#13399: Location of `forall` matters with higher-rank kind polymorphism -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.0.2 Resolution: fixed | 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): Phab:D3860 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: patch => closed * resolution: => fixed * milestone: => 8.4.1 Old description:
The following code fails to compile, but probably should:
{{{ {-# LANGUAGE RankNTypes, TypeInType #-}
import Data.Kind
data Foo :: forall k . (* -> *) -> k -> * -- Decl 1
class C (a :: forall k . k -> *)
instance C (Foo a) -- error on this line }}}
with the error
{{{ • Expected kind ‘forall k. k -> *’, but ‘Foo a’ has kind ‘k0 -> *’ • In the first argument of ‘C’, namely ‘Foo a’ In the instance declaration for ‘C (Foo a)’ }}}
Similarly, the following declarations of `Foo` also cause a similar error at the instance declaration:
Decl 2: `data Foo :: (* -> *) -> k -> *`
Decl 3: `data Foo (a :: * -> *) (b :: k)`
However, if I move the `forall` to a point ''after'' the first type parameter (which is where the instance is partially applied) thusly:
Decl 4: `data Foo :: (* -> *) -> forall k . k -> *`
then GHC happily accepts the instance of `C`.
From my (admittedly negligible) knowledge of type theory, the signatures for Decls 1 and 4 (and 2) are identical, since the `forall` can be floated to the front of Decl 4. GHC should accept any of the four versions of `Foo`, since they are all equivalent.
New description: The following code fails to compile, but probably should: {{{#!hs {-# LANGUAGE RankNTypes, TypeInType #-} import Data.Kind data Foo :: forall k . (* -> *) -> k -> * -- Decl 1 class C (a :: forall k . k -> *) instance C (Foo a) -- error on this line }}} with the error {{{ • Expected kind ‘forall k. k -> *’, but ‘Foo a’ has kind ‘k0 -> *’ • In the first argument of ‘C’, namely ‘Foo a’ In the instance declaration for ‘C (Foo a)’ }}} Similarly, the following declarations of `Foo` also cause a similar error at the instance declaration: Decl 2: `data Foo :: (* -> *) -> k -> *` Decl 3: `data Foo (a :: * -> *) (b :: k)` However, if I move the `forall` to a point ''after'' the first type parameter (which is where the instance is partially applied) thusly: Decl 4: `data Foo :: (* -> *) -> forall k . k -> *` then GHC happily accepts the instance of `C`. From my (admittedly negligible) knowledge of type theory, the signatures for Decls 1 and 4 (and 2) are identical, since the `forall` can be floated to the front of Decl 4. GHC should accept any of the four versions of `Foo`, since they are all equivalent. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13399#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler