#8779: Exhaustiveness checks for pattern synonyms -------------------------------------+------------------------------------- Reporter: nomeata | Owner: mpickering 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): Phab:D2669 Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): And just to say it explicitly, GHC understanding that really doesn't have anything to do with pattern synonyms at all. Even if you write a pattern match like {{{#!hs f (g -> True) = 1 f (g -> False) = 2 }}} the exhaustiveness checker thinks there are unmatched cases, but it should be able to see that there aren't. (And the desugarer already turns this into a `case` that evaluates `g` only once, so it really seems like the exhaustiveness checker ought to be able to handle this!) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8779#comment:50 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler