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Diffstat (limited to 'docs/users_guide/8.12.1-notes.rst')
| -rw-r--r-- | docs/users_guide/8.12.1-notes.rst | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/docs/users_guide/8.12.1-notes.rst b/docs/users_guide/8.12.1-notes.rst index 38b20df1f8..22b9e0b571 100644 --- a/docs/users_guide/8.12.1-notes.rst +++ b/docs/users_guide/8.12.1-notes.rst @@ -103,6 +103,27 @@ Language instantiated through visible type application. More information can be found here: :ref:`Manually-defining-inferred-variables`. +* GADT constructor types now properly adhere to :ref:`forall-or-nothing`. As + a result, GHC will now reject some GADT constructors that previous versions + of GHC would accept, such as the following: :: + + data T where + MkT1 :: (forall a. a -> b -> T) + MkT2 :: (forall a. a -> T) + + ``MkT1`` and ``MkT2`` are rejected because the lack of an outermost + ``forall`` triggers implicit quantification, making the explicit ``forall``s + nested. Furthermore, GADT constructors do not permit the use of nested + ``forall``s, as explained in :ref:`formal-gadt-syntax`. + + In addition to rejecting nested ``forall``s, GHC is now more stringent about + rejecting uses of nested *contexts* in GADT constructors. For example, the + following example, which previous versions of GHC would accept, is now + rejected: + + data U a where + MkU :: (Show a => U a) + Compiler ~~~~~~~~ |