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authorsheaf <sam.derbyshire@gmail.com>2021-10-28 16:25:08 +0200
committerMarge Bot <ben+marge-bot@smart-cactus.org>2021-10-29 16:59:13 -0400
commit2a4581ffcf3dd80e5933684f46ddfe30447a3ced (patch)
tree37be881b9116a50f1bca2aafc56070ff55e6d4d2
parentc8d89f62a32f05bb2f7aa9a8edda4651301cb76f (diff)
downloadhaskell-2a4581ffcf3dd80e5933684f46ddfe30447a3ced.tar.gz
User's guide: data family kind-inference changes
Explain that the kind of a data family instance must now be fully determined by the header of the instance, and how one might migrate code to account for this change. Fixes #20527
-rw-r--r--docs/users_guide/9.2.1-notes.rst15
1 files changed, 12 insertions, 3 deletions
diff --git a/docs/users_guide/9.2.1-notes.rst b/docs/users_guide/9.2.1-notes.rst
index 6a6dec1ed9..fa9214aa8d 100644
--- a/docs/users_guide/9.2.1-notes.rst
+++ b/docs/users_guide/9.2.1-notes.rst
@@ -39,9 +39,18 @@ Language
.. _Unlifted Datatypes Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0265-unlifted-datatypes.rst
-* Kind inference for data/newtype instance declarations is slightly
- more restrictive than before. See the user manual :ref:`kind-inference-data-family-instances`.
- This is a breaking change, albeit a fairly obscure one that corrects a specification bug.
+* Kind inference for data/newtype instance declarations is slightly more restrictive than before.
+ In particular, GHC now requires that the kind of a data family instance be fully determined
+ by the header of the instance, without looking at the definition of the constructor.
+
+ This means that data families that dispatched on an invisible parameter might now require this parameter
+ to be made explicit, as in the following example: ::
+
+ data family DF :: forall (r :: RuntimeRep). TYPE r
+ newtype instance DF @IntRep = MkDF2 Int#
+ newtype instance DF @FloatRep = MkDF1 Float#
+
+ See the user manual :ref:`kind-inference-data-family-instances`.
* GHC is stricter about checking for out-of-scope type variables on the
right-hand sides of associated type family instances that are not bound on