#9223: Type equality makes type variable untouchable -------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 (Type checker) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by tvynr): Ah ha! This makes perfect sense; thank you for the excellent example and explanation. In particular, this points out that whether a principled type exists is dependent upon some pretty incidental factors, like whether the output type and the input type happen to have anything in common. I can see why GHC conservatively rejects both of these, since getting it to work would result in a somewhat fragile system. I had previously supposed that the problem was somehow related to the fact that I had two data -- the `TokenType` and the payload -- and that the `a` in the first parameter of `SomeToken` (`t a`) was not being unified with the `a` in the second parameter (`a`) for the purposes of inference. I think I had gotten than impression by the names `a` and `a1` without realizing that one of them arose merely due to the use of `+` in the body. Thanks again! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9223#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler