[jim-jim-jim]

One practical example: if you have a union of A|B, and you decide to add a third shape C to it, your program won't compile until logic for C is written in all the places where A and B are already being matched against. Refactoring often means changing data models, then letting the errors walk you through the actual implementation details.

[garethrowlands]

Yeah that's how it's supposed to work. Historically, Haskell didn't always make incomplete patterns an error, even now the checker isn't perfect, and even if it was people can still put wildcard patterns.