#10797: Kind-level functional dependencies are not resolved properly -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by danilo2): As an related question - I'm trying to determine the "base type" of a value. I mean, for {{{Proxy Int}}} I want to get {{{Proxy Int}}} but for something like {{{Proxy :: Maybe a}}} (for a known {{{a}}}) I want to get {{{Proxy :: Maybe}}}. I was trying to do this using closed Type Families as well, but without success: {{{ type family BaseTypeTF (t :: *) where BaseTypeTF (Proxy ( (a :: k -> l) (b :: k) )) = BaseTypeTF (Proxy (a :: k -> l) ) BaseTypeTF (Proxy (a :: x) ) = Proxy (a :: x) }}} error: {{{ Family instance purports to bind type variable ‘k’ but the real LHS (expanding synonyms) is: BaseTypeTF (Proxy (a b)) = ... In the equations for closed type family ‘BaseTypeTF’ In the type family declaration for ‘BaseTypeTF’ }}} That could be understandable for two reasons (but I'm not sure if that are good guesses): 1) We've go no mechanism like OverlappingInstances in TypeFamilies, but on the other hand closed type families provides us with similar functionality, but for a closed world, which is fine in this case 2) We've got no syntax to explicitly type the result of this family, because it's kind is {{{*}}}, but it is in fact {{{Proxy :: something}}}, which we are impossible to type if GHC has any ambiguity (which again maybe not related to this question). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10797#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler