#11806: GHC does not warn for mistakenly empty case -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by gkaracha): Replying to [ticket:11806 dfeuer]:
{{{#!hs {-# LANGUAGE EmptyCase #-} {-# OPTIONS_GHC -Wall -fwarn-incomplete-uni-patterns #-}
oops :: Int -> a oops x = case x of {} }}}
Since `Int` is inhabited, I would expect a warning, but I get none. Since empty case is typically used in situations where programmers want GHC to see an absurdity, this seems most unfortunate.
Indeed. Yet, I feel like there is a big misconception here, from the you said
Since `Int` is inhabited
It is indeed inhabited, but **like any other type in Haskell**, where every type is inhabited. That is, for the following (see #11390) a warning should also be expected: {{{#!hs silly1 :: Void -> Void silly1 x = case x of {} }}} There have been separate discussions on this (see related tickets #10746, #7669, #11390) and I think that we have a design choice to make here. Unless you force the argument in some other way like: {{{#!hs silly2 :: Void -> Void silly2 x = x `seq` case x of {} }}} or {{{#!hs silly3 :: Void -> Void silly3 (!x) = case x of {} }}} then it is indeed non-exhaustive. This means that in almost all cases, an empty case expression will issue a warning (which I think is the right thing to do). Of course, in `silly2` and `silly3` the warning is too conservative but remember that type inhabitation is undecidable. Hence, I see two choices here. We can: 1. Always issue a non-exhaustive warning for empty case expressions, or 2. Never issue a non-exhaustive warning for empty case expressions. I am pointing this out because before #7669 we would issue the warning you seek here and it has been intentionally changed. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11806#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler