#12088: Type/data family instances in kind checking -------------------------------------+------------------------------------- Reporter: alexvieth | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #11348, #12239 | Differential Rev(s): Phab:D2272 Wiki Page: | -------------------------------------+------------------------------------- Comment (by int-index): Another example, where a newtype complicates things ever further. I tried adding TH sections to enforce an order in which type family instances are added but it didn't help: {{{ {-# LANGUAGE TemplateHaskell, TypeInType, GADTs, TypeFamilies #-} {-# OPTIONS -Wno-unticked-promoted-constructors #-} import Data.Kind (Type) data N = Z | S N data Fin :: N -> Type where FZ :: Fin (S n) FS :: Fin n -> Fin (S n) type family FieldCount (t :: Type) :: N type family FieldType (t :: Type) (i :: Fin (FieldCount t)) :: Type newtype Field (t :: Type) (i :: Fin (FieldCount t)) = Field (FieldType t i) return [] -- split TH section data L type instance FieldCount L = S Z return [] -- split TH section type instance FieldType L FZ = () return [] -- split TH section -- newtype not involved, compiles. l_fz :: FieldType L FZ l_fz = () -- newtype involvoed, doesn't compile. field_l_fz :: Field L FZ field_l_fz = Field l_fz }}} The error I'm getting is: {{{ field.hs:36:20: error: • Couldn't match type ‘FieldType L 'FZ’ with ‘()’ • In the first argument of ‘Field’, namely ‘l_fz’ In the expression: Field l_fz In an equation for ‘field_l_fz’: field_l_fz = Field l_fz }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12088#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler