<?xml version='1.0'?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
  "../dtd/4.1.2/docbookx.dtd">
<section id="dsssl.expr.loop"><title>Loops</title>
<para>
<indexterm><primary>for loop, DSSSL and</primary></indexterm>
<indexterm><primary>loops, implementing (DSSSL)</primary></indexterm>
<indexterm><primary>tail recursion (DSSSL)</primary></indexterm>
<acronym>DSSSL</acronym> doesn't have any construct that resembles the
for loop that occurs in most imperative languages like C
and Java. Instead, <acronym>DSSSL</acronym> employs a common trick in
functional languages for implementing a loop: tail recursion.
</para>
<para>
Loops in <acronym>DSSSL</acronym> use a special form of
<literal>let</literal>. This loop counts from 1 to 10:
<screen>
(let <co id="dl1"/>loopvar <co id="dl2"/>((count 1))
  <co id="dl3"/>(if (> count 10)
    <co id="dl4"/>#t
    (<co id="dl5"/>loopvar <co id="dl6"/>(+ count 1))))</screen></para>
<calloutlist>
<callout arearefs="dl1">
<para>This variable controls the loop. It is declared without an
initial value, immediately after the <literal>let</literal>
operand.</para>
</callout>
<callout arearefs="dl2">
<para>
<indexterm><primary>variables (DSSSL)</primary>
  <secondary>local, defining after loop variable</secondary></indexterm>
Any number of additional local variables can be defined after
the loop variable, just as they can in any other
<literal>let</literal> expression.</para>
</callout>
<callout arearefs="dl3">
<para>If you ever want the loop to end, you have to put some sort of a
test in it.</para>
</callout>
<callout arearefs="dl4">
<para>This is the value that will be returned.</para>
</callout>
<callout arearefs="dl5">
<para>Note that you iterate the loop by using the loop variable as if
it was a function name.</para>
</callout>
<callout arearefs="dl6">
<para>The arguments to this function are the values that
you want the local variables declared in <xref linkend="dl2"/> to have
in the next iteration.</para>
</callout>
</calloutlist>
</section>