.. _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

* Kind inference for data/newtype instance declarations is slightly
  more restrictive than before.  See the user manual :ref:`kind-inference-data-family-instances`.
  This is a breaking change, albeit a fairly obscure one that corrects a specification bug.

* 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
~~~~~~~~~~~~~~~~

- 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!
