Re: [GHC] #13972: GHC 8.2 error message around indexes for associated type instances is baffling

#13972: GHC 8.2 error message around indexes for associated type instances is baffling -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1-rc2 checker) | Keywords: TypeFamilies, Resolution: | TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3820 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott ): In [changeset:"38260a9e9f8c38edd25f4b4c06e0ea5d88fc6bf2/ghc" 38260a9/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="38260a9e9f8c38edd25f4b4c06e0ea5d88fc6bf2" Fix #13972 by producing tidier errors Summary: Previously, one could experience an error message like this: ``` Expected: T (a -> Either a b) Actual: T (a -> Either a b) ``` This makes the error message an iota clearer by tidying it first, which will instead produce: ``` Expected: T (a1 -> Either a1 b1) Actual: T (a -> Either a b) ``` Which steers users towards the understanding that the two sets of tyvars are actually different. Test Plan: make test TEST=T13972 Reviewers: simonpj, austin, bgamari, goldfire Reviewed By: goldfire Subscribers: goldfire, rwbarton, thomie GHC Trac Issues: #13972 Differential Revision: https://phabricator.haskell.org/D3820 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13972#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler