#13484: Incorrect Exhaustivity Checking -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): This is expected behavior. The first implementation of `impossibleChunk01` you gave is not exhaustive because the second argument might be `⊥`, which may involve neither constructor! If you don't believe me, try evaluating `impossibleChunk01 ChunkNil (let x = x in x)`. So the most correct solution is to adopt your suggestion and write this second case: {{{#!hs impossibleChunk01 ChunkNill x = case x of {} }}} And, as you noted, GHC 8.2 will verify that that empty-case on `x` really is exhaustive, so you can be assured that `x` isn't inhabited by a (terminating) `Chunk` value. Is that a satisfying answer? If so, please feel free to close this ticket. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13484#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler