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.. _magic-hash:
The magic hash
--------------
.. extension:: MagicHash
:shortdesc: Allow ``#`` as a postfix modifier on identifiers.
:since: 6.8.1
Enables the use of the hash character (``#``) as an identifier suffix.
The language extension :extension:`MagicHash` allows ``#`` as a postfix modifier
to identifiers. Thus, ``x#`` is a valid variable, and ``T#`` is a valid type
constructor or data constructor.
The hash sign does not change semantics at all. We tend to use variable
names ending in "#" for unboxed values or types (e.g. ``Int#``), but
there is no requirement to do so; they are just plain ordinary
variables. Nor does the :extension:`MagicHash` extension bring anything into
scope. For example, to bring ``Int#`` into scope you must import
``GHC.Exts`` (see :ref:`primitives`); the :extension:`MagicHash` extension then
allows you to *refer* to the ``Int#`` that is now in scope. Note that
with this option, the meaning of ``x#y = 0`` is changed: it defines a
function ``x#`` taking a single argument ``y``; to define the operator
``#``, put a space: ``x # y = 0``.
The :extension:`MagicHash` also enables some new forms of literals (see
:ref:`glasgow-unboxed`):
- ``'x'#`` has type ``Char#``
- ``"foo"#`` has type ``Addr#``
- ``3#`` has type ``Int#``. In general, any Haskell integer lexeme
followed by a ``#`` is an ``Int#`` literal, e.g. ``-0x3A#`` as well as
``32#``.
- ``3##`` has type ``Word#``. In general, any non-negative Haskell
integer lexeme followed by ``##`` is a ``Word#``.
- ``3.2#`` has type ``Float#``.
- ``3.2##`` has type ``Double#``
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