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.

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/basicTypes/NameEnv.hs b/compiler/basicTypes/NameEnv.hs
index 5f6bdc5846..03c2150102 100644
--- a/compiler/basicTypes/NameEnv.hs
+++ b/compiler/basicTypes/NameEnv.hs
@@ -11,7 +11,7 @@ module NameEnv (
         NameEnv,
 
         -- ** Manipulating these environments
-        mkNameEnv,
+        mkNameEnv, mkNameEnvWith,
         emptyNameEnv, isEmptyNameEnv,
         unitNameEnv, nameEnvElts,
         extendNameEnv_C, extendNameEnv_Acc, extendNameEnv,
@@ -92,6 +92,7 @@ type NameEnv a = UniqFM a       -- Domain is Name
 emptyNameEnv       :: NameEnv a
 isEmptyNameEnv     :: NameEnv a -> Bool
 mkNameEnv          :: [(Name,a)] -> NameEnv a
+mkNameEnvWith      :: (a -> Name) -> [a] -> NameEnv a
 nameEnvElts        :: NameEnv a -> [a]
 alterNameEnv       :: (Maybe a-> Maybe a) -> NameEnv a -> Name -> NameEnv a
 extendNameEnv_C    :: (a->a->a) -> NameEnv a -> Name -> a -> NameEnv a
@@ -121,6 +122,7 @@ extendNameEnvList x l = addListToUFM x l
 lookupNameEnv x y     = lookupUFM x y
 alterNameEnv          = alterUFM
 mkNameEnv     l       = listToUFM l
+mkNameEnvWith f       = mkNameEnv . map (\a -> (f a, a))
 elemNameEnv x y          = elemUFM x y
 plusNameEnv x y          = plusUFM x y
 plusNameEnv_C f x y      = plusUFM_C f x y