.. _release-8-6-1:
Release notes for version 8.6.1
===============================
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 8.4.1 release.
Highlights
----------
The highlights, since the 8.4.1 release, are:
- Support for :extension:`QuantifiedConstraints`.
- A new flexible deriving scheme, :extension:`DerivingVia`.
- A new plugin mechanism and support for plugins to modify their effect on GHC's
recompilation checking logic.
- :ref:`Valid hole fits <typed-hole-valid-hole-fits>` in error messages.
- A number of syntactic language extensions.
- Programs are no longer constrained by the Windows ``MAX_PATH`` file path
length limit. The file path limit is now approximately 32,767 characters. Note
that GHC itself is still somewhat limited due to GCC not supporting file
namespaced paths. Paths that are passed directly to the compiler, linker or
other GNU tools are currently still constrained. See :ref:`windows-file-paths`
for details.
- Many, many bug fixes.
Full details
------------
Language
~~~~~~~~
- Use of quantified type variables in constraints is now allowed via the
:extension:`QuantifiedConstraints` language extension. This long-awaited feature
enables users to encode significantly more precision in their types. For instance,
the common ``MonadTrans`` typeclass could now make the expectation that an
applied transformer is must be a ``Monad`` ::
class (forall m. Monad m => Monad (t m)) => MonadTrans t where {- ... -}
Additionally, quantification can enable terminating instance resolution
where this previously was not possible. See :ref:`quantified-constraints` for
details.
- A new :extension:`DerivingVia` language extension has been added which allows
the use of the ``via`` deriving strategy. For instance: ::
newtype T = MkT Int
deriving Monoid via (Sum Int)
See :ref:`deriving-via` for more information.
- A new :extension:`StarIsType` language extension has been added which controls
whether ``*`` is parsed as ``Data.Kind.Type`` or a regular type operator.
:extension:`StarIsType` is enabled by default.
- GHC now permits the use of a wildcard type as the context of a standalone
``deriving`` declaration with the use of the
:extension:`PartialTypeSignatures` language extension. For instance, this
declaration: ::
deriving instance _ => Eq (Foo a)
Denotes a derived ``Eq (Foo a)`` instance, where the context is inferred in
much the same way as ordinary ``deriving`` clauses do.
See :ref:`partial-type-signatures`.
- Data declarations with empty ``where`` clauses are no longer valid without the
extension :extension:`GADTSyntax` enabled. For instance, consider the
following, ::
data T where
The grammar is invalid in Haskell2010. Previously it could be compiled successfully
without ``GADTs``. As of GHC 8.6.1, this is a parse error.
- Incomplete patterns warning :ghc-flag:`-Wincomplete-patterns` is extended to
guards in pattern bindings and ``if`` alternatives of :extension:`MultiWayIf`.
For instance, consider the following, ::
foo :: Bool -> Int
foo b = if | b -> 1
In GHC 8.6.1, it will raise the warning: ::
<interactive>:2:12: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In a multi-way if alternative:
Guards do not cover entire pattern space
See :ghc-ticket:`14773`.
- Scoped type variables now work in default methods of class declarations
and in pattern synonyms in Template Haskell. See :ghc-ticket:`14885`.
- ``do`` expressions, lambda expressions, etc. to be directly used as
a function argument, enabled with :extension:`BlockArguments`.
See :ref:`More liberal syntax for function arguments <block-arguments>`
for the full details.
- Underscores in numeric literals (e.g. ``1_000_000``), enabled with
:extension:`NumericUnderscores`.
See :ref:`Numeric underscores <numeric-underscores>`
for the full details.
- CUSKs now require all kind variables to be explicitly quantified. This was
already the case with :extension:`TypeInType`, but now :extension:`PolyKinds`
also exhibits this behavior. This means that the following example is no
longer considered to have a CUSK::
data T1 :: k -> Type -- No CUSK: `k` is not explicitly quantified
- Functionality of :extension:`TypeInType` has been subsumed by
:extension:`PolyKinds`, and it is now merely a shorthand for
:extension:`PolyKinds`, :extension:`DataKinds`, and :extension:`NoStarIsType`.
The users are advised to avoid :extension:`TypeInType` due to its misleading
name: the ``Type :: Type`` axiom holds regardless of whether it is enabled.
- GHC has become more diligent about catching illegal uses of kind polymorphism.
For instance, GHC 8.4 would accept the following without the use of
:extension:`PolyKinds`::
f :: forall k (a :: k). Proxy a
f = Proxy
This is now an error unless :extension:`PolyKinds` is enabled.
- Type literals now could be used in type class instances without the extension
:extension:`FlexibleInstances`.
See :ghc-ticket:`13833`.
- :extension:`MonadFailDesugaring` is now enabled by default. See
`MonadFail Proposal (MFP)
<https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail>`__
for more details.
