Your code should just work as before when ApplicativeDo is enabled,

#15344: ApplicativeDo seems to prevent the fail method from being used -------------------------------------+------------------------------------- Reporter: kccqzy | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I am not sure if this is intended, but I came across this issue when debugging a runtime exception. It seems like an incomplete pattern match in a do-block with ApplicativeDo enabled will not use the fail method. If I have a module like this {{{#!hs {-# LANGUAGE ApplicativeDo #-} {-# OPTIONS_ghc -ddump-simpl #-} module M where f :: Maybe (Maybe Int) -> Maybe Int -> Maybe Int f mgs mid = do _ <- mid (Just moi) <- mgs pure (moi + 42) main :: IO () main = print (f (Just Nothing) (Just 2)) }}} This will result in a runtime exception: {{{ Just *** Exception: repro.hs:(6,13)-(9,17): Non-exhaustive patterns in lambda }}} But if I remove the ApplicativeDo extension, this code works as normal, producing Nothing as the output. On a theoretical level I understand why this is happening, but I still find it quite unexpected, especially since the documentation at https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... #things-to-watch-out-for claims that provided you use conventional Applicative instances. If this is not a defect in GHC itself, I'd say it is a defect in documentation in not pointing out this gotcha. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15344 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler