Offer a compiler warning for failable pattern matches

This started off with a mailing list discussion: http://www.mail-archive.com/haskell-cafe@haskell.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.

Trac metadata

Trac field	Value
Version	7.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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