Catch-all considered harmful? ============================= I have been thinking about a potential source of bugs from catch-all pattern matches on sum types and would like to know your thoughts. Motivation ---------- Totality is usually a desirable property of a function and the catch-all can conveniently buy us totality. But at what price? I have been indoctrinated that rigour goes above convenience (think along the lines of: "Once we indulge in the impurities of I/O, there is no redemption.") I would like to evaluate the trade-offs between convenience for the programmer and a potential source of bugs. My questions to the community— 1. Are there real world examples of bugs caused by catch-alls? 2. Do you think that a language extension that disallows catch-alls (and annotations to opt back in at pattern match sites or type declaration) could be useful for certain code bases? 3. If this is a potential problem, then can you think of any better solutions a compiler could provide (i.e. that don't rely on an IDE / structured editing) other than disallowing catch-alls? Feel free to chip in with your 2p (or 2¢), but please only if you have any concrete experience (or compelling theoretical evidence). Example ------- Consider the sum type: data Answer = No | Yes and the function: foo : Answer -> String foo Yes = "Woo-hoo!" foo _ = "Bother." Say we need to extend our sum type: data Answer = No | Perhaps | Yes However, we forget to handle the new case appropriately in `foo`. The compiler is happy, but at runtime `foo Perhaps` would evaluate to `"Bother."`—with potentially catastrophic consequences. (Please imagine this happening in a large codebase with several contributors, no single one of whom knows the entire codebase.)