Interesting point. Getting the type annotation inferred for you there is definitely useful. I've used it a few times myself. The `undefined` trick is also imme

Interesting point. Getting the type annotation inferred for you there is definitely useful. I've used it a few times myself. The `undefined` trick is also immensely useful. I use it a lot when starting a new module. Rust also has a notion of bottom, indicated by `!`, which will unify with all types. I frequently use this in Rust in a similar way that I use `undefined` in Haskell. (In Rust, you would speak `fail!()` or `unreachable!()` or `unimplemented!()` or define-your-own.) > (if you've ever written a little wrapper around a function from a complicated library, you know what I mean) Yes, absolutely. I haven't really run into this problem with Rust yet though. Types are generally pretty simple. If and when Rust gets higher-kinded types, that would assuredly change.