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<span id="Scripts"></span><div class="header">
<p>
Next: <a href="Tweening.html" accesskey="n" rel="next">Tweening</a>, Previous: <a href="Agendas.html" accesskey="p" rel="prev">Agendas</a>, Up: <a href="Scripting.html" accesskey="u" rel="up">Scripting</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html" title="Index" rel="index">Index</a>]</p>
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<hr />
<span id="Scripts-1"></span><h4 class="subsection">5.5.2 Scripts</h4>
<p>Now that we can schedule tasks, let’s take things to the next level.
It sure would be great if we could make procedures that described a
series of actions that happened over time, especially if we could do
so without contorting our code into a nest of callback procedures.
This is where scripts come in. With scripts we can write code in a
linear way, in a manner that appears to be synchronous, but with the
ability to suspend periodically in order to let other scripts have a
turn and prevent blocking the game loop. Building on top of the
scheduling that agendas provide, here is a script that models a child
trying to get their mother’s attention:
</p>
<div class="lisp">
<pre class="lisp"><span class="syntax-open">(</span><span class="syntax-symbol">script</span>
<span class="syntax-open">(</span><span class="syntax-symbol">while</span> <span class="syntax-symbol">#t</span>
<span class="syntax-open">(</span><span class="syntax-symbol">display</span> <span class="syntax-string">"mom!"</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">newline</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">sleep</span> <span class="syntax-symbol">60</span><span class="syntax-close">)</span><span class="syntax-close">)</span><span class="syntax-close">)</span> <span class="syntax-comment">; where 60 = 1 second of real time
</span></pre></div>
<p>This code runs in an endless loop, but the <code>sleep</code> procedure
suspends the script and schedules it to be run later by the agenda.
So, after each iteration of the loop, control is returned back to the
game loop and the program is not stuck spinning in a loop that will
never exit. Pretty neat, eh?
</p>
<p>Scripts can suspend to any capable handler, not just the agenda.
The <code>yield</code> procedure will suspend the current script and pass
its “continuation” to a handler procedure. This handler procedure
could do anything. Perhaps the handler stashes the continuation
somewhere where it will be resumed when the user presses a specific
key on the keyboard, or maybe it will be resumed when the player picks
up an item off of the dungeon floor; the sky is the limit.
</p>
<p>Sometimes it is necessary to abruptly terminate a script after it has
been started. For example, when an enemy is defeated their AI routine
needs to be shut down. When a script is spawned, a handle to that
script is returned that can be used to cancel it when desired.
</p>
<div class="lisp">
<pre class="lisp"><span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-symbol">script</span> <span class="syntax-open">(</span><span class="syntax-symbol">script</span> <span class="syntax-open">(</span><span class="syntax-symbol">while</span> <span class="syntax-symbol">#t</span> <span class="syntax-open">(</span><span class="syntax-symbol">display</span> <span class="syntax-string">"hey\n"</span><span class="syntax-close">)</span> <span class="syntax-open">(</span><span class="syntax-symbol">sleep</span> <span class="syntax-symbol">60</span><span class="syntax-close">)</span><span class="syntax-close">)</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
<span class="syntax-comment">;; sometime later
</span><span class="syntax-open">(</span><span class="syntax-symbol">cancel-script</span> <span class="syntax-symbol">script</span><span class="syntax-close">)</span>
</pre></div>
<dl>
<dt id="index-spawn_002dscript">Procedure: <strong>spawn-script</strong> <em>thunk</em></dt>
<dd><p>Apply <var>thunk</var> as a script and return a handle to it.
</p></dd></dl>
<dl>
<dt id="index-script">Syntax: <strong>script</strong> <em>body …</em></dt>
<dd><p>Evaluate <var>body</var> as a script and return a handle to it.
</p></dd></dl>
<dl>
<dt id="index-script_003f">Procedure: <strong>script?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is a script handle.
</p></dd></dl>
<dl>
<dt id="index-script_002dcancelled_003f">Procedure: <strong>script-cancelled?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> has been cancelled.
</p></dd></dl>
<dl>
<dt id="index-script_002drunning_003f">Procedure: <strong>script-running?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> has not yet terminated or been
cancelled.
</p></dd></dl>
<dl>
<dt id="index-script_002dcomplete_003f">Procedure: <strong>script-complete?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> has terminated.
</p></dd></dl>
<dl>
<dt id="index-cancel_002dscript">Procedure: <strong>cancel-script</strong> <em>co</em></dt>
<dd><p>Prevent further execution of the script <var>co</var>.
</p></dd></dl>
<dl>
<dt id="index-yield">Procedure: <strong>yield</strong> <em>handler</em></dt>
<dd><p>Suspend the current script and pass its continuation to the
procedure <var>handler</var>.
</p></dd></dl>
<dl>
<dt id="index-join">Procedure: <strong>join</strong> <em>script</em></dt>
<dd><p>Suspend the current script until <var>script</var> has terminated.
</p></dd></dl>
<dl>
<dt id="index-sleep">Procedure: <strong>sleep</strong> <em>duration</em></dt>
<dd><p>Wait <var>duration</var> before resuming the current script.
</p></dd></dl>
<dl>
<dt id="index-wait_002duntil">Syntax: <strong>wait-until</strong> <em>condition</em></dt>
<dd><p>Wait until <var>condition</var> is met before resuming the current script.
</p>
<div class="lisp">
<pre class="lisp"><span class="syntax-open">(</span><span class="syntax-symbol">script</span>
<span class="syntax-open">(</span><span class="syntax-symbol">wait-until</span> <span class="syntax-open">(</span><span class="syntax-symbol">key-pressed?</span> <span class="syntax-symbol">'z</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">display</span> <span class="syntax-string">"you pressed the Z key!\n"</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
</pre></div>
</dd></dl>
<dl>
<dt id="index-forever">Syntax: <strong>forever</strong> <em>body …</em></dt>
<dd><p>Evaluate <var>body</var> in an endless loop.
</p></dd></dl>
<hr />
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<p>
Next: <a href="Tweening.html" accesskey="n" rel="next">Tweening</a>, Previous: <a href="Agendas.html" accesskey="p" rel="prev">Agendas</a>, Up: <a href="Scripting.html" accesskey="u" rel="up">Scripting</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html" title="Index" rel="index">Index</a>]</p>
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