If you have a type error message at the very start of a "do" statement, the result can be rather confusing, because the typechecker doesn't "know" that do's are almost always have type (IO (something)) and so tries to shoehorn the monad to fit the type. This has actually happened to me several times; the worst case is when the result is (for example) a list, which allows the error to propagate further. Is there some way of doing something about this?