Stable name type role

Make the `StableName#` parameter phantom:

There is actually never any reason to care about the type of
the underlying object of a `StableName#`. The underlying object
type shouldn't really even *be* a parameter. But at least we
can mark it as phantom.

Reviewers: hvr, bgamari, erikd, simonmar

Reviewed By: simonmar

Subscribers: ekmett, rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5117

3 files changed, 13 insertions, 1 deletions

diff --git a/compiler/prelude/TysPrim.hs b/compiler/prelude/TysPrim.hs
index c5af4a5121..4a69df8e3e 100644
--- a/compiler/prelude/TysPrim.hs
+++ b/compiler/prelude/TysPrim.hs
@@ -943,7 +943,7 @@ mkStablePtrPrimTy ty = TyConApp stablePtrPrimTyCon [ty]
 -}
 
 stableNamePrimTyCon :: TyCon
-stableNamePrimTyCon = pcPrimTyCon stableNamePrimTyConName [Representational] UnliftedRep
+stableNamePrimTyCon = pcPrimTyCon stableNamePrimTyConName [Phantom] UnliftedRep
 
 mkStableNamePrimTy :: Type -> Type
 mkStableNamePrimTy ty = TyConApp stableNamePrimTyCon [ty]
diff --git a/docs/users_guide/8.8.1-notes.rst b/docs/users_guide/8.8.1-notes.rst
index 6bdde400bb..a27aee721e 100644
--- a/docs/users_guide/8.8.1-notes.rst
+++ b/docs/users_guide/8.8.1-notes.rst
@@ -104,6 +104,10 @@ Template Haskell
   eventlog events similar to ``traceBinaryEvent#`` but allows the user to pass
   the event payload as a binary blob instead of a ``String``.
 
+- The ``StableName#`` type parameter now has a phantom role instead of
+  a representational one. There is really no reason to care about the
+  type of the underlying object.
+
 ``ghc`` library
 ~~~~~~~~~~~~~~~
 
@@ -114,6 +118,10 @@ Template Haskell
 - Support the characters from recent versions of Unicode (up to v. 12) in literals
     (see :ghc-ticket:`5518`).
 
+- The ``StableName`` type parameter now has a phantom role instead of
+  a representational one. There is really no reason to care about the
+  type of the underlying object.
+
 Build system
 ~~~~~~~~~~~~
 
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 8964edd7ff..3709a0a1a2 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -6,6 +6,10 @@
   * Support the characters from recent versions of Unicode (up to v. 12) in
     literals (#5518).
 
+  * The `StableName` type parameter now has a phantom role instead of
+    a representational one. There is really no reason to care about the
+    type of the underlying object.
+
 ## 4.12.0.0 *TBA*
   * Bundled with GHC *TBA*