[thinkpad20]

One advantage I could see of Rust over D for language stuff is sum types (enums in Rust). So for example you can write:

    enum MyLanguage {
      Var(String),
      Int(i32),
      Sum(Box<MyLang>, Box<MyLang>),
      Let(String, Box<MyLang>, Box<MyLang>)
    }

(Apologies if I got a few things wrong; I just mean the general idea)

Seems like without a construct like this, you'd have to use subclasses or something similar, which (to me) isn't quite as nice.

[jeremiep]

There's std.variant in D which provides type-safe sum types.

alias MyLanguage = VariantN!(<insert types here>);

[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.

[felixangell1024]

Yeah, tagged unions and the pattern matching make things so much nicer. I love a lot of things about Rust, it's got a lot of things right.