path: root/docs/users_guide/8.2.1-notes.rst

.. _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
- SCC annotations can now be used for declarations.
- Heap overflow throws an exception in certain circumstances.

Full details
------------

- Heap overflow throws a catchable exception, provided that it was detected
  by the RTS during a GC cycle due to the program exceeding a limit set by
  ``+RTS -M``, and not due to an allocation being refused by the operating
  system.  This exception is thrown to the same thread that receives
  ``UserInterrupt`` exceptions, and may be caught by user programs.

Language
~~~~~~~~

-  TODO FIXME.

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

Compiler
~~~~~~~~

-  TODO FIXME.

- Old profiling flags ``-auto-all``, ``-auto``, and ``-caf-all`` are deprecated
  and their usage provokes a compile-time warning.

- 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.

- 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`).

- 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.

- :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>`.

- :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)

- 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.

- 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::

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

- The Mingw-w64 toolchain for the Windows version of GHC has been updated. GHC now uses
  `GCC 6.2.0` and `binutils 2.27`.

- Previously, :ghc-flag:`-Wmissing-methods` would not warn whenever a type
  class method beginning with an underscore was not implemented in an instance.
  For instance, this code would compile without any warnings: ::

     class Foo a where
       _Bar :: a -> Int

     instance Foo Int

  :ghc-flag:`-Wmissing-methods` will now warn that ``_Bar`` is not implemented
  in the ``Foo Int`` instance.

GHCi
~~~~

-  TODO FIXME.

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

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

-  TODO FIXME.

-  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``.

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

-  Make quoting and reification return the same types.  (:ghc-ticket:`11629`)

-  More kind annotations appear in the left-hand sides of reified closed
   type family equations, in order to disambiguate types that would otherwise
   be ambiguous in the presence of :ghc-flag:`-XPolyKinds`.
   (:ghc-ticket:`12646`)

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

- TODO FIXME.

- Added support for *Compact Regions*, which offer a way to manually
  move long-lived data outside of the heap so that the garbage
  collector does not have to trace it repeatedly.  Compacted data can
  also be serialized, stored, and deserialized again later by the same
  program.  For more details see the :compact-ref:`Data.Compact
  <Data-Compact.html>` module.

- There is new support for improving performance on machines with a
  Non-Uniform Memory Architecture (NUMA).  See :rts-flag:`--numa`.
  This is supported on Linux and Windows systems.

- The garbage collector can be told to use fewer threads than the
  global number of capabilities set by :rts-flag:`-N`.  See
  :rts-flag:`-qn`, and a `blog post
  <http://simonmar.github.io/posts/2016-12-08-Haskell-in-the-datacentre.html>`_
  that describes this.

- 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`).

- Some bugs have been fixed in the stack-trace implementation in the
  profiler that sometimes resulted in incorrect stack traces and
  costs attributed to the wrong cost centre stack (see :ghc-ticket:`5654`).

- 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`)

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)

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

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

- 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.

- Add ``type family AppendSymbol (m :: Symbol) (n :: Symbol) :: Symbol`` to
  ``GHC.TypeLits``

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
~~~

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

-  This is an internal package. Use with caution.

-  TODO FIXME.

ghc-prim
~~~~~~~~

-  Version number XXXXX (was 0.3.1.0)

-  Added new ``isByteArrayPinned#`` and ``isMutableByteArrayPinned#`` operation.

-  New function ``noinline`` in ``GHC.Magic`` lets you mark that a function
   should not be inlined.  It is optimized away after the simplifier runs.

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)

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

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

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