#9761: Foldable cannot be derived by GND due to redundant contexts -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: bug | Status: patch Priority: normal | Milestone: Component: Core | Version: 7.9 Libraries | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: rejects valid program | Test Case: | Blocking: | Differential Revisions: Phab:D425 | -------------------------------------+------------------------------------- Comment (by goldfire): As Mr. Role, I agree with the analysis reported in comment:2. (But, just to be pedantic, I disagree with the phrasing in comment:3. `Foldable` should most certainly have a nominal role on its parameter, because we don't want to be able to coerce an existentially-bound `Foldable` dictionary, as `Foldable MyList` and `Foldable []` might be different. However, the extra `Foldable` constraints do cause GND to fail, because the constraint gets instantiated before coercing, and then it looks like we're trying to coerce `Foldable []` to `Foldable MyList`, which is bad.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9761#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler