.. _release-9-2-1:

Version 9.2.1
==============

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.

* 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)

Compiler
~~~~~~~~

- New ``-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 ``-finline-generics`` and ``-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`.

- Type checker plugins which work with the natural numbers now
  should use ``naturalTy`` kind instead of ``typeNatKind``, which has been removed.

- GHCi's ``:kind!`` command now expands through type synonyms in addition to type
  families. See :ghci-cmd:`:kind`.

- 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.

- 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.

``ghc-prim`` library
~~~~~~~~~~~~~~~~~~~~

- ``Void#`` is now a type synonym for the unboxed tuple ``(# #)``.
  Code using ``Void#`` now has to enable :extension:`UnboxedTuples`.

``ghc`` library
~~~~~~~~~~~~~~~

- 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
~~~~~~~~~~~~~~~~

- 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``.