Even better, if you're using a language with pattern matching (rust, any ML, haskell), just put it in a single match with the happy case on top, and the error c

Even better, if you're using a language with pattern matching (rust, any ML, haskell), just put it in a single match with the happy case on top, and the error cases below. To do this, you need to encode the distinction between happy and sad path in your types. That's good for all sorts of other reasons, so it's not really a drawback.