Structural exhaustive matching

[Justin Whear via Digitalmars-d-learn]

On Tue, 21 Apr 2015 15:36:27 +0000, 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.

std.variant.Algebraic implements ADTs:

import std.variant, std.string;
struct Foo { ... }
alias A = Algebraic!(int, double, Foo);
A a = 1;

// std.variant.visit enforces that all possible types are handled, so this
//  is an error:
auto res = a.visit!(
	(int x) => format("Got an int: %s", x),
	(double x) => format("Got a double: %s", x),
	(Foo x) => "Got a Foo"
);

You can also dispatch to a function with the appropriate overloads/
template instantiations like so:

foreach (T; A.AllowedTypes)
	if (a.type is typeid(T))
		myfunc(a.get!T);

This also exhaustively guarantees that myfunc can be called with all 
possible types of a.