!)

  import std.typecons;
  import std.socket;
  import std.variant;

  struct Disconnected {}
  struct Connecting {}

  alias Connected = Typedef!(TcpSocket, null, "connected");
  alias Transferring = Typedef!(TcpSocket, null, "transferring");

  alias ConnState = Algebraic!(Disconnected, Connecting, Connected, Transferring);

You could use plain structs instead of the typedef template for the same result.

[p0nce]

I see what you mean.

You can create such new types with std.typecons.Typedef https://dlang.org/phobos/std_typecons.html#.Typedef

Though it's less pretty.