> Match needs to be exhaustive. When you add something to the enum you were matching, it chokes. This is good. There’s a reason why ML and Haskell compilers ge

> Match needs to be exhaustive. When you add something to the enum you were matching, it chokes. This is good. There’s a reason why ML and Haskell compilers generally have that as a warning by default and not an error: when you need a pipeline of small transformations of very similar languages, the easiest way to go is usually declare one tree type that’s the union of all of them, then ignore the impossible cases at each stage. This takes the problem entirely out of the type system, true, but an ergonomic alternative for that hasn’t been invented, as far as I know. Well, aside from the micropass framework in Scheme, I guess, but that requires exactly the kind of rich macros that Rust goes out of its way to make ugly. (There have been other attempts in the Haskell world, like SYB, but I haven’t seen one that wouldn’t be awkward.)