Embrace -XTypeInType, add -XStarIsType

Summary:
Implement the "Embrace Type :: Type" GHC proposal,
.../ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst

GHC 8.0 included a major change to GHC's type system: the Type :: Type
axiom. Though casual users were protected from this by hiding its
features behind the -XTypeInType extension, all programs written in GHC
8+ have the axiom behind the scenes. In order to preserve backward
compatibility, various legacy features were left unchanged. For example,
with -XDataKinds but not -XTypeInType, GADTs could not be used in types.
Now these restrictions are lifted and -XTypeInType becomes a redundant
flag that will be eventually deprecated.

* Incorporate the features currently in -XTypeInType into the
  -XPolyKinds and -XDataKinds extensions.
* Introduce a new extension -XStarIsType to control how to parse * in
  code and whether to print it in error messages.

Test Plan: Validate

Reviewers: goldfire, hvr, bgamari, alanz, simonpj

Reviewed By: goldfire, simonpj

Subscribers: rwbarton, thomie, mpickering, carter

GHC Trac Issues: #15195

Differential Revision: https://phabricator.haskell.org/D4748

1 files changed, 3 insertions, 21 deletions

diff --git a/compiler/prelude/TysWiredIn.hs b/compiler/prelude/TysWiredIn.hs
index 9ba2f1f45a..1156d810b9 100644
--- a/compiler/prelude/TysWiredIn.hs
+++ b/compiler/prelude/TysWiredIn.hs
@@ -91,9 +91,8 @@ module TysWiredIn (
         -- * Kinds
         typeNatKindCon, typeNatKind, typeSymbolKindCon, typeSymbolKind,
         isLiftedTypeKindTyConName, liftedTypeKind, constraintKind,
-        starKindTyCon, starKindTyConName,
-        unicodeStarKindTyCon, unicodeStarKindTyConName,
         liftedTypeKindTyCon, constraintKindTyCon,
+        liftedTypeKindTyConName,
 
         -- * Equality predicates
         heqTyCon, heqClass, heqDataCon,
@@ -224,8 +223,6 @@ wiredInTyCons = [ -- Units are not treated like other tuples, because then
                 , vecElemTyCon
                 , constraintKindTyCon
                 , liftedTypeKindTyCon
-                , starKindTyCon
-                , unicodeStarKindTyCon
                 ]
 
 mkWiredInTyConName :: BuiltInSyntax -> Module -> FastString -> Unique -> TyCon -> Name
@@ -389,11 +386,8 @@ typeSymbolKindConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "Symbol")
 constraintKindTyConName :: Name
 constraintKindTyConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "Constraint") constraintKindTyConKey   constraintKindTyCon
 
-liftedTypeKindTyConName, starKindTyConName, unicodeStarKindTyConName
-  :: Name
+liftedTypeKindTyConName :: Name
 liftedTypeKindTyConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "Type") liftedTypeKindTyConKey liftedTypeKindTyCon
-starKindTyConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "*") starKindTyConKey starKindTyCon
-unicodeStarKindTyConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "★") unicodeStarKindTyConKey unicodeStarKindTyCon
 
 runtimeRepTyConName, vecRepDataConName, tupleRepDataConName, sumRepDataConName :: Name
 runtimeRepTyConName = mkWiredInTyConName UserSyntax gHC_TYPES (fsLit "RuntimeRep") runtimeRepTyConKey runtimeRepTyCon
@@ -1063,25 +1057,13 @@ mk_class tycon sc_pred sc_sel_id
 runtimeRepTy :: Type
 runtimeRepTy = mkTyConTy runtimeRepTyCon
 
-liftedTypeKindTyCon, starKindTyCon, unicodeStarKindTyCon :: TyCon
-
 -- Type synonyms; see Note [TYPE and RuntimeRep] in TysPrim
 -- type Type = tYPE 'LiftedRep
--- type *    = tYPE 'LiftedRep
--- type *    = tYPE 'LiftedRep  -- Unicode variant
-
+liftedTypeKindTyCon :: TyCon
 liftedTypeKindTyCon   = buildSynTyCon liftedTypeKindTyConName
                                        [] liftedTypeKind []
                                        (tYPE liftedRepTy)
 
-starKindTyCon         = buildSynTyCon starKindTyConName
-                                       [] liftedTypeKind []
-                                       (tYPE liftedRepTy)
-
-unicodeStarKindTyCon  = buildSynTyCon unicodeStarKindTyConName
-                                       [] liftedTypeKind []
-                                       (tYPE liftedRepTy)
-
 runtimeRepTyCon :: TyCon
 runtimeRepTyCon = pcTyCon runtimeRepTyConName Nothing []
                           (vecRepDataCon : tupleRepDataCon :