#13761: Can't create poly-kinded GADT with TypeInType enabled, but can without -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: invalid | Keywords: TypeInType 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: | -------------------------------------+------------------------------------- Comment (by simonpj): I'm only questioning whether we need new syntax. Let me re-state what I think you are saying. * The kind of a type constructor can be ''inferred'' from its definition, or ''specified'' by a CUSK. * A CUSK ''specifies'' the kind of a type constructor, fully. No need to look at any other definitions etc; it all comes from the CUSK. * In type declarations you can add kind signatures but they merely specify additional constraints that may guide the inference process. For example {{{ data T (a :: k -> *) = ... }}} constraints `a` to have a kind of the specified form, but it does no more than that. The thing you say is "incredibly confusing" is that the switch from inferred to specified is based on some fairly subtle syntactic rules, and you'd like to have a clearer syntactic distinction. It's a software engineering issue, not a technical one. Separate kind signatures would be one solution. Simplifying the CUSK rules might be another. The [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... #complete-user-supplied-kind-signatures-and-polymorphic-recursion existing rules] are not bad, actually, except for the mysterious second bullet which depends on explicit quantification of kind variables. Otherwise the rule is ''it has a CUSK if every argument variable, and result kind have explicit signatures''. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13761#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler