I think this is a similar 'failure to deduce' partiality as last week's thread; but the other way round (and with ViewPatterns) -- at GHC 8.10.7:
GHCi> :set -XViewPatterns GHCi> :set -Wincomplete-patterns -- also warns overlapping GHCi> (\x -> case x of {((\(Just j) -> j) -> j2) -> j2; Nothing -> 'N'}) Nothing
===> <interactive>: warning: [-Woverlapping-patterns] Pattern match is redundant In a case alternative: Nothing -> ... *** Exception: <interactive>:16:21-34: Non-exhaustive patterns in lambda
(Same Warning then same Exception if I compile the code.)
I think what we see here is an instance of the fact that all checks must break down in presence of view patterns. If I parsed your code correctly, the case analysis has two branches. The first uses a view pattern with a function, call it v for the moment. The result of v is bound to another pattern, in your case the wildcard j2 and that is returned as-is, so in pseudo-Haskell: case (v x) of j2 -> j2 case x of Nothing -> 'N' Now your view function v happens to be partial, it pattern-matches its argument and only provides a branch for the Just case. v = \(Just j) -> j = Data.Maybe.fromJust That is why you see the exception when you call this function with 'Nothing'. And indeed the second match is redundant insofar as the view pattern will evaluate v for any x. Since v could be anything, like unsafePerformIO (lauchMissiles >> return j) GHC does not examine this and thus can not decide that v does not match the cases that the Nothing branch does. If you swap the two branches and make the view pattern the last one, then both the exception and the warning go away. Olaf