35de998871
In RestrutruedText, double backtick quotes (instead of single backtick quotes in markdown) are used for code span.
57 lines
1.8 KiB
ReStructuredText
57 lines
1.8 KiB
ReStructuredText
==========
|
|
loop/recur
|
|
==========
|
|
|
|
.. versionadded:: 0.10.0
|
|
|
|
The ``loop`` / ``recur`` macro gives programmers a simple way to use
|
|
tail-call optimization (TCO) in their Hy code.
|
|
|
|
A tail call is a subroutine call that happens inside another
|
|
procedure as its final action; it may produce a return value which
|
|
is then immediately returned by the calling procedure. If any call
|
|
that a subroutine performs, such that it might eventually lead to
|
|
this same subroutine being called again down the call chain, is in
|
|
tail position, such a subroutine is said to be tail-recursive,
|
|
which is a special case of recursion. Tail calls are significant
|
|
because they can be implemented without adding a new stack frame
|
|
to the call stack. Most of the frame of the current procedure is
|
|
not needed any more, and it can be replaced by the frame of the
|
|
tail call. The program can then jump to the called
|
|
subroutine. Producing such code instead of a standard call
|
|
sequence is called tail call elimination, or tail call
|
|
optimization. Tail call elimination allows procedure calls in tail
|
|
position to be implemented as efficiently as goto statements, thus
|
|
allowing efficient structured programming.
|
|
|
|
-- Wikipedia (https://en.wikipedia.org/wiki/Tail_call)
|
|
|
|
Macros
|
|
======
|
|
|
|
.. _loop:
|
|
|
|
loop
|
|
-----
|
|
|
|
``loop`` establishes a recursion point. With ``loop``, ``recur``
|
|
rebinds the variables set in the recursion point and sends code
|
|
execution back to that recursion point. If ``recur`` is used in a
|
|
non-tail position, an exception is raised.
|
|
|
|
Usage: ``(loop bindings &rest body)``
|
|
|
|
Example:
|
|
|
|
.. code-block:: hy
|
|
|
|
(require hy.contrib.loop)
|
|
|
|
(defn factorial [n]
|
|
(loop [[i n] [acc 1]]
|
|
(if (zero? i)
|
|
acc
|
|
(recur (dec i) (* acc i)))))
|
|
|
|
(factorial 1000)
|