[Haskell-cafe] Avoiding "Non-exhaustive patterns in function f"

Haskell is known for its very strong static type checking, which eliminates a lot of runtime errors. But the following simple program crashes at runtime: data D = A | B f A = True main = print (f B) I understand this has nothing to do with type checking, but why can't the compiler give a warning about this? Or is this by design or because it is impossible to check with more complex recursive data types? Thanks, Peter