#8779: Exhaustiveness checks for pattern synonyms -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.1 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by dfeuer): * cc: dfeuer (added) Comment: I suspect making this really work right may require some modifications to the pattern synonym concept. For example, {{{#!hs pattern Empty = Seq EmptyT pattern x :<| xs <- (viewl -> x :< xs) pattern xs :|> x <- (viewr -> xs :> x) }}} To see that either `x :<| xs` or `xs :|> x` is complete when combined with `Empty`, the exhaustiveness checker would have to recognize that `viewl` and `viewr` will give non-empty results under the same circumstances. This may be feasible in this case (I'm not sure), but in principle it seems rather hard. I think a good target would be to ensure that multiple pattern synonyms using the ''same'' view are handled properly. An alternative blunt instrument: let the user promise that a certain combination of pattern synonyms will always be exhaustive when applied to a particular type. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8779#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler