Add section about macros in tutorial

relates #926
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Tuukka Turto 2015-10-03 13:24:43 +03:00 committed by Ryan Gonzalez
parent 5a34285b08
commit 7c36fe9a14

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@ -503,6 +503,64 @@ In Hy:
address (models.TextField)
notes (models.TextField)])
Macros
======
One really powerful feature of Hy are macros. They are small functios that are
used to generate code (or data). When program written in Hy is started, the
macros are executed and their output is placed in program source. After this,
the program starts executing normally. Very simple example:
.. code-block:: clj
=> (defmacro hello [person]
... `(print "Hello there," ~person))
=> (Hello "Tuukka")
Hello there, Tuukka
The thing to notice here is that hello macro doesn't output anything on
screen. Instead it creates piece of code that is then executed and prints on
screen. Macro writes a piece of program that looks like this (provided that
we used "Tuukka" as parameter:
.. code-block:: clj
(print "Hello there," Tuukka)
We can also manipulate code with macros:
.. code-block:: clj
=> (defmacro rev [code]
... (let [op (last code) params (list (butlast code))]
... `(~op ~@params)))
=> (rev (1 2 3 +))
6
The code that was generated with this macro just switched around some the
elements, so by the time program started executing, it actually red:
.. code-block:: clj
(+ 1 2 3)
Sometimes it's nice to have a very short name for macro that doesn't take much
space or use extra parentheses. Reader macros can be pretty useful in these
situations (and since Hy operates well with unicode, we aren't running out of
characters that soon):
.. code-block:: clj
=> (defreader ↻ [code]
... (let [op (last code) params (list (butlast code))]
... `(~op ~@params)))
=> #↻(1 2 3 +)
6
Macros are useful when one wished to extend the Hy or write their own
language on top of that. Many features of Hy are macros, like ``when``,
``cond`` and ``->``.
Hy <-> Python interop
=====================