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.. _release-8-2-1:

Release notes for version 8.2.1
===============================

The significant changes to the various parts of the compiler are listed
in the following sections. There have also been numerous bug fixes and
performance improvements over the 8.0 branch.

Highlights
----------

The highlights since the 8.0 branch are:

- TODO FIXME
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- SCC annotations can now be used for declarations.
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Full details
------------

Language
~~~~~~~~

-  TODO FIXME.

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- Pattern synonym signatures can now be applied to multiple patterns, just like
  value-level binding signatures. See :ref:`patsyn-typing` for details.

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Compiler
~~~~~~~~

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-  TODO FIXME.
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- Old profiling flags ``-auto-all``, ``-auto``, and ``-caf-all`` are deprecated
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  and their usage provokes a compile-time warning.
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- Support for adding cost centres to declarations is added. The same `SCC`
  syntax can be used, in addition to a new form for specifying the cost centre
  name. See :ref:`scc-pragma` for examples.
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- It is now possible to explicitly pick a strategy to use when deriving a
  class instance using the :ghc-flag:`-XDerivingStrategies` language extension
  (see :ref:`deriving-strategies`).

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- GHC now allows standalone deriving using :ghc-flag:`-XDeriveAnyClass` on
  any data type, even if its data constructors are not in scope. This is
  consistent with the fact that this code (in the presence of
  :ghc-flag:`-XDeriveAnyClass`): ::

      deriving instance C T

  is exactly equivalent to: ::

      instance C T

  and the latter code has no restrictions about whether the data constructors
  of ``T`` are in scope.

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- :ghc-flag:`-XGeneralizedNewtypeDeriving` now supports deriving type classes
  with associated type families. See the section on
  :ref:`GeneralizedNewtypeDeriving and associated type families
  <gnd-and-associated-types>`.

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- :ghc-flag:`-XGeneralizedNewtypeDeriving` will no longer infer constraints
  when deriving a class with no methods. That is, this code: ::

      class Throws e
      newtype Id a = MkId a
        deriving Throws

  will now generate this instance: ::

      instance Throws (Id a)

  instead of this instance: ::

      instance Throws a => Throws (Id a)

  This change was motivated by the fact that the latter code has a strictly
  redundant ``Throws a`` constraint, so it would emit a warning when compiled
  with :ghc-flag:`-Wredundant-constraints`. The latter instance could still
  be derived if so desired using :ghc-flag:`-XStandaloneDeriving`: ::

      deriving instance Throws a => Throws (Id a)

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- Add warning flag :ghc-flag:`-Wcpp-undef` which passes ``-Wundef`` to the C
  pre-processor causing the pre-processor to warn on uses of the ``#if``
  directive on undefined identifiers.

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- GHC will no longer automatically infer the kind of higher-rank type synonyms;
  you must explicitly explicitly annotate the synonym with a kind signature.
  For example, given::

    data T :: (forall k. k -> Type) -> Type

  to define a synonym of ``T``, you must write::

    data TSyn = (T :: (forall k. k -> Type) -> Type)

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GHCi
~~~~

-  TODO FIXME.

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- Added :ghc-flag:`-flocal-ghci-history` which uses current directory for `.ghci-history`.

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Template Haskell
~~~~~~~~~~~~~~~~

-  TODO FIXME.

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-  Reifying types that contain unboxed tuples now works correctly. (Previously,
   Template Haskell reified unboxed tuples as boxed tuples with twice their
   appropriate arity.)

-  Splicing singleton unboxed tuple types (e.g., ``(# Int #)``) now works
   correctly. Previously, Template Haskell would implicitly remove the
   parentheses when splicing, which would turn ``(# Int #)`` into ``Int``.

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-  Add support for type signatures in patterns. (:ghc-ticket:`12164`)

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Runtime system
~~~~~~~~~~~~~~

-  TODO FIXME.

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- The :ref:`heap profiler <prof-heap>` can now emit heap census data to the GHC
  event log, allowing heap profiles to be correlated with other tracing events
  (see :ghc-ticket:`11094`).

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- Added NUMA support to Windows.

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- Added processor group support for Windows. This allows the runtime to allocate
  threads to all cores in systems which have multiple processor groups.
  (e.g. > 64 cores, see :ghc-ticket:`11054`)

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Build system
~~~~~~~~~~~~

-  TODO FIXME.

Package system
~~~~~~~~~~~~~~

-  TODO FIXME.

hsc2hs
~~~~~~

-  TODO FIXME.

Libraries
---------

array
~~~~~

-  Version number XXXXX (was 0.5.0.0)


.. _lib-base:

base
~~~~

See ``changelog.md`` in the ``base`` package for full release notes.

-  Version number 4.10.0.0 (was 4.9.0.0)

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- ``Data.Either`` now provides ``fromLeft`` and ``fromRight``

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- ``Data.Type.Coercion`` now provides ``gcoerceWith``, which is analogous to
  ``gcastWith`` from ``Data.Type.Equality``.

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- The ``Read1`` and ``Read2`` classes in ``Data.Functor.Classes`` have new
  methods, ``liftReadList(2)`` and ``liftReadListPrec(2)``, that are defined in
  terms of ``ReadPrec`` instead of ``ReadS``. This matches the interface
  provided in GHC's version of the ``Read`` class, and allows users to write
  more efficient ``Read1`` and ``Read2`` instances.

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binary
~~~~~~

-  Version number XXXXX (was 0.7.1.0)

bytestring
~~~~~~~~~~

-  Version number XXXXX (was 0.10.4.0)

Cabal
~~~~~

-  Version number XXXXX (was 1.18.1.3)

containers
~~~~~~~~~~

-  Version number XXXXX (was 0.5.4.0)

deepseq
~~~~~~~

-  Version number XXXXX (was 1.3.0.2)

directory
~~~~~~~~~

-  Version number XXXXX (was 1.2.0.2)

filepath
~~~~~~~~

-  Version number XXXXX (was 1.3.0.2)

ghc
~~~

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-
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ghc-boot
~~~~~~~~

-  This is an internal package. Use with caution.

-  TODO FIXME.

ghc-prim
~~~~~~~~

-  Version number XXXXX (was 0.3.1.0)

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-  Added new ``isByteArrayPinned#`` and ``isMutableByteArrayPinned#`` operation.
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-  New function ``noinline`` in ``GHC.Magic`` lets you mark that a function
   should not be inlined.  It is optimized away after the simplifier runs.

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haskell98
~~~~~~~~~

-  Version number XXXXX (was 2.0.0.3)

haskell2010
~~~~~~~~~~~

-  Version number XXXXX (was 1.1.1.1)

hoopl
~~~~~

-  Version number XXXXX (was 3.10.0.0)

hpc
~~~

-  Version number XXXXX (was 0.6.0.1)

integer-gmp
~~~~~~~~~~~

-  Version number XXXXX (was 0.5.1.0)

old-locale
~~~~~~~~~~

-  Version number XXXXX (was 1.0.0.6)

old-time
~~~~~~~~

-  Version number XXXXX (was 1.1.0.2)

process
~~~~~~~

-  Version number XXXXX (was 1.2.0.0)

template-haskell
~~~~~~~~~~~~~~~~

-  Version number XXXXX (was 2.9.0.0)

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-  Added support for unboxed sums :ghc-ticket:`12478`.

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-  Added support for visible type applications :ghc-ticket:`12530`.

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time
~~~~

-  Version number XXXXX (was 1.4.1)

unix
~~~~

-  Version number XXXXX (was 2.7.0.0)

Win32
~~~~~

-  Version number XXXXX (was 2.3.0.1)

Known bugs
----------

-  TODO FIXME