57 lines
1.8 KiB
ReStructuredText
57 lines
1.8 KiB
ReStructuredText
==========
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loop/recur
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==========
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.. versionadded:: 0.9.13
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The loop/recur macro gives programmers a simple way to use tail-call
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optimization (TCO) in their Hy code.
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A tail call is a subroutine call that happens inside another
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procedure as its final action; it may produce a return value which
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is then immediately returned by the calling procedure. If any call
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that a subroutine performs, such that it might eventually lead to
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this same subroutine being called again down the call chain, is in
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tail position, such a subroutine is said to be tail-recursive,
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which is a special case of recursion. Tail calls are significant
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because they can be implemented without adding a new stack frame
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to the call stack. Most of the frame of the current procedure is
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not needed any more, and it can be replaced by the frame of the
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tail call. The program can then jump to the called
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subroutine. Producing such code instead of a standard call
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sequence is called tail call elimination, or tail call
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optimization. Tail call elimination allows procedure calls in tail
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position to be implemented as efficiently as goto statements, thus
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allowing efficient structured programming.
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-- Wikipedia (http://en.wikipedia.org/wiki/Tail_call)
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Macros
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======
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.. _loop:
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loop
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-----
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``loop`` establishes a recursion point. With ``loop``, ``recur``
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rebinds the variables set in the recursion point and sends code
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execution back to that recursion point. If ``recur`` is used in a
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non-tail position, an exception is thrown.
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Usage: `(loop bindings &rest body)`
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Example:
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.. code-block:: hy
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(require hy.contrib.loop)
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(defn factorial [n]
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(loop [[i n] [acc 1]]
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(if (zero? i)
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acc
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(recur (dec i) (* acc i)))))
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(factorial 1000)
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