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, 16 insertions, 21 deletions

diff --git a/compiler/hsSyn/Convert.hs b/compiler/hsSyn/Convert.hs
index 7487983419..7b721ed1f2 100644
--- a/compiler/hsSyn/Convert.hs
+++ b/compiler/hsSyn/Convert.hs
@@ -963,7 +963,7 @@ the trees to reflect the fixities of the underlying operators:
 This is done by the renamer (see @mkOppAppRn@, @mkConOppPatRn@, and
 @mkHsOpTyRn@ in RnTypes), which expects that the input will be completely
 right-biased for types and left-biased for everything else. So we left-bias the
-trees of @UInfixP@ and @UInfixE@ and use HsAppsTy for UInfixT.
+trees of @UInfixP@ and @UInfixE@ and right-bias the trees of @UInfixT@.
 
 Sample input:
 
@@ -1332,10 +1332,8 @@ cvtTypeKind ty_str ty
                    }
 
            UInfixT t1 s t2
-             -> do { t1' <- cvtType t1
-                   ; t2' <- cvtType t2
-                   ; s'  <- tconName s
-                   ; return $ cvtOpAppT t1' s' t2'
+             -> do { t2' <- cvtType t2
+                   ; cvtOpAppT t1 s t2'
                    } -- Note [Converting UInfix]
 
            ParensT t
@@ -1445,23 +1443,20 @@ cvtTyLit :: TH.TyLit -> HsTyLit
 cvtTyLit (TH.NumTyLit i) = HsNumTy NoSourceText i
 cvtTyLit (TH.StrTyLit s) = HsStrTy NoSourceText (fsLit s)
 
-{- | @cvtOpAppT x op y@ takes converted arguments and flattens any HsAppsTy
-   structure in them.
+{- | @cvtOpAppT x op y@ converts @op@ and @y@ and produces the operator
+application @x `op` y@. The produced tree of infix types will be right-biased,
+provided @y@ is.
+
+See the @cvtOpApp@ documentation for how this function works.
 -}
-cvtOpAppT :: LHsType GhcPs -> RdrName -> LHsType GhcPs -> LHsType GhcPs
-cvtOpAppT t1@(L loc1 _) op t2@(L loc2 _)
-  = L (combineSrcSpans loc1 loc2) $
-    HsAppsTy noExt (t1' ++ [noLoc $ HsAppInfix noExt (noLoc op)] ++ t2')
-  where
-    t1' | L _ (HsAppsTy _ t1s) <- t1
-        = t1s
-        | otherwise
-        = [noLoc $ HsAppPrefix noExt t1]
-
-    t2' | L _ (HsAppsTy _ t2s) <- t2
-        = t2s
-        | otherwise
-        = [noLoc $ HsAppPrefix noExt t2]
+cvtOpAppT :: TH.Type -> TH.Name -> LHsType GhcPs -> CvtM (LHsType GhcPs)
+cvtOpAppT (UInfixT x op2 y) op1 z
+  = do { l <- cvtOpAppT y op1 z
+       ; cvtOpAppT x op2 l }
+cvtOpAppT x op y
+  = do { op' <- tconNameL op
+       ; x' <- cvtType x
+       ; returnL (mkHsOpTy x' op' y) }
 
 cvtKind :: TH.Kind -> CvtM (LHsKind GhcPs)
 cvtKind = cvtTypeKind "kind"