Structural exhaustive matching

[Martin Nowak via Digitalmars-d-learn]

On Tuesday, 21 April 2015 at 15:36:28 UTC, Jadbox wrote:
> What's the best equivalent to Rust's structural enum/pattern 
> (match)ing? Is it also possible to enforce exhaustive matches? 
> Basically, I'm curious on what the best way to do ADTs in D.

If it needs to be really fast, use final switch on the tag of a 
discriminated union.

enum Tag { A, B, C }
struct Val
{
   Tag tag;
   union
   {
     A a;
     B b;
     C c;
   }
}

void too(Val val)
{
   final switch (val.tag)
   {
   case Tag.A: writeln(val.a); break;
   case Tag.B: writeln(val.b); break;
   case Tag.C: writeln(val.c); break;
   }
}