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authorOleg Grenrus <oleg.grenrus@iki.fi>2019-05-08 21:22:19 +0300
committerMarge Bot <ben+marge-bot@smart-cactus.org>2019-05-29 10:37:54 -0400
commitb58b389b538b6fb9cbb3517cff83808f2a690a0e (patch)
tree99bca95aa55b84024ba4f59ea98fcda7bd1992fe
parent32acecc29d4766fd2b168cbd654667ba6be03dbb (diff)
downloadhaskell-b58b389b538b6fb9cbb3517cff83808f2a690a0e.tar.gz
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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 release include:
-
-- A new, more expressive ``Typeable`` mechanism, ``Type.Reflection``
-
-- Colorful error messages with caret diagnostics
-
-- SCC annotations can now be used for declarations.
-
-- Heap overflow throws an exception in certain circumstances.
-
-- Improved code generation of join points
-
-- Deriving strategies
-
-- Compact regions support, allowing efficient garbage collection of large heaps
-
-- More reliable DWARF debug information
-
-Full details
-------------
-
-Package system
-~~~~~~~~~~~~~~
-
-- The long awaited Backpack module system is now fully usable. See
- :ghc-wiki:`the GHC Wiki <backpack>` for details.
-
-Language
-~~~~~~~~
-
-- Pattern synonym signatures can now be applied to multiple patterns, just like
- value-level binding signatures. See :ref:`patsyn-typing` for details.
-
-- 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`).
-
-- The new :ghc-flag:`-XUnboxedSums` extension allows more efficient representation
- of sum data. Some future GHC release will have support for worker/wrapper
- transformation of sum arguments and constructor unpacking.
-
-- Support for overloaded record fields via a new ``HasField`` class and
- associated compiler logic (see :ref:`record-field-selector-polymorphism`)
-
-- GHC now recognizes the ``COMPLETE`` language pragma, allowing the user to
- specify sets of patterns (including pattern synonyms) which constitute a
- complete pattern match. See :ref:`complete-pragma` for details.
-
-Compiler
-~~~~~~~~
-
-- GHC will now use ``ld.gold`` or ``ld.lld`` instead of the system's default
- ``ld``, if available. Linker availability will be evaluated at ``configure``
- time. The user can manually override which linker to use by passing the ``LD``
- variable to ``configure``. You can revert to the old behavior of using the
- system's default ``ld`` by passing the ``--disable-ld-override`` flag to
- ``configure``.
-
-- GHC now uses section splitting (i.e. :ghc-flag:`-split-sections`) instead of
- object splitting (i.e. :ghc-flag:`-split-objs`) as the default mechanism for
- linker-based dead code removal. While the effect is the same, split sections
- tends to produce significantly smaller objects than split objects and more
- closely mirrors the approach used by other compilers. Split objects will
- be deprecated and eventually removed in a future GHC release.
-
- Note that some versions of the ubiquitous BFD linker exhibit performance
- trouble with large libraries with section splitting enabled (see
- :ghc-ticket:`13739`). It is recommended that you use either the ``gold`` or
- ``lld`` linker if you observe this. This will require that you install one of
- these compilers, rerun ``configure``, and reinstall GHC.
-
- Split sections is enabled by default in the official binary distributions for
- platforms that support it.
-
-- 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.
-
-- GHC is now much more particular about :ghc-flag:`-XDefaultSignatures`. The
- type signature for a default method of a type class must now be the same as
- the corresponding main method's type signature modulo differences in the
- signatures' contexts. Otherwise, the typechecker will reject that class's
- definition. See :ref:`class-default-signatures` for further details.
-
-- :ghc-flag:`-XDeriveAnyClass` is no longer limited to type classes whose
- argument is of kind ``*`` or ``* -> *``.
-
-- The means by which :ghc-flag:`-XDeriveAnyClass` infers instance contexts has
- been completely overhauled. The instance context is now inferred using the
- type signatures (and default type signatures) of the derived class's methods
- instead of using the datatype's definition, which often led to
- over-constrained instances or instances that didn't typecheck (or worse,
- triggered GHC panics). See the section on
- :ref:`DeriveAnyClass <derive-any-class>` for more details.
-
-- 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 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.
-
-- A new flag :ghc-flag:`-ddump-json` has been added. This flag dumps compiler
- output as JSON documents. It is experimental and will be refined depending
- on feedback from tooling authors for the next release.
-
-- GHC is now able to better optimize polymorphic expressions by using known
- superclass dictionaries where possible. Some examples: ::
-
- -- uses of `Monad IO` or `Applicative IO` here are improved
- foo :: MonadBaseControl IO m => ...
-
- -- uses of `Monoid MyMonoid` here are improved
- bar :: MonadWriter MyMonoid m => ...
-
-- GHC now derives the definition of ``<$`` when using :ghc-flag:`-XDeriveFunctor`
- rather than using the default definition. This prevents unnecessary
- allocation and a potential space leak when deriving ``Functor`` for
- a recursive type.
-
-- The :ghc-flag:`-XExtendedDefaultRules` extension now defaults multi-parameter
- typeclasses. See :ghc-ticket:`12923`.
-
-- GHC now ignores ``RULES`` for data constructors (:ghc-ticket:`13290`).
- Previously, it accepted::
-
- {-# RULES "NotAllowed" forall x. Just x = e #-}
-
- That rule will no longer take effect, and a warning will be issued. ``RULES``
- may still mention data constructors, but not in the outermost position::
-
- {-# RULES "StillWorks" forall x. f (Just x) = e #-}
-
-- Type synonyms can no longer appear in the class position of an instance.
- This means something like this is no longer allowed: ::
-
- type ReadShow a = (Read a, Show a)
- instance Read Foo
- instance Show Foo
- instance ReadShow Foo -- illegal
-
- See :ghc-ticket:`13267`.
-
-- Validity checking for associated type family instances has tightened
- somewhat. Before, this would be accepted: ::
-
- class Foo a where
- type Bar a
-
- instance Foo (Either a b) where
- type Bar (Either c d) = d -> c
-
- This is now disallowed, as the type variables used in the `Bar` instance do
- not match those in the instance head. This instance can be fixed by changing
- it to: ::
-
- instance Foo (Either a b) where
- type Bar (Either a b) = b -> a
-
- See the section on :ref:`associated type family instances <assoc-data-inst>`
- for more information.
-
-- A bug involving the interaction between :ghc-flag:`-XMonoLocalBinds` and
- :ghc-flag:`-XPolyKinds` has been fixed. This can cause some programs to fail
- to typecheck in case explicit kind signatures are not provided. See
- :ref:`kind-generalisation` for an example.
-
-GHCi
-~~~~
-
-- Added :ghc-flag:`-flocal-ghci-history` which uses current directory for `.ghci-history`.
-
-- Added support for :ghc-flag:`-XStaticPointers` in interpreted modules. Note, however,
- that ``static`` expressions are still not allowed in expressions evaluated in the REPL.
-
-- Added support for :ghci-cmd:`:type +d` and :ghci-cmd:`:type +v`. (:ghc-ticket:`11975`)
-
-Template Haskell
-~~~~~~~~~~~~~~~~
-
-- 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`)
-
-- Quoted type signatures are more accurate with respect to implicitly
- quantified type variables. Before, if you quoted this: ::
-
- [d| id :: a -> a
- id x = x
- |]
-
- then the code that Template Haskell would give back to you would actually be
- this instead: ::
-
- id :: forall a. a -> a
- id x = x
-
- That is, quoting would explicitly quantify all type variables, even ones
- that were implicitly quantified in the source. This could be especially
- harmful if a kind variable was implicitly quantified. For example, if
- you took this quoted declaration: ::
-
- [d| idProxy :: forall proxy (b :: k). proxy b -> proxy b
- idProxy x = x
- |]
-
- and tried to splice it back in, you'd get this instead: ::
-
- idProxy :: forall k proxy (b :: k). proxy b -> proxy b
- idProxy x = x
-
- Now ``k`` is explicitly quantified, and that requires turning on
- :ghc-flag:`-XTypeInType`, whereas the original declaration did not!
-
- Template Haskell quoting now respects implicit quantification in type
- signatures, so the quoted declarations above now correctly leave the
- type variables ``a`` and ``k`` as implicitly quantified.
- (:ghc-ticket:`13018` and :ghc-ticket:`13123`)
-
-- Looking up type constructors with symbol names (e.g., ``+``) now works
- as expected (:ghc-ticket:`11046`)
-
-
-Runtime system
-~~~~~~~~~~~~~~
-
-- 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`` (see :rts-flag:`-M ⟨size⟩`), 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.
