#13814: Unable to resolve instance for polykinded superclass constraint on associated-type-family. -------------------------------------+------------------------------------- Reporter: isovector | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: polykinds, | type families Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by isovector: @@ -10,1 +10,1 @@ - class Back k + class Back t @@ -12,2 +12,2 @@ - class Back (FrontBack k) => Front k where - type FrontBack k :: k' + class Back (FrontBack t) => Front t where + type FrontBack t :: k New description: The following program doesn't compile: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TypeFamilies #-} module Test where class Back t class Back (FrontBack t) => Front t where type FrontBack t :: k instance Back Bool instance Front Int where type FrontBack Int = Bool }}} with the error message: {{{ • No instance for (Back (FrontBack Int)) arising from the superclasses of an instance declaration • In the instance declaration for ‘Front Int’ }}} The example successfully compiles if the kind annotation on FrontBack is removed. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13814#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler