hy/docs/language/syntax.rst

==============
Syntax
==============

Hy maintains, over everything else, 100% compatibility in both directions
with Python itself. All Hy code follows a few simple rules. Memorize
this, as it's going to come in handy.

These rules help ensure that Hy code is idiomatic and interfaceable in both
languages.

  * Symbols in earmuffs will be translated to the upper-cased version of that
    string. For example, ``foo`` will become ``FOO``.

  * UTF-8 entities will be encoded using
    `punycode <https://en.wikipedia.org/wiki/Punycode>`_ and prefixed with
    ``hy_``. For instance, ``⚘`` will become ``hy_w7h``, ``♥`` will become
    ``hy_g6h``, and ``i♥u`` will become ``hy_iu_t0x``.

  * Symbols that contain dashes will have them replaced with underscores. For
    example, ``render-template`` will become ``render_template``. This means
    that symbols with dashes will shadow their underscore equivalents, and vice
    versa.

numeric literals
----------------

In addition to regular numbers, standard notation from Python 3 for non-base 10
integers is used. ``0x`` for Hex, ``0o`` for Octal, ``0b`` for Binary.

.. code-block:: clj
    
    (print 0x80 0b11101 0o102 30)

Underscores and commas can appear anywhere in a numeric literal except the very
beginning. They have no effect on the value of the literal, but they're useful
for visually separating digits.

.. code-block:: clj

    (print 10,000,000,000 10_000_000_000)

Unlike Python, Hy provides literal forms for NaN and infinity: ``NaN``,
``Inf``, and ``-Inf``.

string literals
---------------

Hy allows double-quoted strings (e.g., ``"hello"``), but not single-quoted
strings like Python. The single-quote character ``'`` is reserved for
preventing the evaluation of a form (e.g., ``'(+ 1 1)``), as in most Lisps.

Python's so-called triple-quoted strings (e.g., ``'''hello'''`` and
``"""hello"""``) aren't supported. However, in Hy, unlike Python, any string
literal can contain newlines. Furthermore, Hy supports an alternative form of
string literal called a "bracket string" similar to Lua's long brackets.
Bracket strings have customizable delimiters, like the here-documents of other
languages. A bracket string begins with ``#[FOO[`` and ends with ``]FOO]``,
where ``FOO`` is any string not containing ``[`` or ``]``, including the empty
string. For example::

   => (print #[["That's very kind of yuo [sic]" Tom wrote back.]])
   "That's very kind of yuo [sic]" Tom wrote back.
   => (print #[==[1 + 1 = 2]==])
   1 + 1 = 2

A bracket string can contain newlines, but if it begins with one, the newline
is removed, so you can begin the content of a bracket string on the line
following the opening delimiter with no effect on the content. Any leading
newlines past the first are preserved.

Plain string literals support :ref:`a variety of backslash escapes
<py:strings>`. To create a "raw string" that interprets all backslashes
literally, prefix the string with ``r``, as in ``r"slash\not"``. Bracket
strings are always raw strings and don't allow the ``r`` prefix.

Whether running under Python 2 or Python 3, Hy treats all string literals as
sequences of Unicode characters by default, and allows you to prefix a plain
string literal (but not a bracket string) with ``b`` to treat it as a sequence
of bytes. So when running under Python 3, Hy translates ``"foo"`` and
``b"foo"`` to the identical Python code, but when running under Python 2,
``"foo"`` is translated to ``u"foo"`` and ``b"foo"`` is translated to
``"foo"``.

.. _syntax-keywords:

keywords
--------

An identifier headed by a colon, such as ``:foo``, is a keyword. Keywords
evaluate to a string preceded by the Unicode non-character code point U+FDD0,
like ``"\ufdd0:foo"``, so ``:foo`` and ``":foo"`` aren't equal. However, if a
literal keyword appears in a function call, it's used to indicate a keyword
argument rather than passed in as a value. For example, ``(f :foo 3)`` calls
the function ``f`` with the keyword argument named ``foo`` set to ``3``. Hence,
trying to call a function on a literal keyword may fail: ``(f :foo)`` yields
the error ``Keyword argument :foo needs a value``. To avoid this, you can quote
the keyword, as in ``(f ':foo)``, or use it as the value of another keyword
argument, as in ``(f :arg :foo)``.

discard prefix
--------------

Hy supports the Extensible Data Notation discard prefix, like Clojure.
Any form prefixed with ``#_`` is discarded instead of compiled.
This completely removes the form so it doesn't evaluate to anything,
not even None.
It's often more useful than linewise comments for commenting out a
form, because it respects code structure even when part of another
form is on the same line. For example:

.. code-block:: clj

   => (print "Hy" "cruel" "World!")
   Hy cruel World!
   => (print "Hy" #_"cruel" "World!")
   Hy World!
   => (+ 1 1 (print "Math is hard!"))
   Math is hard!
   Traceback (most recent call last):
      ...
   TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
   => (+ 1 1 #_(print "Math is hard!"))
   2