#13990: Core Lint error on empty case -------------------------------------+------------------------------------- Reporter: mbieleck | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.2 Component: Compiler | Version: 8.2.1-rc3 Resolution: | Keywords: core-lint | case Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
Or find out why exprIsHNF scrut is True here.
It's because in a case alternative {{{ K x -> ...x... }}} where `K` is strict, we know that when we unpack `K x`, the field `x` must be evaluated. So it is marked as such, and hence responds True to `exprIsHNF`. Joachim's solition is right; but it's very much a corner case and a bit hard to get right. E.g. {{{ data T a = T1 !(F a) Int | ... }}} When we pattern match on `T1 x y :: T X`, is that pattern impossible? WEll it depends on whether `(F X)` evaluates to an empty type. Hard to be sure it is fully nailed. Better, I think, to say that {{{ case x of {} }}} is fine if `x :: T` where `T` has no data constructors. That would silence the warning and is obviously correct. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13990#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler