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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`.
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.. _release-8-4-1:
Release notes for version 8.4.1
===============================
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 8.2.1 release.
Highlights
----------
The highlights, since the 8.2.1 release, are:
- GHC is now capable of deriving more instances
- More refinement of the :ghc-flag:`-XTypeInType` story and improvements in type
error messages.
- Further improvements in code generation
- Many, many bug fixes.
Full details
------------
Language
~~~~~~~~
- Data families have been generalised a bit: a data family declaration can now
end with a kind variable ``k`` instead of ``Type``. Additionally, data/newtype
instance no longer need to list all the patterns of the family if they don't
wish to; this is quite like how regular datatypes with a kind signature can omit
some type variables.
- There are now fewer restrictions regarding whether kind variables can appear
on the right-hand sides of type and data family instances. Before, there was
a strict requirements that all kind variables on the RHS had to be explicitly
bound by type patterns on the LHS. Now, kind variables can be *implicitly*
bound, which allows constructions like these: ::
data family Nat :: k -> k -> *
-- k is implicitly bound by an invisible kind pattern
newtype instance Nat :: (k -> *) -> (k -> *) -> * where
Nat :: (forall xx. f xx -> g xx) -> Nat f g
class Funct f where
type Codomain f :: *
instance Funct ('KProxy :: KProxy o) where
-- o is implicitly bound by the kind signature
-- of the LHS type pattern ('KProxy)
type Codomain 'KProxy = NatTr (Proxy :: o -> *)
- Implicitly bidirectional pattern synonyms no longer allow bang patterns
(``!``) or irrefutable patterns (``~``) on the right-hand side. Previously,
this was allowed, although the bang patterns and irrefutable patterns would
be silently ignored when used in an expression context. This is now a proper
error, and explicitly bidirectional pattern synonyms should be used in their
stead. That is, instead of using this (which is an error): ::
pattern StrictJust a = Just !a
Use this: ::
pattern StrictJust a <- Just !a where
StrictJust !a = Just a
- GADTs with kind-polymorphic type arguments now require :ghc-flag:`-XTypeInType`.
For instance, consider the following, ::
data G :: k -> * where
GInt :: G Int
GMaybe :: G Maybe
In previous releases this would compile with :ghc-flag:`-XPolyKinds` alone due
to bug :ghc-ticket:`13391`. As of GHC 8.4, however, this requires
:ghc-flag:`-XTypeInType`. Note that since GADT kind signatures aren't generalized,
this will also require that you provide a :ref:`CUSK
<complete-kind-signatures>` by explicitly quantifying over the kind argument,
``k``, ::
data G :: forall k. k -> * where
GInt :: G Int
GMaybe :: G Maybe
- The order in which type variables are quantified in GADT constructor type
signatures has changed. Before, if you had ``MkT`` as below: ::
data T a where
MkT :: forall b a. b -> T a
Then the type of ``MkT`` would (counterintuitively) be
``forall a b. b -> T a``! Now, GHC quantifies the type variables in the
order that the users writes them, so the type of ``MkT`` is now
``forall b a. b -> T a`` (this matters for :ghc-flag:`-XTypeApplications`).
- The new :ghc-flag:`-XEmptyDataDeriving` extension allows deriving ``Eq``,
``Ord``, ``Read``, and ``Show`` instances directly for empty data types, as
in ``data Empty deriving Eq``. (Previously, this would require the use of
:ghc-flag:`-XStandaloneDeriving` to accomplish.)
One can also now derive ``Data`` instances directly for empty data types (as
in ``data Empty deriving Data``) without needing to use
:ghc-flag:`-XStandaloneDeriving`. However, since already requires a GHC
extension (:ghc-flag:`-XDeriveDataTypeable`), one does not need to enable
:ghc-flag:`-XEmptyDataDeriving` to do so. This also goes for other classes
which require extensions to derive, such as :ghc-flag:`-XDeriveFunctor`.
- Hexadecimal floating point literals (e.g. ``0x0.1p4``), enabled with
:ghc-flag:`-XHexFloatLiterals`. See
:ref:`Hexadecimal floating point literals <hex-float-literals>`
for the full details.
Compiler
~~~~~~~~
- LLVM code generator (e.g. :ghc-flag:`-fllvm`) compatible with LLVM releases in
the |llvm-version| series.
- Add warning flag :ghc-flag:`-Wmissing-export-lists` which causes the type
checker to warn when a module does not include an explicit export list.
- The ``configure`` script now no longer accepts ``--with-TOOL`` flags (e.g.
``--with-nm``, ``--with-ld``, etc.). Instead, these are taken from environment
variables, as is typical in ``autoconf`` scripts. For instance,
``./configure --with-nm=/usr/local/bin/nm`` turns into
``./configure NM=/usr/local/bin/nm``.
- Derived ``Functor``, ``Foldable``, and ``Traversable`` instances are now
optimized when their last type parameters have phantom roles.
Specifically, ::
fmap _ = coerce
traverse _ x = pure (coerce x)
foldMap _ _ = mempty
These definitions of ``foldMap`` and ``traverse`` are lazier than the ones we
would otherwise derive, as they may produce results without inspecting their
arguments at all.
See also :ref:`deriving-functor`, :ref:`deriving-foldable`, and
:ref:`deriving-traversable`.
- Derived instances for empty data types are now substantially different
than before. Here is an overview of what has changed. These examples will
use a running example of ``data Empty a`` to describe what happens when an
instance is derived for ``Empty``:
- Derived ``Eq`` and ``Ord`` instances would previously emit code that used
``error``: ::
instance Eq (Empty a) where
(==) = error "Void =="
instance Ord (Empty a) where
compare = error "Void compare"
Now, they emit code that uses maximally defined, lazier semantics: ::
instance Eq (Empty a) where
_ == _ = True
instance Ord (Empty a) where
compare _ _ = EQ
- Derived ``Read`` instances would previous emit code that used
``parens``: ::
instance Read (Empty a) where
readPrec = parens pfail
But ``parens`` forces parts of the parsed string that it doesn't need to.
Now, the derived instance will not use ``parens`` (that it, parsing
``Empty`` will always fail, without reading *any* input): ::
instance Read (Empty a) where
readPrec = pfail
- Derived ``Show`` instances would previously emit code that used
``error``: ::
instance Show (Empty a) where
showsPrec = error "Void showsPrec"
Now, they emit code that inspects the argument. That is, if the argument
diverges, then showing it will also diverge: ::
instance Show (Empty a) where
showsPrec _ x = case x of {}
- Derived ``Functor``, ``Foldable``, ``Traversable``, ``Generic``,
``Generic1``, ``Lift``, and ``Data`` instances previously emitted code that
used ``error``: ::
instance Functor Empty where
fmap = error "Void fmap"
instance Foldable Empty where
foldMap = error "Void foldMap"
instance Traversable Empty where
traverse = error "Void traverse"
instance Generic (Empty a) where
from = M1 (error "No generic representation for empty datatype Empty")
to (M1 _) = error "No values for empty datatype Empty"
-- Similarly for Generic1
instance Lift (Empty a) where
lift _ = error "Can't lift value of empty datatype Empty"
instance Data a => Data (Empty a) where
gfoldl _ _ _ = error "Void gfoldl"
toConstr _ = error "Void toConstr"
...
Now, derived ``Functor``, ``Traversable, ``Generic``, ``Generic1``,
``Lift``, and ``Data`` instances emit code which inspects their
arguments: ::
instance Functor Empty where
fmap _ x = case x of {}
instance Traversable Empty where
traverse _ x = pure (case x of {})
instance Generic (Empty a) where
from x = M1 (case x of {})
to (M1 x) = case x of {}
-- Similarly for Generic1
instance Lift (Empty a) where
lift x = pure (case x of {})
instance Data a => Data (Empty a) where
gfoldl _ x = case x of {}
toConstr x = case x of {}
...
Derived ``Foldable`` instances now are maximally lazy: ::
instance Foldable Empty where
foldMap _ _ = mempty
- Derived ``Foldable`` instances now derive custom definitions for ``null``
instead of using the default one. This leads to asymptotically better
performance for recursive types not shaped like cons-lists, and allows ``null``
to terminate for more (but not all) infinitely large structures.
- :ghc-flag:`-fsplit-sections` is now supported on x86_64 Windows and is on by default.
See :ghc-ticket:`12913`.
- Configure on Windows now supports the ``--enable-distro-toolchain``
``configure`` flag, which can be used to build a GHC using compilers on your
``PATH`` instead of using the bundled bindist. See :ghc-ticket:`13792`
- GHC now enables :ghc-flag:`-fllvm-pass-vectors-in-regs` by default. This means
that GHC will now use native vector registers to pass vector arguments across
function calls.
- The optional ``instance`` keyword is now usable in type family instance
declarations. See :ghc-ticket:`13747`
- Lots of other bugs. See `Trac <https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.4.1&col=id&col=summary&col=status&col=type&col=priority&col=milestone&col=component&order=priority>`_
for a complete list.
- New flags :ghc-flag:`-fignore-optim-changes` and
:ghc-flag:`-fignore-hpc-changes` allow GHC to reuse previously compiled
modules even if they were compiled with different optimisation or HPC
flags. These options are enabled by default by :ghc-flag:`--interactive`.
See :ghc-ticket:`13604`
Runtime system
~~~~~~~~~~~~~~
- Function ``hs_add_root()`` was removed. It was a no-op since GHC-7.2.1
where module initialisation stopped requiring a call to ``hs_add_root()``.
- Proper import library support added to GHC which can handle all of the libraries produced
by ``dlltool``. The limitation of them needing to be named with the suffix
``.dll.a`` is also removed. See :ghc-ticket:`13606`, :ghc-ticket:`12499`,
:ghc-ticket:`12498`
- The GHCi runtime linker on Windows now supports the ``big-obj`` file format.
- The runtime system's :ref:`native stack backtrace <backtrace-signal>` support
on POSIX platforms is now triggered by ``SIGQUIT`` instead of ``SIGUSR2`` as
it was in previous releases. This change is to bring GHC's behavior into
compliance with the model set by the most Java virtual machine
implementations.
- The GHC runtime on Windows now uses Continue handlers instead of Vectorized
handlers to trap exceptions. This change gives other exception handlers a chance
to handle the exception before the runtime does. Furthermore The RTS flag
:rts-flag:`--install-seh-handlers=<yes|no>` Can be used on Wndows to
completely disable the runtime's handling of exceptions. See
:ghc-ticket:`13911`, :ghc-ticket:`12110`.
- The GHC runtime on Windows can now generate crash dumps on unhandled exceptions
using the RTS flag :rts-flag:`--generate-crash-dumps`.
- The GHCi runtime linker now avoid calling GCC to find libraries as much as possible by caching
the list of search directories of GCC and querying the file system directly. This results in
much better performance, especially on Windows.
- The GHC runtime on Windows can now generate stack traces on unhandled exceptions.
When running in GHCi more information is displayed about the symbols if available.
This behavior can be controlled with the RTS flag `--generate-stack-traces=<yes|no>`.
Template Haskell
~~~~~~~~~~~~~~~~
- Template Haskell now reifies data types with GADT syntax accurately.
Previously, TH used heuristics to determine whether a data type
should be reified using GADT syntax, which could lead to incorrect results,
such as ``data T1 a = (a ~ Int) => MkT1`` being reified as a GADT and
``data T2 a where MkT2 :: Show a => T2 a`` *not* being reified as a GADT.
In addition, reified GADT constructors now more accurately track the order in
which users write type variables. Before, if you reified ``MkT`` as below: ::
data T a where
MkT :: forall b a. b -> T a
Then the reified type signature of ``MkT`` would have been headed by
``ForallC [PlainTV a, PlainTV b]``. Now, reifying ``MkT`` will give a type
headed by ``ForallC [PlainTV b, PlainTV a]``, as one would expect.
- ``Language.Haskell.TH.FamFlavour``, which was deprecated in GHC 8.2,
has been removed.
``ghc`` library
~~~~~~~~~~~~~~~
- hsSyn Abstract Syntax Tree (AST) is now extensible via the mechanism described in `Trees that Grow <http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf>`_
The main change for users of the GHC API is that the AST is no longer indexed
by the type used as the identifier, but by a specific index type, ::
type GhcPs = GhcPass 'Parsed -- Old 'RdrName' type param
type GhcRn = GhcPass 'Renamed -- Old 'Name' type param
type GhcTc = GhcPass 'Typechecked -- Old 'Id' type para,
type GhcTcId = GhcTc -- Old 'TcId' type param
The simplest way to support the current GHC as well as earlier ones is to define ::
#if MIN_VERSION_ghc(8,3,0)
type ParseI = GhcPs
type RenameI = GhcRn
type TypecheckI = GhcTc
#else
type ParseI = RdrName
type RenameI = Name
type TypecheckI = Var
#endif
and then replace all hardcoded index types accordingly. For polymorphic types,
the constraint ::
#if MIN_VERSION_ghc(8,3,0)
-- |bundle up the constraints required for a trees that grow pass
type IsPass pass = (DataId pass, OutputableBndrId pass, SourceTextX pass)
else
type IsPass pass = (DataId pass, OutputableBndrId pass)
#endif
can be used.
``base`` library
~~~~~~~~~~~~~~~~
- Blank strings can now be used as values for environment variables using the
``System.Environment.Blank`` module. See :ghc-ticket:`12494`
- ``Data.Type.Equality.==`` is now a closed type family. It works for all kinds
out of the box. Any modules that previously declared instances of this family
will need to remove them. Whereas the previous definition was somewhat ad
hoc, the behavior is now completely uniform. As a result, some applications
that used to reduce no longer do, and conversely. Most notably, ``(==)`` no
longer treats the ``*``, ``j -> k``, or ``()`` kinds specially; equality is
tested structurally in all cases.
Build system
~~~~~~~~~~~~
- ``dll-split`` has been removed and replaced with an automatic partitioning utility ``gen-dll``.
This utility can transparently split and compile any DLLs that require this. Note that the ``rts`` and
``base`` can not be split at this point because of the mutual recursion between ``base`` and ``rts``.
There is currently no explicit dependency between the two in the build system and such there is no way
to notify ``base`` that the ``rts`` has been split, or vice versa.
(see :ghc-ticket:`5987`).
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
Win32
~~~~~
.. attention::
This release is a backwards incompatible release which corrects the type of
certain APIs. See issue `#24 <https://github.com/haskell/win32/issues/24>`_.
docs/users_guide/8.4.2-notes.rst deleted 100644 → 0
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.. _release-8-4-2:
Release notes for version 8.4.2
===============================
TODO
Highlights
----------
The highlights, since the 8.4.1 release, are:
- TODO
Full details
------------
Language
~~~~~~~~
- Fix a regression in which derived ``Read`` instances for record data types
with field names ending with ``#`` (by way of :ghc-flag:`-XMagicHash`) would
no longer parse valid output.
Compiler
~~~~~~~~
Runtime system
~~~~~~~~~~~~~~
Template Haskell
~~~~~~~~~~~~~~~~
``ghc`` library
~~~~~~~~~~~~~~~
``base`` library
~~~~~~~~~~~~~~~~
- Add the ``readFieldHash`` function to ``GHC.Read`` which behaves like
``readField``, but for a field that ends with a ``#`` symbol.
``integer-gmp`` library
~~~~~~~~~~~~~~~~~~~~~~~
- Define ``powModSecInteger``, a "secure" version of ``powModInteger`` using
the ``mpz_powm_sec`` function.
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
docs/users_guide/index.rst
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......@@ -12,7 +12,6 @@ Contents:
license
intro
8.4.2-notes
8.6.1-notes
ghci
runghc
......
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