It probably doesn't belong in -Wall, as it is a fairly common idiom to use
fail intentionally this way, but it could pretty easily be added to the
'do' and list/monad comprehension desugaring to issue a separate warning
that we don't turn on by default.
Making it possible to see where you use
Can -Wall be extended to report pattern match failures in do
expressions, like it does for case expressions?
Prelude :set -Wall
Prelude let f = do Just x - return Nothing; return x
Prelude let g = case Nothing of Just x - x
interactive:9:9: Warning:
Pattern match(es) are non-exhaustive
