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.. _release-9-6-1:
Version 9.6.1
==============
Language
~~~~~~~~
- Record updates for GADTs and other existential datatypes are now
fully supported.
For example: ::
data D b where
MkD :: { fld1 :: a -> a, fld2 :: a -> (), fld3 :: b } -> D b
foo :: D b -> D b
foo d = d { fld1 = id, fld2 = const () }
In this example, we have an existential variable ``a``, and we update
all fields whose type involves ``a`` at once, so the update is valid.
A side-effect of this change is that GHC now rejects some record updates
involving fields whose types contain type families (these record updates
were previously erroneously accepted).
Example: ::
type family F a where
F Int = Char
F Float = Char
data T b = MkT { x :: [Int], y :: [F b] }
emptyT :: forall b. T b
emptyT = MkT [] []
bar :: T Int
bar = emptyT { x = [3] }
In this example, we can't infer the type of ``emptyT`` in ``bar``: it could be
``T Int``, but it could also be ``T Float`` because the type family ``F``
is not injective and ``T Float ~ T Int``. Indeed, the following typechecks ::
baz :: T Int
baz = case ( emptyT :: T Float ) of { MkT _ y -> MkT [3] y }
This means that the type of ``emptyT`` is ambiguous in the definition
of ``bar`` above, and thus GHC rejects the record update: ::
Couldn't match type `F b0' with `Char'
Expected: [F Int]
Actual: [F b0]
NB: âFâ is a non-injective type family
The type variable âb0â is ambiguous
To fix these issues, add a type signature to the expression that the
record update is applied to (``emptyT`` in the example above), or
add an injectivity annotation to the type family in the case that
the type family is in fact injective.
Compiler
~~~~~~~~
- The :extension:`TypeInType` is now marked as deprecated. Its meaning has been included
in :extension:`PolyKinds` and :extension:`DataKinds`.
``base`` library
~~~~~~~~~~~~~~~~
- Exceptions thrown by weak pointer finalizers are now caught and reported
via a global exception handler. By default this handler reports the error
to ``stderr`` although this can be changed using
``GHC.Weak.Finalize.setFinalizerExceptionHandler``.
``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~
``ghc`` library
~~~~~~~~~~~~~~~
``ghc-heap`` library
~~~~~~~~~~~~~~~
- The ``link`` field of ``GHC.Exts.Heap.WeakClosure`` has been replaced with a
``weakLink`` field which is ``Nothing`` if and only if ``link`` would have
been NULL.
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