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authorRyan Scott <ryan.gl.scott@gmail.com>2019-10-13 06:58:44 -0400
committerMarge Bot <ben+marge-bot@smart-cactus.org>2019-10-14 08:39:12 -0400
commit4aba72d62da2fbb499a5eaace361c49c04533600 (patch)
tree3356ce10a36220ccd9db20a26e1bf94e9e738a02
parent78463fc5a238fa01f73bbed236ad17d0a2c37c0b (diff)
downloadhaskell-4aba72d62da2fbb499a5eaace361c49c04533600.tar.gz
Mention changes from #16980, #17213 in 8.10.1 release notes
The fixes for these issues both have user-facing consequences, so it would be good to mention them in the release notes for GHC 8.10.1. While I'm in town, also mention `UnboxedSums` in the release notes entry related to `-fobject-code`.
-rw-r--r--docs/users_guide/8.10.1-notes.rst64
1 files changed, 59 insertions, 5 deletions
diff --git a/docs/users_guide/8.10.1-notes.rst b/docs/users_guide/8.10.1-notes.rst
index 3251b326d5..0d7be2e6d8 100644
--- a/docs/users_guide/8.10.1-notes.rst
+++ b/docs/users_guide/8.10.1-notes.rst
@@ -114,6 +114,28 @@ Language
class C a
newtype T a = MkT a deriving C
+- GHC now performs more validity checks on inferred type signatures. One
+ consequence of this change is that some programs that used to be accepted
+ will no longer compile without enabling the required language extensions.
+ For example, in these two modules: ::
+
+ {-# LANGUAGE RankNTypes #-}
+ module A where
+
+ foo :: (forall a. a -> a) -> b -> b
+ foo f x = f x
+
+ module B where
+
+ import A
+
+ bar = foo
+
+ Notice that ``A`` enables :ghc-flag:`RankNTypes`, but ``B`` does not.
+ Previous versions of GHC would allow ``bar`` to typecheck, even though its
+ inferred type is higher-rank. GHC 8.10 will now reject this, as one must now
+ enable :ghc-flag:`RankNTypes` in ``B`` to accept the inferred type signature.
+
Compiler
~~~~~~~~
@@ -131,11 +153,11 @@ Compiler
process, as long as there are no native dependencies that rely on
global state.
-- When loading modules that use :extension:`UnboxedTuples` into GHCi,
- it will now automatically enable :ghc-flag:`-fobject-code` for
- these modules and all modules they depend on. Before this change,
- attempting to load these modules into the interpreter would just
- fail, and the only convenient workaround was to enable
+- When loading modules that use :extension:`UnboxedTuples` or
+ :extension:`UnboxedSums` into GHCi, it will now automatically enable
+ :ghc-flag:`-fobject-code` for these modules and all modules they depend on.
+ Before this change, attempting to load these modules into the interpreter
+ would just fail, and the only convenient workaround was to enable
:ghc-flag:`-fobject-code` for all modules. See the
:ref:`GHCi FAQ <ghci-faq>` for further details.
@@ -175,6 +197,38 @@ Template Haskell
without flattening. In most of the cases these will be obtained using
Template Haskell since it is uncommon to deal with 1-tuples in the source.
+- GHC's constraint solver now solves constraints in each top-level group
+ sooner. This has practical consequences for Template Haskell, as TH splices
+ necessarily separate top-level groups. For example, the following program
+ would compile in previous versions of GHC, but not in GHC 8.10: ::
+
+ data T = MkT
+
+ tStr :: String
+ tStr = show MkT
+
+ $(return [])
+
+ instance Show T where
+ show MkT = "MkT"
+
+ This is because each top-level group's constraints are solved before moving
+ on to the next, and since the top-level group for ``tStr`` appears before the
+ top-level group that defines a ``Show T`` instance, GHC 8.10 will throw an
+ error about a missing ``Show T`` instance in the expression ``show MkT``. The
+ issue can be fixed by rearranging the order of declarations. For instance,
+ the following will compile: ::
+
+ data T = MkT
+
+ instance Show T where
+ show MkT = "MkT"
+
+ $(return [])
+
+ tStr :: String
+ tStr = show MkT
+
``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~