#9695: GADT Constraint breaks type inference -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Resolution: invalid | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: GHC | Blocked By: rejects valid program | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by goldfire): * status: new => closed * resolution: => invalid Comment:
Is that expected behavior for GADTs?
Yes. Type inference in the presence of an equality constraint -- even a seemingly-irrelevant one -- is incomplete. The error message definitely needs to get better (#9109), but I'm not sure the behavior can be fixed. The [http://research.microsoft.com/~simonpj/papers/constraints/jfp- outsidein.pdf OutsideIn] paper, section 5.2, gives a nice description of the problem. This section is readable without reading the rest of the paper (although the short, preceding section 5.1 helps somewhat). I'm closing as "invalid", but if you think there's something deeper going on here, please reopen. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9695#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler