hy/docs/language/syntax.rst
2018-05-15 23:29:07 -04:00

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==============
Syntax
==============
identifiers
-----------
An identifier consists of a nonempty sequence of Unicode characters that are not whitespace nor any of the following: ``( ) [ ] { } ' "``. Hy first tries to parse each identifier into a numeric literal, then into a keyword if that fails, and finally into a symbol if that fails.
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. 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)``.
.. _mangling:
symbols
-------
Symbols are identifiers that are neither legal numeric literals nor legal
keywords. In most contexts, symbols are compiled to Python variable names. Some
example symbols are ``hello``, ``+++``, ``3fiddy``, ``$40``, ``just✈wrong``,
and ``🦑``.
Since the rules for Hy symbols are much more permissive than the rules for
Python identifiers, Hy uses a mangling algorithm to convert its own names to
Python-legal names. The rules are:
- Convert all hyphens (``-``) to underscores (``_``). Thus, ``foo-bar`` becomes
``foo_bar``.
- If the name ends with ``?``, remove it and prepend ``is_``. Thus, ``tasty?``
becomes ``is_tasty``.
- If the name still isn't Python-legal, make the following changes. A name
could be Python-illegal because it contains a character that's never legal in
a Python name, it contains a character that's illegal in that position, or
it's equal to a Python reserved word.
- Prepend ``hyx_`` to the name.
- Replace each illegal character with ``XfooX``, where ``foo`` is the Unicode
character name in lowercase, with spaces replaced by underscores and
hyphens replaced by ``H``. Replace ``X`` itself the same way. If the
character doesn't have a name, use ``U`` followed by its code point in
lowercase hexadecimal.
Thus, ``green☘`` becomes ``hyx_greenXshamrockX`` and ``if`` becomes
``hyx_if``.
- Finally, any added ``hyx_`` or ``is_`` is added after any leading
underscores, because leading underscores have special significance to Python.
Thus, ``_tasty?`` becomes ``_is_tasty`` instead of ``is__tasty``.
Mangling isn't something you should have to think about often, but you may see
mangled names in error messages, the output of ``hy2py``, etc. A catch to be
aware of is that mangling, as well as the inverse "unmangling" operation
offered by the ``unmangle`` function, isn't one-to-one. Two different symbols
can mangle to the same string and hence compile to the same Python variable.
The chief practical consequence of this is that ``-`` and ``_`` are
interchangeable in all symbol names, so you shouldn't assign to the
one-character name ``_`` , or else you'll interfere with certain uses of
subtraction.
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