hy/docs/language/api.rst

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=================
Hy (the language)
=================
.. warning::
This is incomplete; please consider contributing to the documentation
effort.
Theory of Hy
============
Hy maintains, over everything else, 100% compatibility in both directions
with Python it's self. All Hy code follows a few simple rules. Memorize
this, it's going to come in handy.
These rules help make sure code is idiomatic and interface-able in both
languages.
* Symbols in earmufs will be translated to the uppercased version of that
string. For example, `*foo*` will become `FOO`.
* UTF-8 entities will be encoded using
`punycode <http://en.wikipedia.org/wiki/Punycode>`_ and prefixed with
`hy_`. For instance, `⚘` will become `hy_w7h`, and `♥` will become
`hy_g6h`.
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* Symbols that contain dashes will have them replaced with underscores. For
example, `render-template` will become `render_template`.
Builtins
========
Hy features a number special forms that are used to help generate
correct Python AST. The following are "special" forms, which may have
behavior that's slightly unexpected in some situations.
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import
------
`import` is used to import modules, like in Python. There are several forms
of import you can use.
.. code-block:: clj
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;; Imports each of these modules
;;
;; Python:
;; import sys
;; import os.path
(import sys os.path)
;; Import from a module
;;
;; Python: from os.path import exists, isdir, isfile
(import [os.path [exists isdir isfile]])
;; Import with an alias
;;
;; Python: import sys as systest
(import [sys :as systest])
;; You can list as many imports as you like of different types.
(import [tests.resources [kwtest function-with-a-dash]]
[os.path [exists isdir isfile]]
[sys :as systest])
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do / progn
----------
the `do` or `progn` forms can be used in full code branches. What that means
is basically `(do)` and `(progn)` can only be used where a Python expression
can be used. These forms don't actually allow you to break Pythonic internals
such as `lambda` or `list-comp`, where you can only have one expression.
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Some example usage
.. code-block:: clj
(if true
(do (print "Side effects rock!")
(print "Yeah, really!")))
`do` can accept any number of arguments, from 1 to n.
throw / raise
-------------
the `throw` or `raise` forms can be used to raise an Exception at runtime.
Example usage
.. code-block:: clj
(throw)
; re-rase the last exception
(throw IOError)
; Throw an IOError
(throw (IOError "foobar"))
; Throw an IOError("foobar")
`throw` can acccept a single argument (an `Exception` class or instance), or
no arguments to re-raise the last Exception.
try
---
.. TODO::
Document the else / finally syntax.
the `try` form is used to start a `try` / `catch` block. The form is used
as follows
.. code-block:: clj
(try
(error-prone-function)
(catch [e SomeException] (err "It sucks!")))
`try` must contain at least one `catch` block, and may optionally have an
`else` or `finally` block.