#14440: Duplicate type family instances are permitted -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: TypeFamilies 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 AntC): Replying to [ticket:14440 RyanGlScott]:
My intuition screams "no", since if we offer //class// instance coherence, ...
And the short answer to that is we '''don't''' offer and can't guarantee class instance coherence/consistency, if you're talking about FunDeps. See SPJ on [https://ghc.haskell.org/trac/ghc/ticket/10675#comment:15 'bogus consistency check']. (Sorry, but there's plenty of code abusing that consistency check, pioneered in GHC in the 2004 HList paper. Contrast Hugs never allowed such abuse. There is [https://github.com/ghc-proposals/ghc- proposals/pull/56 a way out]. Please upvote.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14440#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler