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<a name="Agendas"></a>
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<hr>
<a name="Agendas-1"></a>
<h4 class="subsection">2.4.1 Agendas</h4>

<p>To schedule a task to be performed later, an &ldquo;agenda&rdquo; is used.
There is a default, global agenda that is ready to be used, or
additional agendas may be created for different purposes.  The
following example prints the text &ldquo;hello&rdquo; when the agenda has
advanced to time unit 10.
</p>
<div class="example">
<pre class="example">(at 10 (display &quot;hello\n&quot;))
</pre></div>

<p>Most of the time it is more convenient to schedule tasks relative to
the current time.  This is where <code>after</code> comes in handy:
</p>
<div class="example">
<pre class="example">(after 10 (display &quot;hello\n&quot;))
</pre></div>

<p>Time units in the agenda are in no way connected to real time.  It&rsquo;s
up to the programmer to decide what agenda time means.  A simple and
effective approach is to map each call of the update procedure
(see <a href="Kernel.html#Kernel">Kernel</a>) to 1 unit of agenda time, like so:
</p>
<div class="example">
<pre class="example">(define (update dt)
  (update-agenda 1))
</pre></div>

<p>It is important to call <code>update-agenda</code> periodically, otherwise
no tasks will ever be run!
</p>
<p>In addition to using the global agenda, it is useful to have multiple
agendas for different purposes.  For example, the game world can use a
different agenda than the user interface, so that pausing the game is
a simple matter of not updating the world&rsquo;s agenda while continuing to
update the user interface&rsquo;s agenda.  The current agenda is dynamically
scoped and can be changed using the <code>with-agenda</code> special form:
</p>
<div class="example">
<pre class="example">(define game-world-agenda (make-agenda))

(with-agenda game-world-agenda
  (at 60 (spawn-goblin))
  (at 120 (spawn-goblin))
  (at 240 (spawn-goblin-king)))
</pre></div>

<dl>
<dt><a name="index-make_002dagenda"></a>Procedure: <strong>make-agenda</strong></dt>
<dd><p>Return a new task scheduler.
</p></dd></dl>

<dl>
<dt><a name="index-agenda_003f"></a>Procedure: <strong>agenda?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is an agenda.
</p></dd></dl>

<dl>
<dt><a name="index-current_002dagenda"></a>Procedure: <strong>current-agenda</strong></dt>
<dt><a name="index-current_002dagenda-1"></a>Procedure: <strong>current-agenda</strong> <em>agenda</em></dt>
<dd><p>When called with no arguments, return the current agenda.  When called
with one argument, set the current agenda to <var>agenda</var>.
</p></dd></dl>

<dl>
<dt><a name="index-with_002dagenda"></a>Syntax: <strong>with-agenda</strong> <em>agenda body &hellip;</em></dt>
<dd><p>Evaluate <var>body</var> with the current agenda set to <var>agenda</var>.
</p></dd></dl>

<dl>
<dt><a name="index-agenda_002dtime"></a>Procedure: <strong>agenda-time</strong></dt>
<dd><p>Return the current agenda time.
</p></dd></dl>

<dl>
<dt><a name="index-update_002dagenda"></a>Procedure: <strong>update-agenda</strong> <em>dt</em></dt>
<dd><p>Advance the current agenda by <var>dt</var>.
</p></dd></dl>

<dl>
<dt><a name="index-schedule_002dat"></a>Procedure: <strong>schedule-at</strong> <em>time thunk</em></dt>
<dd><p>Schedule <var>thunk</var>, a procedure of zero arguments, to be run at
<var>time</var>.
</p></dd></dl>

<dl>
<dt><a name="index-schedule_002dafter"></a>Procedure: <strong>schedule-after</strong> <em>delay thunk</em></dt>
<dd><p>Schedule <var>thunk</var>, a procedure of zero arguments, to be run after
<var>delay</var>.
</p></dd></dl>

<dl>
<dt><a name="index-schedule_002devery"></a>Procedure: <strong>schedule-every</strong> <em>interval thunk [n]</em></dt>
<dd><p>Schedule <var>thunk</var>, a procedure of zero arguments, to be run every
<var>interval</var> amount of time.  Repeat this <var>n</var> times, or
indefinitely if not specified.
</p></dd></dl>

<dl>
<dt><a name="index-at"></a>Syntax: <strong>at</strong> <em>time body &hellip;</em></dt>
<dd><p>Schedule <var>body</var> to be evaluated at <var>time</var>.
</p></dd></dl>

<dl>
<dt><a name="index-after"></a>Syntax: <strong>after</strong> <em>delay body &hellip;</em></dt>
<dd><p>Schedule <var>body</var> to be evaluated after <var>delay</var>.
</p></dd></dl>

<dl>
<dt><a name="index-every"></a>Syntax: <strong>every</strong> <em>interval body &hellip;</em></dt>
<dt><a name="index-every-1"></a>Syntax: <strong>every</strong> <em>(interval n) body &hellip;</em></dt>
<dd><p>Schedule <var>body</var> to be evaluated every <var>interval</var> amount of
time.  Repeat this <var>n</var> times, or indefinitely if not specified.
</p></dd></dl>

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