#9725: Constraint deduction failure -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 (Type checker) | 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: | -------------------------------------+------------------------------------- Comment (by rwbarton): The thread starting at http://www.haskell.org/pipermail/glasgow-haskell- users/2013-December/024466.html may be relevant. In short, GHC does not decompose `f x ~ g y` into `f ~ g, x ~ y` unless it knows already that `f` and `g` (or equivalently `x` and `y`) have the same kind. It seems to me that GHC should be able to ''conclude'' from `f x ~ g y` that `f` and `g` have the same kind, but maybe it has no way to represent this? I think there is a related Trac ticket also, but I couldn't find it. Maybe adding kind annotations for some of the type variables involved would help? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9725#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler