#11962: Support induction recursion -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 9269 | Blocking: Related Tickets: #13901 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by andrewthad): @goldfire To my understanding, the priority of this ticket is getting induction recursion supported for GADTs. I'm unclear on how the "same recursive subgroup" error in the example you gave is related to the "same recursive subgroup" error that I run into with typeclasses. For example: {{{ class Grounded v where type Rep (a :: v) :: RuntimeRep type Lifted (a :: v) :: Type type Unlifted (a :: v) :: TYPE (Rep a) ... some methods in here as well }}} This currently errors with: {{{ sorted_int_type.hs:100:35: error: • Type constructor ‘Rep’ cannot be used here (it is defined and used in the same recursive group) • In the first argument of ‘TYPE’, namely ‘(Rep a)’ In the kind ‘TYPE (Rep a)’ | 100 | type Unlifted (a :: v) :: TYPE (Rep a) | ^^^ }}} I have to split the class up: {{{ class GroundSuper v where type Rep (a :: v) :: RuntimeRep class GroundSuper v => Grounded v where type Lifted (a :: v) :: Type type Unlifted (a :: v) :: TYPE (Rep a) }}} It's not the worst thing ever, but it's kind of silly that what really is logically one typeclass needs to be split up into two typeclasses. It's not totally clear to me if this is really even the same issue as what you describe above or if fixing one automatically fixes the other, but it gives the same error message, which leads me to believe that they are somehow related. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11962#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler