.. _release-9-4-1:

Version 9.4.1
==============

Compiler
~~~~~~~~

- New :ghc-flag:`-Wredundant-strictness-flags` that checks for strictness flags
  (``!``) applied to unlifted types, which are always strict.

- A new type of plugin: defaulting plugins. These plugins can propose
  defaults for ambiguous variables that would otherwise cause errors
  just like the built-in defaulting mechanism.

- The way GHC checks for representation polymorphism has been overhauled:
  all the checks are now done during typechecking. The error messages
  now contain more detailed information about the specific check that was performed.

``base`` library
~~~~~~~~~~~~~~~~

- ``GHC.Exts.magicDict`` has been renamed to ``withDict`` and given a more
  specific type: ::

        withDict :: forall {rr :: RuntimeRep} st dt (r :: TYPE rr). st -> (dt => r) -> r

  Unlike ``magicDict``, ``withDict`` can be used without defining an
  intermediate data type. For example, the ``withTypeable`` function from the
  ``Data.Typeable`` module can now be defined as: ::

        withTypeable :: forall k (a :: k) rep (r :: TYPE rep). ()
                     => TypeRep a -> (Typeable a => r) -> r
        withTypeable rep k = withDict @(TypeRep a) @(Typeable a) rep k

  Note that the explicit type applications are required, as the call to
  ``withDict`` would be ambiguous otherwise.

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

- ``GHC.Exts.mkWeak#``, ``GHC.Exts.mkWeakNoFinalizer#``, ``GHC.Exts.touch#``
  and ``GHC.Exts.keepAlive#`` are now levity-polymorphic instead of
  representation-polymorphic. For instance: ::

        mkWeakNoFinalizer#
          :: forall {l :: Levity} (a :: TYPE (BoxedRep l)) (b :: Type)
          .  a -> b -> State# RealWorld -> (# State# RealWorld, Weak# b #)

  That is, the type signature now quantifies over a variable of type ``GHC.Exts.Levity``
  instead of ``GHC.Exts.RuntimeRep``. In addition, this variable is now inferred,
  instead of specified, meaning that it is no longer eligible for visible type application.

- The ``GHC.Exts.RuntimeRep`` parameter to ``GHC.Exts.raise#`` is now inferred: ::

        raise# :: forall (a :: Type) {r :: RuntimeRep} (b :: TYPE r). a -> b

- ``GHC.Exts.reallyUnsafePtrEquality#`` has been made more general, as it is now
  both levity-polymorphic and heterogeneous: ::

        reallyUnsafePtrEquality#
          :: forall {l :: Levity} (a :: TYPE (BoxedRep l))
                    {k :: Levity} (b :: TYPE (BoxedRep k))
          . a -> b -> Int#

  This means that ``GHC.Exts.reallyUnsafePtrEquality#`` can be used
  on primitive arrays such as ``GHC.Exts.Array#`` and ``GHC.Exts.ByteArray#``.
  It can also be used on values of different types, without needing to call
  ``GHC.Exts.unsafeCoerce#``.

- Added ``GHC.Exts.reallyUnsafePtrEquality`` which recovers the
  previous behaviour of ``GHC.Exts.reallyUnsafePtrEquality#``: ::

        reallyUnsafePtrEquality :: forall (a :: Type). a -> a -> Int#

- Added ``GHC.Exts.sameArray#``, ``GHC.Exts.sameSmallArray#``,
  ``GHC.Exts.sameByteArray#`` and ``GHC.Exts.sameArrayArray#``: ::

        sameArray# :: Array# a -> Array# a -> Int#
        sameSmallArray# :: SmallArray# a -> SmallArray# a -> Int#
        sameByteArray# :: ByteArray# -> ByteArray# -> Int#
        sameArrayArray# :: ArrayArray# -> ArrayArray# -> Int#

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

- A new ``GHC.Hs.Syn.Type`` module has been introduced which defines functions
  for computing the ``Type`` of an ``HsExpr GhcTc`` in a pure fashion.
  The ``hsLitType`` and ``hsPatType`` functions that previously lived in
  ``GHC.Tc.Utils.Zonk`` have been moved to this module.
- A ``Typeable`` constraint has been added to ``fromStaticPtr`` in the
  class ``GHC.StaticPtr.IsStatic``. GHC automatically wraps each use of
  the ``static`` keyword with ``fromStaticPtr``. Because ``static`` requires
  its argument to be an instance of ``Typeable``, ``fromStaticPtr`` can
  safely carry this constraint as well.