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| -rw-r--r-- | docs/users_guide/9.2.1-notes.rst | 15 |
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 |