Offer a compiler warning for failable pattern matches
This started off with a mailing list discussion: http://firstname.lastname@example.org/msg96517.html. The problem is that the following code produces no compile-time warning and results in a runtime error:
data MyType = Foo | Bar deriving Show test :: Monad m => m MyType -> m () test myType = do Foo <- myType return () main :: IO () main = test $ return Bar
This is in contrast to the rest of pattern matching, which would warn about unmatched patterns, e.g.:
data MyType = Foo | Bar deriving Show test :: Monad m => m MyType -> m () test myType = do x <- myType case x of Foo -> return () main :: IO () main = test $ return Bar
I understand that this style of code may be very useful in some circumstances when paired with a Monad providing a sensible fail implementation, and is especially used in list comprehensions. However, this is allowing an easily catchable static error to slip through our fingers.
I recommend we add a new compiler warning to catch incomplete patterns in do-notation binding. I believe this warning should not apply to list comprehensions. Ideally, this warning would be turned on by -Wall.