[commit: ghc] master: Comments only (b37f216)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/b37f2164c08bfc1623ca4d9cab7cc4c2ebbc2e40/ghc

>---------------------------------------------------------------

commit b37f2164c08bfc1623ca4d9cab7cc4c2ebbc2e40
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Wed Dec 23 10:11:56 2015 +0000

    Comments only


>---------------------------------------------------------------

b37f2164c08bfc1623ca4d9cab7cc4c2ebbc2e40
 compiler/rename/RnTypes.hs | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/compiler/rename/RnTypes.hs b/compiler/rename/RnTypes.hs
index 1731ef7..52a164f 100644
--- a/compiler/rename/RnTypes.hs
+++ b/compiler/rename/RnTypes.hs
@@ -349,6 +349,35 @@ and
 as our lists. We can then do normal fixity resolution on these. The fixities
 must come along for the ride just so that the list stays in sync with the
 operators.
+
+Note [QualTy in kinds]
+~~~~~~~~~~~~~~~~~~~~~~
+I was wondering whether QualTy could occur only at TypeLevel.  But no,
+we can have a qualified type in a kind too. Here is an example:
+
+  type family F a where
+    F Bool = Nat
+    F Nat  = Type
+
+  type family G a where
+    G Type = Type -> Type
+    G ()   = Nat
+
+  data X :: forall k1 k2. (F k1 ~ G k2) => k1 -> k2 -> Type where
+    MkX :: X 'True '()
+
+See that k1 becomes Bool and k2 becomes (), so the equality is
+satisfied. If I write MkX :: X 'True 'False, compilation fails with a
+suitable message:
+
+  MkX :: X 'True '()
+    • Couldn't match kind ‘G Bool’ with ‘Nat’
+      Expected kind: G Bool
+        Actual kind: F Bool
+
+However: in a kind, the constraints in the QualTy must all be
+equalities; or at least, any kinds with a class constraint are
+uninhabited.
 -}
 
 data RnTyKiEnv
@@ -436,9 +465,8 @@ rnHsTyKi env ty@(HsForAllTy { hst_bndrs = tyvars, hst_body  = tau })
        ; return ( HsForAllTy { hst_bndrs = tyvars', hst_body =  tau' }
                 , fvs) } }
 
-rnHsTyKi env ty@(HsQualTy { hst_ctxt = lctxt
-                          , hst_body = tau })
-  = do { checkTypeInType env ty
+rnHsTyKi env ty@(HsQualTy { hst_ctxt = lctxt, hst_body = tau })
+  = do { checkTypeInType env ty  -- See Note [QualTy in kinds]
        ; (ctxt', fvs1) <- rnTyKiContext env lctxt
        ; (tau',  fvs2) <- rnLHsTyKi env tau
        ; return (HsQualTy { hst_ctxt = ctxt', hst_body =  tau' }