docs/users_guide/9.4.1-notes.rst


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.. _release-9-4-1:

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

``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.