Fix the handling of uninhabited types in pattern matching. by canndrew · Pull Request #1872 · rust-lang/rfcs (original) (raw)

Hmm, I just realized if one has a tuple of &!, and only some of them are valid, punning on _ isn't so good. Let me amend my alternative so ! is a pattern that asserts the thing it is matching is "trivially" uninhabited.

Examples:

1. unsafe {
   match get(): &! {
   &!, // no arm needed
   }
   }
2. unsafe {
   match get(): Void {
   !, // overkill, but accepted.
   }
   }
3. unsafe {
   match get(): Result<usize, !> {
   Ok(n) => println!("going strong {}", n);
   Err(_) => println!("still alive!");
   }
   }
4. unsafe {
   match get(): Result<(usize, !), (Void, !)> {
   Ok((n, )) => println!("still alive! {}", n);
   Err((, !)), // only 1 ! needed
   }
   }

This is great for those coming from Haskell [haha, what a pedagogical priority], because if you really really drunkenly squint, asserting uninhabitedness is like forcing a binding (with !), and and @arielb1's trap representations are like the bottom value.