#10097: GHC 7.11 errors on dictionary casting tricks -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by goldfire): I don't like that there's been a typechecker regression here, but this doesn't seem like an outright bug to me. The one bit I'm quite unsure of is the eta-contraction... and my reasoning-about-higher-rank-eta- contracted-functions neuron seems to be enjoying the snow day here. So I'll eta-expand: {{{ iFeelDirty x = unsafeCoerce x }}} Now, I can see why your error appears. When you say `x`, GHC will try to instantiate the implicit parameters of `x`, namely `ts` and `List f ts`. But, GHC has no guidance as to what these should be, because it can't glean anything useful from `unsafeCoerce`. Some more type annotations might be helpful, as might be newtype wrappers to accurately control what you're `unsafeCoerce`ing. So, I have two conclusions: * I'm happy enough with an error in the eta-expanded version, but I agree that the eta-contracted one should probably work. * I don't think this bug shows up a problem with the dirty dictionary- casting trick. That should still be valid. It's just a typechecker behavior change, which (I believe) could be overcome with more types. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10097#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler