From my perspective, these really are more like enums than unions. Rust enums cover C-style enums as a limiting case: enum Result { Ok, Err,

From my perspective, these really are more like enums than unions. Rust enums cover C-style enums as a limiting case: enum Result { Ok, Err, } If this was all you got, you'd need to return something like `(status, value, err)` to model fallible functions. This is not unlike Go's `value, err` convention, except that `status` is additionally inlined into `value` as a sentinel `nil`. This does nothing to prevent you from using the `value` if an error has occurred, or vice versa. Instead, we like to carry data _alongside_ the enum tags: // using named fields instead of tuple structs, for clarity enum Result<T, E> { Ok {value: T}, Err {err: E}, } Now we can return a single `Result,` and you can only use `value` if the function succeeded, and you can only use `err` if the function failed.