#13788: TypeInType fails to compile old code -------------------------------------+------------------------------------- Reporter: br1 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: 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 goldfire): This looks like correct behavior to me. With `-XTypeInType`, GHC will generalize kinds, including the kind of `HEq` (now `forall k. k -> Constraint`) and of `Field` (now `forall k. k -> Type -> Type`). However, `label` can't be generalized; it has type `forall (l :: Type) (v :: Type). Field @Type l v -> l`. (I'm using `@` to denote a normally-elided parameter.) Because the instance at the end doesn't constraint the kind of `l`, the type of `f` is `Field @k l v`, where `k` is implicitly bound in the instance declaration. Accordingly, it can't be passed to `label`. The solution: add a kind signature: `instance HEq l => HListGet (Field (l :: Type) v) where ...`. (You will have to import `Type` from `Data.Kind`.) Please close the ticket if you agree with my analysis. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler