.. _release-9-2-1:
Version 9.2.1
==============
The significant changes to the various parts of the compiler are listed in the
following sections.
The :ghc-flag:`LLVM backend <-fllvm>` of this release is to be used with LLVM
10 or 11.
Language
~~~~~~~~
* :extension:`ImpredicativeTypes`: Finally, polymorphic types have become first class!
GHC 9.2 includes a full implementation of the Quick Look approach to type inference for
impredicative types, as described in in the paper
`A quick look at impredicativity
<https://www.microsoft.com/en-us/research/publication/a-quick-look-at-impredicativity/>`__
(Serrano et al, ICFP 2020). More information here: :ref:`impredicative-polymorphism`.
This replaces the old (undefined, flaky) behaviour of the :extension:`ImpredicativeTypes` extension.
* The first stage of the `Pointer Rep Proposal`_ has been implemented. All
boxed types, both lifted and unlifted, now have representation kinds of
the shape ``BoxedRep r``. Code that references ``LiftedRep`` and ``UnliftedRep``
will need to be updated.
.. _Pointer Rep Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst
* :extension:`UnliftedDatatypes`: The `Unlifted Datatypes Proposal`_ has been
implemented. That means GHC Haskell now offers a way to define algebraic
data types with strict semantics like in OCaml or Idris! The distinction to
ordinary lifted data types is made in the kind system: Unlifted data types
live in kind ``TYPE (BoxedRep Unlifted)``. :extension:`UnliftedDatatypes`
allows giving data declarations such result kinds, such as in the following
example with the help of :extension:`StandaloneKindSignatures`: ::
type IntSet :: UnliftedType -- type UnliftedType = TYPE (BoxedRep Unlifted)
data IntSet = Branch IntSet !Int IntSet | Leaf
See :extension:`UnliftedDatatypes` for what other declarations are
possible. Slight caveat: Most functions in ``base`` (including ``$``)
are not levity-polymorphic (yet) and hence won't work with unlifted
data types.
.. _Unlifted Datatypes Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0265-unlifted-datatypes.rst
* GHC now supports visible type applications in patterns when :extension:`TypeApplications` is enabled . This allows
you to use the ``@variable`` syntax to bind types in patterns. For instance, instead of ::
foo (Just (x :: ty)) = …
You can now use
foo (Just @ty x) = …
See the `Type Applications in Patterns Proposal`_ for more details
.. _Type Applications in Patterns Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0126-type-applications-in-patterns.rst
* Kind inference for data/newtype instance declarations is slightly more restrictive than before.
In particular, GHC now requires that the kind of a data family instance be fully determined
by the header of the instance, without looking at the definition of the constructor.
This means that data families that dispatched on an invisible parameter might now require this parameter
to be made explicit, as in the following example: ::
data family DF :: forall (r :: RuntimeRep). TYPE r
newtype instance DF @IntRep = MkDF2 Int#
newtype instance DF @FloatRep = MkDF1 Float#
See the user manual :ref:`kind-inference-data-family-instances`.
* GHC is stricter about checking for out-of-scope type variables on the
right-hand sides of associated type family instances that are not bound on
the left-hand side. As a result, some programs that were accidentally
accepted in previous versions of GHC will now be rejected, such as this
example: ::
class Funct f where
type Codomain f
instance Funct ('KProxy :: KProxy o) where
type Codomain 'KProxy = NatTr (Proxy :: o -> Type)
Where: ::
data Proxy (a :: k) = Proxy
data KProxy (t :: Type) = KProxy
data NatTr (c :: o -> Type)
GHC will now reject the ``o`` on the right-hand side of the ``Codomain``
instance as being out of scope, as it does not meet the requirements for
being explicitly bound (as it is not mentioned on the left-hand side) nor
implicitly bound (as it is not mentioned in an *outermost* kind signature,
as required by :ref:`scoping-class-params`). This program can be repaired in
a backwards-compatible way by mentioning ``o`` on the left-hand side: ::
instance Funct ('KProxy :: KProxy o) where
type Codomain ('KProxy @o) = NatTr (Proxy :: o -> Type)
-- Alternatively,
-- type Codomain ('KProxy :: KProxy o) = NatTr (Proxy :: o -> Type)
* Previously, ``-XUndecidableInstances`` accidentally implied ``-XFlexibleContexts``.
This is now fixed, but it means that some programs will newly require
``-XFlexibleContexts``.
* The :extension:`GHC2021` language is supported now. It builds on top of
Haskell2010, adding several stable and conservative extensions, and removing
deprecated ones. It is now also the “default†language set that is active
when no other language set, such as :extension:`Haskell98` or
:extension:`Haskell2010`, is explicitly loaded (e.g via Cabal’s
``default-language``).
Because :extension:`GHC2021` includes
:extension:`GeneralizedNewtypeDeriving`, which is not safe for Safe Haskell,
users of Safe Haskell are advised to use :extension:`Haskell2010` explicitly.
The default mode of GHC until 9.0 included
:extension:`NondecreasingIndentation`, but :extension:`GHC2021` does not.
This may break code implicitly using this extension.
* The `Record Dot Syntax Proposal`_ has been implemented:
- A new extension :extension:`OverloadedRecordDot` provides record ``.`` syntax e.g. ``x.foo``
- A new extension :extension:`OverloadedRecordUpdate` provides record ``.``
syntax in record updates e.g. ``x{foo.bar = 1}``. *The design of this
extension may well change in the future.*
.. _Record Dot Syntax Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0282-record-dot-syntax.rst
* Various records-related extensions have been improved:
- A new extension :extension:`NoFieldSelectors` hides record field selector
functions, so it is possible to define top-level bindings with the same names.
- The :extension:`DisambiguateRecordFields` extension now works for updates.
An update ``expr { field = value }`` will be accepted if there is a single
field called ``field`` in scope, regardless of whether there are non-fields
in scope with the same name.
- The :extension:`DuplicateRecordFields` extension now applies to fields in
record pattern synonyms. In particular, it is possible for a single module
to define multiple pattern synonyms using the same field names.
* Because of simplifications to the way that GHC typechecks operator sections,
operators with nested ``forall``\ s or contexts in their type signatures might
not typecheck when used in a section. For instance, the ``g`` function below,
which was accepted in previous GHC releases, will no longer typecheck: ::
f :: a -> forall b. b -> a
f x _ = x
g :: a -> a
g = (`f` "hello")
``g`` can be made to typecheck once more by eta expanding it to
``\x -> x `f` "hello"``. For more information, see
:ref:`simple-subsumption`.
* :extension:`LinearTypes` can now infer multiplicity for ``case``
expressions. Previously, the scrutinee of a ``case`` (the bit between ``case``
and ``of``) was always assumed to have a multiplicity of ``Many``. Now, GHC
will allow the scrutinee to have a multiplicity of ``One``, using its best-effort
inference algorithm.
* Support for matching on GADT constructors in arrow notation has been removed,
as the current implementation of :extension:`Arrows` doesn't handle GADT
evidence correctly.
One possible workaround, for the time being, is to perform GADT matches
inside let bindings: ::
data G a where
MkG :: Show a => a -> G a
foo :: G a -> String
foo = proc x -> do
let res = case x of { MkG a -> show a }
returnA -< res
Compiler
~~~~~~~~
- GHC now has an ARMv8 native code generator, significantly improving compilation
performance for ARM targets and eliminating a dependency on LLVM.
- Performance of the compiler in :ghc-flag:`--make` mode with :ghc-flag:`-j[⟨n⟩]`
is significantly improved by improvements to the parallel
garbage collector noted below.
Benchmarks show a 20% decrease in wall clock time, and a 40% decrease in cpu
time, when compiling Cabal with ``-j4`` on linux. Improvements are more dramatic
with higher parallelism, and we no longer see significant degradation in wall
clock time as parallelism increases above 4.
- New :ghc-flag:`-Wredundant-bang-patterns` flag that enables checks for "dead" bangs.
For instance, given this program: ::
f :: Bool -> Bool
f True = False
f !x = x
GHC would report that the bang on ``x`` is redundant and can be removed
since the argument was already forced in the first equation. For more
details see :ghc-flag:`-Wredundant-bang-patterns`.
- New :ghc-flag:`-Wimplicit-lift` flag which warns when a Template Haskell quote
implicitly uses ``lift``.
- New :ghc-flag:`-finline-generics` and
:ghc-flag:`-finline-generics-aggressively` flags for improving performance of
generics-based algorithms.
For more details see :ghc-flag:`-finline-generics` and
:ghc-flag:`-finline-generics-aggressively`.
- GHC now supports a flag, :ghc-flag:`-fprof-callers=⟨name⟩`, for requesting
that the compiler automatically insert cost-centres on all call-sites of
the named function.
- The heap profiler can now be controlled from within a Haskell program using
functions in ``GHC.Profiling``. Profiling can be started and stopped or a heap
census requested at a specific point in the program.
There is a new RTS flag :rts-flag:`--no-automatic-heap-samples` which can be
used to stop heap profiling starting when a program starts.
- A new debugging facility, :ghc-flag:`-finfo-table-map`, which embeds a mapping
from the address of an info table to information about that info table, including
an approximate source position. :ghc-flag:`-fdistinct-constructor-tables` is
also useful with this flag to give each usage of a data constructor its own
unique info table so they can be distinguished in gdb and heap profiles.
GHCi
~~~~
- GHCi's ``:kind!`` command now expands through type synonyms in addition to
type families. See :ghci-cmd:`:kind`.
- GHCi's :ghci-cmd:`:edit` command now looks for an editor in
the :envvar:`VISUAL` environment variable before
:envvar:`EDITOR`, following UNIX convention.
(:ghc-ticket:`19030`)
- GHC now follows by default the XDG Base Directory Specification. If
``$HOME/.ghc`` is found it will fallback to the old paths to give you
time to migrate. This fallback will be removed in three releases.
- New debugger command :ghci-cmd:`:ignore` to set an ``ignore count`` for a
specified breakpoint. The next ``ignore count`` times the program hits this
breakpoint, the breakpoint is ignored, and the program doesn't stop.
- New optional parameter added to the command :ghci-cmd:`:continue` to set the
``ignore count`` for the current breakpoint.
Runtime system
~~~~~~~~~~~~~~
- The parallel garbage collector is now significantly more performant. Heavily
contended spinlocks have been replaced with mutexes and condition variables.
For most programs compiled with the threaded runtime, and run with more than
four capabilities, we expect minor GC pauses and GC cpu time both to be reduced.
For very short running programs (in the order of 10s of milliseconds), we have seen
some performance regressions. We recommend programs affected by this to either
compile with the single threaded runtime, or otherwise to disable the parallel
garbage collector with :rts-flag:`-qg ⟨gen⟩`.
We don't expect any other performance regressions, however only limited
benchmarking has been done. We have only benchmarked GHC and nofib and only on
linux.
Users are advised to reconsider the rts flags that programs are run with. If
you have been mitigating poor parallel GC performance by: using large
nurseries (:rts-flag:`-A <-A ⟨size⟩>`), disabling load balancing (:rts-flag:`-qb ⟨gen⟩`), or
limiting parallel GC to older generations (:rts-flag:`-qg ⟨gen⟩`); then you may
find these mitigations are no longer necessary.
- The heap profiler now has proper treatment of pinned ``ByteArray#``\ s. Such
heap objects will now be correctly attributed to their appropriate cost
centre instead of merely being lumped into the ``PINNED`` category.
Moreover, we now correctly account for the size of the array, meaning that
space lost to fragmentation is no longer counted as live data.
- The ``-xt`` RTS flag has been removed. Now STACK and TSO closures are always
included in heap profiles. Tooling can choose to filter out these closure types
if necessary.
- A new heap profiling mode, :rts-flag:`-hi`, profile by info table allows for
fine-grain banding by the info table address of a closure. The profiling
mode is intended to be used with :ghc-flag:`-finfo-table-map` and can best
be consumed with ``eventlog2html``. This profiling mode does not require a
profiling build.
- The RTS will now gradually return unused memory back to the OS rather than
retaining a large amount (up to 4 * live) indefinitely. The rate at which memory
is returned is controlled by the :rts-flag:`-Fd ⟨factor⟩`. Memory return
is triggered by consecutive idle collections.
- The default nursery size, :rts-flag:`-A <-A ⟨size⟩>`, has been increased from
1mb to 4mb.
Template Haskell
~~~~~~~~~~~~~~~~
- There are two new functions ``putDoc`` and ``getDoc``, which allow Haddock
documentation to be attached and read from module headers, declarations,
function arguments, class instances and family instances.
These functions are quite low level, so the ``withDecDoc`` function provides
a more ergonomic interface for this. Similarly ``funD_doc``, ``dataD_doc``
and friends provide an easy way to document functions and constructors
alongside their arguments simultaneously. ::
$(withDecsDoc "This does good things" [d| foo x = 42 |])
``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~
- ``Void#`` is now a type synonym for the unboxed tuple ``(# #)``.
Code using ``Void#`` now has to enable :extension:`UnboxedTuples`.
Eventlog
~~~~~~~~
- Two new events, :event-type:`BLOCKS_SIZE` tells you about the total size of
all allocated blocks and :event-type:`MEM_RETURN` gives statistics about why
the OS is returning and retaining megablocks.
``ghc`` library
~~~~~~~~~~~~~~~
- There is a significant refactoring in the solver; any type-checker plugins
will have to be updated, as GHC no longer uses flattening skolems or
flattening metavariables.
- Type checker plugins which work with the natural numbers now
should use ``naturalTy`` kind instead of ``typeNatKind``, which has been removed.
- The ``con_args`` field of ``ConDeclGADT`` has been renamed to ``con_g_args``.
This is because the type of ``con_g_args`` is now different from the type of
the ``con_args`` field in ``ConDeclH98``: ::
data ConDecl pass
= ConDeclGADT
{ ...
, con_g_args :: HsConDeclGADTDetails pass -- ^ Arguments; never infix
, ...
}
| ConDeclH98
{ ...
, con_args :: HsConDeclH98Details pass -- ^ Arguments; can be infix
, ...
}
Where: ::
-- Introduced in GHC 9.2; was called `HsConDeclDetails` in previous versions of GHC
type HsConDeclH98Details pass
= HsConDetails (HsScaled pass (LBangType pass)) (XRec pass [LConDeclField pass])
-- Introduced in GHC 9.2
data HsConDeclGADTDetails pass
= PrefixConGADT [HsScaled pass (LBangType pass)]
| RecConGADT (XRec pass [LConDeclField pass])
Unlike Haskell98-style constructors, GADT constructors cannot be declared
using infix syntax, which is why ``HsConDeclGADTDetails`` lacks an
``InfixConGADT`` constructor.
As a result of all this, the ``con_args`` field is now partial, so using
``con_args`` as a top-level field selector is discouraged.
``base`` library
~~~~~~~~~~~~~~~~
- The lifted fixed-width integer and word types (e.g.
``Data.Int.Int8``, ``Data.Word.Word32``) are now represented by their associated
fixed-width unlifted types. For instance, while in previous GHC versions
``Int8`` was defined as: ::
data Int8 = I8# Int#
As of GHC 9.2 it is rather defined as, ::
data Int8 = I8# Int8#
- Character set metadata bumped to Unicode 13.0.0.
- It's possible now to promote the ``Natural`` type: ::
data Coordinate = Mk2D Natural Natural
type MyCoordinate = Mk2D 1 10
The separate kind ``Nat`` is removed and now it is just a type synonym for
``Natural``. As a consequence, one must enable ``TypeSynonymInstances``
in order to define instances for ``Nat``.
The ``Numeric`` module receives ``showBin`` and ``readBin`` to show and
read integer numbers in binary.
- ``Char`` gets type-level support by analogy with strings and natural numbers.
We extend the ``GHC.TypeLits`` module with these built-in type-families: ::
type family CmpChar (a :: Char) (b :: Char) :: Ordering
type family ConsSymbol (a :: Char) (b :: Symbol) :: Symbol
type family UnconsSymbol (a :: Symbol) :: Maybe (Char, Symbol)
type family CharToNat (c :: Char) :: Natural
type family NatToChar (n :: Natural) :: Char
and with the type class ``KnownChar`` (and such additional functions as ``charVal`` and ``charVal'``): ::
class KnownChar (n :: Char)
charVal :: forall n proxy. KnownChar n => proxy n -> Char
charVal' :: forall n. KnownChar n => Proxy# n -> Char
- A new kind-polymorphic ``Compare`` type family was added in ``Data.Type.Ord``
and has type instances for ``Nat``, ``Symbol``, and ``Char``. Furthermore,
the ``(<=?)`` type (and ``(<=)``) from ``GHC.TypeNats`` is now governed by
this type family (as well as new comparison type operators that are exported
by ``Data.Type.Ord``). This has two important repercussions. First, GHC can
no longer deduce that all natural numbers are greater than or equal to zero.
For instance, ::
test1 :: Proxy (0 <=? x) -> Proxy True
test1 = id
which previously type checked will now result in a type error. Second, when
these comparison type operators are used very generically, a kind may need to
be provided. For example, ::
test2 :: Proxy (x <=? x) -> Proxy True
test2 = id
will now generate a type error because GHC does not know the kind of ``x``.
To fix this, one must provide an explicit kind, perhaps by changing the type
to: ::
test2 :: forall (x :: Nat). Proxy (x <=? x) -> Proxy True
- On POSIX, ``System.IO.openFile`` can no longer leak a file descriptor if it
is interrupted by an asynchronous exception (:ghc-ticket:`19114`, :ghc-ticket:`19115`).
- There's a new binding ``GHC.Exts.considerAccessible``. It's equivalent to
``True`` and allows the programmer to turn off pattern-match redundancy
warnings for particular clauses, like the third one here ::
g :: Bool -> Int
g x = case (x, x) of
(True, True) -> 1
(False, False) -> 2
(True, False) | considerAccessible -> 3 -- No warning!
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: Dependency of ``ghc`` library
libraries/Cabal/Cabal/Cabal.cabal: Dependency of ``ghc-pkg`` utility
libraries/containers/containers/containers.cabal: Dependency of ``ghc`` library
libraries/deepseq/deepseq.cabal: Dependency of ``ghc`` library
libraries/directory/directory.cabal: Dependency of ``ghc`` library
libraries/exceptions/exceptions.cabal: Dependency of ``ghc`` and ``haskeline`` 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-boot-th/ghc-boot-th.cabal: Internal compiler library
libraries/ghc-compact/ghc-compact.cabal: Core library
libraries/ghc-heap/ghc-heap.cabal: GHC heap-walking 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/libiserv/libiserv.cabal: Internal compiler library
libraries/mtl/mtl.cabal: Dependency of ``Cabal`` library
libraries/parsec/parsec.cabal: Dependency of ``Cabal`` library
libraries/pretty/pretty.cabal: Dependency of ``ghc`` library
libraries/process/process.cabal: Dependency of ``ghc`` library
libraries/stm/stm.cabal: Dependency of ``haskeline`` library
libraries/template-haskell/template-haskell.cabal: Core library
libraries/terminfo/terminfo.cabal: Dependency of ``haskeline`` 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