diff options
Diffstat (limited to 'docs/users_guide/exts/magic_hash.rst')
| -rw-r--r-- | docs/users_guide/exts/magic_hash.rst | 46 |
1 files changed, 46 insertions, 0 deletions
diff --git a/docs/users_guide/exts/magic_hash.rst b/docs/users_guide/exts/magic_hash.rst new file mode 100644 index 0000000000..3873620959 --- /dev/null +++ b/docs/users_guide/exts/magic_hash.rst @@ -0,0 +1,46 @@ +.. _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.Prim`` (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#`` + + |