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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.
- Error messages are now assigned unique error codes, of the form ``[GHC-12345]``.
- GHC Proposal `#106
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0106-type-data.rst>`_
has been implemented, introducing a new language extension
:extension:`TypeData`. This extension permits ``type data`` declarations
as a more fine-grained alternative to :extension:`DataKinds`.
- GHC now does a better job of solving constraints in the presence of multiple
matching quantified constraints. For example, if we want to solve
``C a b Int`` and we have matching quantified constraints: ::
forall x y z. (Ord x, Enum y, Num z) => C x y z
forall u v. (Enum v, Eq u) => C u v Int
Then GHC will use the second quantified constraint to solve ``C a b Int``,
as it has a strictly weaker precondition.
Compiler
~~~~~~~~
- The :extension:`TypeInType` is now marked as deprecated. Its meaning has been included
in :extension:`PolyKinds` and :extension:`DataKinds`.
- The :ghc-flag:`-Woperator-whitespace` warning no longer ignores constructor symbols
(operators starting with ``:``).
GHCi
~~~~
- GHCi will now accept any file-header pragmas it finds, such as
``{-# OPTIONS_GHC ... #-}`` and ``{-# LANGUAGE ... #-}`` (see :ref:`pragmas`). For example,
instead of using :ghci-cmd:`:set` to enable :ghc-flag:`-Wmissing-signatures`,
you could instead write:
.. code-block:: none
ghci> {-# OPTIONS_GHC -Wmissing-signatures #-}
This can be convenient when pasting large multi-line blocks of code into GHCi.
Runtime system
~~~~~~~~~~~~~~
- The `Delimited continuation primops <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0313-delimited-continuation-primops.rst>`_
proposal has been implemented, adding native support for first-class,
delimited continuations to the RTS. For the reasons given in the proposal,
no safe API to access this functionality is provided anywhere in ``base``.
Instead, the ``prompt#`` and ``control0#`` primops are intended to be consumed
by library authors directly, who may wrap them a safe API that maintains the
necessary invariants. See the documentation in ``GHC.Prim`` for more details.
``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 now provides a set of operations for introspecting on the threads of a
program, ``GHC.Conc.listThreads``, as well as operations for querying a thread's
label (:base-ref:`GHC.Conc.threadLabel`) and status
(:base-ref:`GHC.Conc.threadStatus`).
- Change default ``Ord`` implementation of ``(>=)``, ``(>)``, and ``(<)`` to use
``(<=)`` instead of ``compare`` per CLC proposal:
https://github.com/haskell/core-libraries-committee/issues/24
- Updated to `Unicode 15.0.0 <https://www.unicode.org/versions/Unicode15.0.0/>`_.
``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~
``ghc`` library
~~~~~~~~~~~~~~~
``ghc-heap`` library
~~~~~~~~~~~~~~~~~~~~
Included libraries
------------------
The package database provided with this distribution also contains a number of
packages other than GHC itself. See the changelogs provided with these packages
for further change information.
.. ghc-package-list::
libraries/array/array.cabal: Dependency of ``ghc`` library
libraries/base/base.cabal: Core library
libraries/binary/binary.cabal: Dependency of ``ghc`` library
libraries/bytestring/bytestring.cabal: Dependency of ``ghc`` library
libraries/Cabal/Cabal/Cabal.cabal: Dependency of ``ghc-pkg`` utility
libraries/Cabal/Cabal-syntax/Cabal-syntax.cabal: Dependency of ``ghc-pkg`` utility
libraries/containers/containers/containers.cabal: Dependency of ``ghc`` library
libraries/deepseq/deepseq.cabal: Dependency of ``ghc`` library
libraries/directory/directory.cabal: Dependency of ``ghc`` library
libraries/exceptions/exceptions.cabal: Dependency of ``ghc`` and ``haskeline`` library
libraries/filepath/filepath.cabal: Dependency of ``ghc`` library
compiler/ghc.cabal: The compiler itself
libraries/ghci/ghci.cabal: The REPL interface
libraries/ghc-boot/ghc-boot.cabal: Internal compiler library
libraries/ghc-boot-th/ghc-boot-th.cabal: Internal compiler library
libraries/ghc-compact/ghc-compact.cabal: Core library
libraries/ghc-heap/ghc-heap.cabal: GHC heap-walking library
libraries/ghc-prim/ghc-prim.cabal: Core library
libraries/haskeline/haskeline.cabal: Dependency of ``ghci`` executable
libraries/hpc/hpc.cabal: Dependency of ``hpc`` executable
libraries/integer-gmp/integer-gmp.cabal: Core library
libraries/libiserv/libiserv.cabal: Internal compiler library
libraries/mtl/mtl.cabal: Dependency of ``Cabal`` library
libraries/parsec/parsec.cabal: Dependency of ``Cabal`` library
libraries/pretty/pretty.cabal: Dependency of ``ghc`` library
libraries/process/process.cabal: Dependency of ``ghc`` library
libraries/stm/stm.cabal: Dependency of ``haskeline`` library
libraries/template-haskell/template-haskell.cabal: Core library
libraries/terminfo/terminfo.cabal: Dependency of ``haskeline`` library
libraries/text/text.cabal: Dependency of ``Cabal`` library
libraries/time/time.cabal: Dependency of ``ghc`` library
libraries/transformers/transformers.cabal: Dependency of ``ghc`` library
libraries/unix/unix.cabal: Dependency of ``ghc`` library
libraries/Win32/Win32.cabal: Dependency of ``ghc`` library
libraries/xhtml/xhtml.cabal: Dependency of ``haddock`` executable
|
