162 lines
6.7 KiB
ReStructuredText
162 lines
6.7 KiB
ReStructuredText
==============
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Syntax
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==============
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identifiers
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-----------
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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.
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numeric literals
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----------------
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In addition to regular numbers, standard notation from Python 3 for non-base 10
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integers is used. ``0x`` for Hex, ``0o`` for Octal, ``0b`` for Binary.
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.. code-block:: clj
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(print 0x80 0b11101 0o102 30)
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Underscores and commas can appear anywhere in a numeric literal except the very
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beginning. They have no effect on the value of the literal, but they're useful
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for visually separating digits.
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.. code-block:: clj
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(print 10,000,000,000 10_000_000_000)
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Unlike Python, Hy provides literal forms for NaN and infinity: ``NaN``,
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``Inf``, and ``-Inf``.
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string literals
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---------------
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Hy allows double-quoted strings (e.g., ``"hello"``), but not single-quoted
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strings like Python. The single-quote character ``'`` is reserved for
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preventing the evaluation of a form (e.g., ``'(+ 1 1)``), as in most Lisps.
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Python's so-called triple-quoted strings (e.g., ``'''hello'''`` and
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``"""hello"""``) aren't supported. However, in Hy, unlike Python, any string
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literal can contain newlines. Furthermore, Hy supports an alternative form of
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string literal called a "bracket string" similar to Lua's long brackets.
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Bracket strings have customizable delimiters, like the here-documents of other
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languages. A bracket string begins with ``#[FOO[`` and ends with ``]FOO]``,
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where ``FOO`` is any string not containing ``[`` or ``]``, including the empty
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string. For example::
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=> (print #[["That's very kind of yuo [sic]" Tom wrote back.]])
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"That's very kind of yuo [sic]" Tom wrote back.
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=> (print #[==[1 + 1 = 2]==])
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1 + 1 = 2
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A bracket string can contain newlines, but if it begins with one, the newline
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is removed, so you can begin the content of a bracket string on the line
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following the opening delimiter with no effect on the content. Any leading
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newlines past the first are preserved.
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Plain string literals support :ref:`a variety of backslash escapes
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<py:strings>`. To create a "raw string" that interprets all backslashes
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literally, prefix the string with ``r``, as in ``r"slash\not"``. Bracket
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strings are always raw strings and don't allow the ``r`` prefix.
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Whether running under Python 2 or Python 3, Hy treats all string literals as
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sequences of Unicode characters by default, and allows you to prefix a plain
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string literal (but not a bracket string) with ``b`` to treat it as a sequence
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of bytes. So when running under Python 3, Hy translates ``"foo"`` and
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``b"foo"`` to the identical Python code, but when running under Python 2,
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``"foo"`` is translated to ``u"foo"`` and ``b"foo"`` is translated to
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``"foo"``.
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Unlike Python, Hy only recognizes string prefixes (``r``, etc.) in lowercase.
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.. _syntax-keywords:
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keywords
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--------
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An identifier headed by a colon, such as ``:foo``, is a keyword. If a
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literal keyword appears in a function call, it's used to indicate a keyword
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argument rather than passed in as a value. For example, ``(f :foo 3)`` calls
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the function ``f`` with the keyword argument named ``foo`` set to ``3``. Hence,
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trying to call a function on a literal keyword may fail: ``(f :foo)`` yields
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the error ``Keyword argument :foo needs a value``. To avoid this, you can quote
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the keyword, as in ``(f ':foo)``, or use it as the value of another keyword
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argument, as in ``(f :arg :foo)``.
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Keywords can be called like functions as shorthand for ``get``. ``(:foo obj)``
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is equivalent to ``(get obj :foo)``. An optional ``default`` argument is also
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allowed: ``(:foo obj 2)`` or ``(:foo obj :default 2)`` returns ``2`` if ``(get
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obj :foo)`` raises a ``KeyError``.
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.. _mangling:
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symbols
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-------
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Symbols are identifiers that are neither legal numeric literals nor legal
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keywords. In most contexts, symbols are compiled to Python variable names. Some
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example symbols are ``hello``, ``+++``, ``3fiddy``, ``$40``, ``just✈wrong``,
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and ``🦑``.
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Since the rules for Hy symbols are much more permissive than the rules for
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Python identifiers, Hy uses a mangling algorithm to convert its own names to
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Python-legal names. The rules are:
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- Convert all hyphens (``-``) to underscores (``_``). Thus, ``foo-bar`` becomes
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``foo_bar``.
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- If the name ends with ``?``, remove it and prepend ``is_``. Thus, ``tasty?``
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becomes ``is_tasty``.
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- If the name still isn't Python-legal, make the following changes. A name
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could be Python-illegal because it contains a character that's never legal in
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a Python name, it contains a character that's illegal in that position, or
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it's equal to a Python reserved word.
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- Prepend ``hyx_`` to the name.
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- Replace each illegal character with ``XfooX``, where ``foo`` is the Unicode
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character name in lowercase, with spaces replaced by underscores and
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hyphens replaced by ``H``. Replace ``X`` itself the same way. If the
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character doesn't have a name, use ``U`` followed by its code point in
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lowercase hexadecimal.
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Thus, ``green☘`` becomes ``hyx_greenXshamrockX`` and ``if`` becomes
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``hyx_if``.
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- Finally, any added ``hyx_`` or ``is_`` is added after any leading
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underscores, because leading underscores have special significance to Python.
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Thus, ``_tasty?`` becomes ``_is_tasty`` instead of ``is__tasty``.
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Mangling isn't something you should have to think about often, but you may see
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mangled names in error messages, the output of ``hy2py``, etc. A catch to be
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aware of is that mangling, as well as the inverse "unmangling" operation
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offered by the ``unmangle`` function, isn't one-to-one. Two different symbols
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can mangle to the same string and hence compile to the same Python variable.
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The chief practical consequence of this is that ``-`` and ``_`` are
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interchangeable in all symbol names, so you shouldn't assign to the
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one-character name ``_`` , or else you'll interfere with certain uses of
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subtraction.
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discard prefix
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--------------
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Hy supports the Extensible Data Notation discard prefix, like Clojure.
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Any form prefixed with ``#_`` is discarded instead of compiled.
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This completely removes the form so it doesn't evaluate to anything,
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not even None.
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It's often more useful than linewise comments for commenting out a
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form, because it respects code structure even when part of another
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form is on the same line. For example:
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.. code-block:: clj
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=> (print "Hy" "cruel" "World!")
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Hy cruel World!
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=> (print "Hy" #_"cruel" "World!")
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Hy World!
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=> (+ 1 1 (print "Math is hard!"))
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Math is hard!
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Traceback (most recent call last):
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...
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TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
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=> (+ 1 1 #_(print "Math is hard!"))
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2
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