Compiler
~~~~~~~~
- GHC now no longer adds the current file's directory as a general include path
calling the C compiler. Instead we use :ghc-flag:`-iquote` to only add it as
an include path for `#include ""`. See :ghc-ticket:`14312`.
- GHC now supports British spelling of :extension:`GeneralizedNewtypeDeriving`.
- GHC now does significantly more constant folding in its core-to-core optimiser.
This will result in significantly better code being generated for some
programs. See :ghc-ticket:`9136`.
- GHC now offers significantly more information about typed holes such as valid
hole fits and refinement hole fits. See :ref:`Valid Hole Fits <typed-hole-valid-hole-fits>`
for more information.
- The code-generation effects of :ghc-flag:`-dynamic` can now be
enabled independently by the flag
:ghc-flag:`-fexternal-dynamic-refs`. If you don't know why you might
need this, you don't need it.
- :ghc-flag:`-Wcompat` now includes :ghc-flag:`-Wimplicit-kind-vars` to
provide early detection of breakage that will be caused by implementation of
`GHC proposal #24
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0024-no-kind-vars.rst>`__
in a future release.
- The :ghc-flag:`-package-env ⟨file⟩|⟨name⟩` flag and ``GHC_ENVIRONMENT``
environment variable now accept the ``-`` value, which instructs GHC to ignore
any package environment files.
Plugins
~~~~~~~
- GHC's plugin mechanism now offers plugin authors control over their plugin's
effect on recompilation checking. Specifically the ``Plugin`` record name has
a new field ::
data Plugin = Plugin {
pluginRecompile :: [CommandLineOption] -> IO PluginRecompile
, {- ... -}
}
data PluginRecompile = ForceRecompile | NoForceRecompile | MaybeRecompile Fingerprint
Plugin based on ``defaultPlugin`` will have their previous recompilation
behavior (``ForceRecompile``) preserved. However, plugins that are "pure" are
encouraged to override this to either ``NoForceRecompile`` or ``MaybeRecompile``.
See :ref:`plugin_recompilation` for details.
- GHC now provides a class of new plugins: source plugins. These plugins can
inspect and modify a variety of intermediate representations used by the
compiler's frontend. These include:
* The ability to modify the parser output
* The ability to inspect the renamer output
* The ability to modify the typechecked AST
* The ability to modify Template Haskell splices
* The ability to modify interface files as they are loaded
See :ref:`source-plugins` for details.
GHCi
~~~~
- Added an experimental :ghci-cmd:`:doc` command that displays the
documentation for a declaration.
Runtime system
~~~~~~~~~~~~~~
- The GHC runtime linker now prefers user shared libraries above system ones.
When extra search directories are specified these are searched before anything
else. This fixes ``iuuc`` on Windows given the proper search directories (e.g
``-L/mingw64/lib``).
- The GHC runtime linker now uses ``LIBRARY_PATH`` and the runtime loader now also
searches ``LD_LIBRARY_PATH``.
- The GHC runtime on Windows is no longer constrained by the ``MAX_PATH`` file path
length limitation. See :ref:`windows-file-paths`.
- The runtime now allows use of the :rts-flag:`-hT` profiling variety on
programs built with :ghc-flag:`-prof`.
- The STM assertions mechanism (namely the ``always`` and ``alwaysSucceeds``
functions) has been removed. This happened a bit earlier than proposed in the
deprecation pragma included in GHC 8.4, but due to community feedback we
decided to move ahead with the early removal.
Template Haskell
~~~~~~~~~~~~~~~~
``ghc`` library
~~~~~~~~~~~~~~~
- The ``Plugin`` record now has a several new fields for the new source plugins
and recompilation checking mechanisms.
``base`` library
~~~~~~~~~~~~~~~~
- ``($!)`` is now representation-polymorphic like ``($)``.
- The module ``Data.Functor.Contravariant`` has been moved from the
``contravariant`` package into ``base``. All the other modules in
``contravariant`` (``Data.Functor.Contravariant.Divisible``, etc.)
have not been moved to ``base``, and they still reside in ``contravariant``.
``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~
- Added new ``addWordC#`` operation for unsigned addition with carry.
Build system
~~~~~~~~~~~~
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: Deppendency of ``ghc`` library
libraries/Cabal/Cabal/Cabal.cabal: Dependency of ``ghc-pkg`` utility
libraries/containers/containers.cabal: Dependency of ``ghc`` library
libraries/deepseq/deepseq.cabal: Dependency of ``ghc`` library
libraries/directory/directory.cabal: Dependency of ``ghc`` 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-compact/ghc-compact.cabal: Core 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/mtl/mtl.cabal: Dependency of ``Cabal`` library
libraries/parsec/parsec.cabal: Dependency of ``Cabal`` library
libraries/process/process.cabal: Dependency of ``ghc`` library
libraries/template-haskell/template-haskell.cabal: Core 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