-
-- 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 :ghc-compact-ref:`GHC.Compact.` module.
- Moreover, see the ``compact`` library on `Hackage
- <https://hackage.haskell.org/package/compact>`_ for a high-level interface.
-
-- 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 ⟨x⟩`. By default, the garbage
- collector will use a number of threads equal to the lesser of the global number
- of capabilities or the number of physical cores. See :rts-flag:`-qn ⟨x⟩`, 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`)
-
-- Output of :ref:`Event log <rts-eventlog>` data can now be configured,
- enabling external tools to collect and analyze the event log data while the
- application is still running.
-
-- ``advapi32``, ``shell32`` and ``user32`` are now automatically loaded in GHCi.
- ``libGCC`` is also loaded when a dependency requires it. See
- :ghc-ticket:`13189`.
-
-hsc2hs
-~~~~~~
-
-- Version number 0.68.2
-
-Libraries
----------
-
-array
-~~~~~
-
-- Version number 0.5.2.0 (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``
-
-- Add ``GHC.TypeNats`` module with ``Natural``-based ``KnownNat``. The ``Nat``
- operations in ``GHC.TypeLits`` are a thin compatibility layer on top.
- Note: the ``KnownNat`` evidence is changed from an ``Integer`` to a ``Natural``.
-
-- ``liftA2`` is now a method of the ``Applicative`` class. ``Traversable``
- deriving has been modified to use ``liftA2`` for the first two elements
- traversed in each constructor. ``liftA2`` is not yet in the ``Prelude``,
- and must currently be imported from ``Control.Applicative``. It is likely
- to be added to the ``Prelude`` in the future.
-
-binary
-~~~~~~
-
-- Version number 0.8.5.1 (was 0.7.1.0)
-
-bytestring
-~~~~~~~~~~
-
-- Version number 0.10.8.2 (was 0.10.4.0)
-
-Cabal
-~~~~~
-
-- Version number 2.0.0.0 (was 1.24.2.0)
-
-containers
-~~~~~~~~~~
-
-- Version number 0.5.10.2 (was 0.5.4.0)
-
-deepseq
-~~~~~~~
-
-- Version number 1.4.3.0 (was 1.3.0.2)
-
-directory
-~~~~~~~~~
-
-- Version number 1.3.0.2 (was 1.2.0.2)
-
-filepath
-~~~~~~~~
-
-- Version number 1.4.1.2 (was 1.3.0.2)
-
-ghc
-~~~
-
-- Version number 8.2.1
-
-ghc-boot
-~~~~~~~~
-
-- This is an internal package. Use with caution.
-
-ghc-compact
-~~~~~~~~~~~
-
-The ``ghc-compact`` library provides an experimental API for placing immutable
-data structures into a contiguous memory region. Data in these regions is not
-traced during garbage collection and can be serialized to disk or over the
-network.
-
-- Version number 0.1.0.0 (newly added)
-
-ghc-prim
-~~~~~~~~
-
-- Version number 0.5.1.0 (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.
-
-hoopl
-~~~~~
-
-- Version number 3.10.2.2 (was 3.10.2.1)
-
-hpc
-~~~
-
-- Version number 0.6.0.3 (was 0.6.0.2)
-
-integer-gmp
-~~~~~~~~~~~
-
-- Version number 1.0.0.1 (was 1.0.0.1)
-
-process
-~~~~~~~
-
-- Version number 1.6.1.0 (was 1.4.3.0)
-
-template-haskell
-~~~~~~~~~~~~~~~~
-
-- Version 2.12.0.0 (was 2.11.1.0)
-
-- Added support for unboxed sums :ghc-ticket:`12478`.
-
-- Added support for visible type applications :ghc-ticket:`12530`.
-
-time
-~~~~
-
-- Version number 1.8.0.1 (was 1.6.0.1)
-
-unix
-~~~~
-
-- Version number 2.7.2.2 (was 2.7.2.1)
-
-Win32
-~~~~~
-
-- Version number 2.5.4.1 (was 2.3.1.1)
-
-Known bugs
-----------
-
-- At least one known program regresses in compile time significantly over 8.0.
- See :ghc-ticket:`13535`.
-
-- Some uses of type applications may cause GHC to panic. See :ghc-ticket:`13819`.
-
-- The compiler may loop during typechecking on some modules using
- :ghc-flag:`-XUndecidableInstances`. See :ghc-ticket:`13943`.