Standalone kind signatures (#16794)wip/top-level-kind-signatures

Implements GHC Proposal #54: .../ghc-proposals/blob/master/proposals/0054-kind-signatures.rst

With this patch, a type constructor can now be given an explicit
standalone kind signature:

  {-# LANGUAGE StandaloneKindSignatures #-}
  type Functor :: (Type -> Type) -> Constraint
  class Functor f where
    fmap :: (a -> b) -> f a -> f b

This is a replacement for CUSKs (complete user-specified
kind signatures), which are now scheduled for deprecation.

User-facing changes
-------------------

* A new extension flag has been added, -XStandaloneKindSignatures, which
  implies -XNoCUSKs.

* There is a new syntactic construct, a standalone kind signature:

    type <name> :: <kind>

  Declarations of data types, classes, data families, type families, and
  type synonyms may be accompanied by a standalone kind signature.

* A standalone kind signature enables polymorphic recursion in types,
  just like a function type signature enables polymorphic recursion in
  terms. This obviates the need for CUSKs.

* TemplateHaskell AST has been extended with 'KiSigD' to represent
  standalone kind signatures.

* GHCi :info command now prints the kind signature of type constructors:

    ghci> :info Functor
    type Functor :: (Type -> Type) -> Constraint
    ...

Limitations
-----------

* 'forall'-bound type variables of a standalone kind signature do not
  scope over the declaration body, even if the -XScopedTypeVariables is
  enabled. See #16635 and #16734.

* Wildcards are not allowed in standalone kind signatures, as partial
  signatures do not allow for polymorphic recursion.

* Associated types may not be given an explicit standalone kind
  signature. Instead, they are assumed to have a CUSK if the parent class
  has a standalone kind signature and regardless of the -XCUSKs flag.

* Standalone kind signatures do not support multiple names at the moment:

    type T1, T2 :: Type -> Type   -- rejected
    type T1 = Maybe
    type T2 = Either String

  See #16754.

* Creative use of equality constraints in standalone kind signatures may
  lead to GHC panics:

    type C :: forall (a :: Type) -> a ~ Int => Constraint
    class C a where
      f :: C a => a -> Int

  See #16758.

Implementation notes
--------------------

* The heart of this patch is the 'kcDeclHeader' function, which is used to
  kind-check a declaration header against its standalone kind signature.
  It does so in two rounds:

    1. check user-written binders
    2. instantiate invisible binders a la 'checkExpectedKind'

* 'kcTyClGroup' now partitions declarations into declarations with a
  standalone kind signature or a CUSK (kinded_decls) and declarations
  without either (kindless_decls):

    * 'kinded_decls' are kind-checked with 'checkInitialKinds'
    * 'kindless_decls' are kind-checked with 'getInitialKinds'

* DerivInfo has been extended with a new field:

    di_scoped_tvs :: ![(Name,TyVar)]

  These variables must be added to the context in case the deriving clause
  references tcTyConScopedTyVars. See #16731.

4 files changed, 6 insertions, 1 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib.hs b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
index 86311762a3..7bb4eb50dd 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
@@ -85,7 +85,7 @@ module Language.Haskell.TH.Lib (
     viaStrategy, DerivStrategy(..),
     -- **** Class
     classD, instanceD, instanceWithOverlapD, Overlap(..),
-    sigD, standaloneDerivD, standaloneDerivWithStrategyD, defaultSigD,
+    sigD, kiSigD, standaloneDerivD, standaloneDerivWithStrategyD, defaultSigD,
 
     -- **** Role annotations
     roleAnnotD,
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
index 5ec59b3737..4d3887baf2 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
@@ -435,6 +435,9 @@ instanceWithOverlapD o ctxt ty decs =
 sigD :: Name -> TypeQ -> DecQ
 sigD fun ty = liftM (SigD fun) $ ty
 
+kiSigD :: Name -> KindQ -> DecQ
+kiSigD fun ki = liftM (KiSigD fun) $ ki
+
 forImpD :: Callconv -> Safety -> String -> Name -> TypeQ -> DecQ
 forImpD cc s str n ty
  = do ty' <- ty
diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 792a78b606..98ddd1c2ca 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -341,6 +341,7 @@ ppr_dec _ (InstanceD o ctxt i ds) =
         text "instance" <+> maybe empty ppr_overlap o <+> pprCxt ctxt <+> ppr i
                                   $$ where_clause ds
 ppr_dec _ (SigD f t)    = pprPrefixOcc f <+> dcolon <+> ppr t
+ppr_dec _ (KiSigD f k)  = text "type" <+> pprPrefixOcc f <+> dcolon <+> ppr k
 ppr_dec _ (ForeignD f)  = ppr f
 ppr_dec _ (InfixD fx n) = pprFixity n fx
 ppr_dec _ (PragmaD p)   = ppr p
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 72eadbff91..59cc5dceef 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -2029,6 +2029,7 @@ data Dec
                                   -- ^ @{ instance {\-\# OVERLAPS \#-\}
                                   --        Show w => Show [w] where ds }@
   | SigD Name Type                -- ^ @{ length :: [a] -> Int }@
+  | KiSigD Name Kind              -- ^ @{ type TypeRep :: k -> Type }@
   | ForeignD Foreign              -- ^ @{ foreign import ... }
                                   --{ foreign export ... }@