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-<a name="Signals"></a>
-<div class="header">
-<p>
-Next: <a href="Coroutines.html#Coroutines" accesskey="n" rel="next">Coroutines</a>, Up: <a href="Time.html#Time" accesskey="u" rel="up">Time</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
-</div>
-<hr>
-<a name="Signals-1"></a>
-<h4 class="subsection">4.3.1 Signals</h4>
-
-<div class="example">
-<pre class="example">(use-modules (sly signal))
-</pre></div>
-
-<p>Game state is a function of time. The player&rsquo;s score, the current
-stage, an enemy&rsquo;s hit points, etc. all change in response to events
-that happen at discrete points in time. Typically, this means that a
-number of callback procedures are registered to respond to events
-which mutate the relevant data structures. However, this approach,
-while simple and effective, comes at the price of readability,
-reproducibility, and expression. Instead of explicitly mutating data
-and entering &ldquo;callback hell&rdquo;, Sly abstracts and formalizes the
-process using a functional reactive programming style.
-</p>
-<p>In Sly, time-varying values are called &ldquo;signals&rdquo;, and they are
-defined in a declarative and functional manner. Rather than
-describing the process of mutation procedurally, one describes the
-relationships between signals instead. The result is a &ldquo;signal
-graph&rdquo;, a directed acyclic graph of event responses.
-</p>
-<div class="example">
-<pre class="example">(define-signal position
- (signal-fold v+ (vector2 320 240)
- (signal-map (lambda (v) (v* v 4))
- (signal-sample 1 key-arrows))))
-</pre></div>
-
-<p>This signal describes a relationship between the arrow keys on the
-keyboard and the position of the player. <code>signal-sample</code> is used
-to trigger a signal update upon every game tick that provides the
-current state of the arrow keys. <code>key-arrows</code> is a 2D vector
-that maps to the current state of the arrow keys, allowing for 8
-directional movement. This vector is then scaled 4x to make the
-player move faster. Finally, the scaled vector is added to the
-previous player position via <code>signal-fold</code>. The player&rsquo;s
-position is at (320, 240) initially. As you can see, there are no
-callbacks and explicit mutation needed, and the position seems to
-magically change with the passage of time.
-</p>
-<dl>
-<dt><a name="index-signal_003f"></a>Scheme Procedure: <strong>signal?</strong> <em><var>obj</var></em></dt>
-<dd><p>Return <code>#t</code> if <var>obj</var> is a signal.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-make_002dsignal"></a>Scheme Procedure: <strong>make-signal</strong> <em><var>value</var></em></dt>
-<dd><p>Wrap <var>value</var> in a signal.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-define_002dsignal"></a>Scheme Syntax: <strong>define-signal</strong> <em><var>name</var> <var>value</var></em></dt>
-<dd><p>Create a top-level signal variable called <var>name</var>. If the variable
-already exists and refers to a signal then its outputs will be spliced
-into the new signal. If the given value is not a signal then it will
-be put into one via <code>make-signal</code>.
-</p>
-<p><code>define-signal</code> is particularly useful when working at the REPL.
-A top-level signal variable defined by <code>define-signal</code> can be
-redefined at runtime, and the signals that depended on the old signal
-will continue to work with the new signal.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dref"></a>Scheme Procedure: <strong>signal-ref</strong> <em><var>signal</var></em></dt>
-<dd><p>Return the value stored within <var>signal</var>.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dref_002dmaybe"></a>Scheme Procedure: <strong>signal-ref-maybe</strong> <em>object</em></dt>
-<dd><p>Return the value stored within <var>object</var> if <var>object</var> is a
-signal. Otherwise, return <var>object</var>.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dlet"></a>Scheme Syntax: <strong>signal-let</strong> <em>((<var>var</var> <var>signal</var>) &hellip;) <var>body</var> &hellip;</em></dt>
-<dd><p>Evaluate <var>body</var> in the context of the local bindings defined by
-the two-element lists <code>((var signal) &hellip;)</code>.
-<code>signal-let</code> works like regular <code>let</code>, except that it
-derefences <var>signal</var> before binding to <var>var</var>.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dlet_002a"></a>Scheme Syntax: <strong>signal-let*</strong> <em>((<var>var</var> <var>signal</var>) &hellip;) <var>body</var> &hellip;</em></dt>
-<dd><p>Similar to <code>signal-let</code>, but the variable bindings are performed
-sequentially. This means that all initialization expressions are
-allowed to use the variables defined to the their left in the binding
-list.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dset_0021"></a>Scheme Procedure: <strong>signal-set!</strong> <em>signal-box value</em></dt>
-<dd><p>Change the contents of <var>signal</var> to <var>value</var>. This procedure
-should almost never be used, except to bootstrap a root node of a
-signal graph.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-hook_002d_003esignal"></a>Scheme Procedure: <strong>hook-&gt;signal</strong> <em><var>hook</var> <var>init</var> <var>proc</var></em></dt>
-<dd><p>Create a new signal whose initial value is <var>init</var> and whose future
-values are calculated by applying <var>proc</var> to the arguments passed
-when <var>hook</var> is run.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dmerge"></a>Scheme Procedure: <strong>signal-merge</strong> <em><var>signal1</var> <var>signal2</var> . <var>rest</var></em></dt>
-<dd><p>Create a new signal whose value is the that of the most recently
-updated signal in <var>signal1</var>, <var>signal2</var>, etc. The initial
-value is that of <var>signal1</var>.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dzip"></a>Scheme Procedure: <strong>signal-zip</strong> <em>. <var>signals</var></em></dt>
-<dd><p>Create a new signal whose value is a list of the values stored in
-<var>signals</var>.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dmap"></a>Scheme Procedure: <strong>signal-map</strong> <em><var>proc</var> <var>signal</var> . <var>rest</var></em></dt>
-<dd><p>Create a new signal that applies <var>proc</var> to the values of
-<var>SIGNAL</var>. More than one input signal may be specified, in which
-case <var>proc</var> must accept as many arguments as there are input
-signals.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dsample_002don"></a>Scheme Procedure: <strong>signal-sample-on</strong> <em><var>value-signal</var> <var>sample-signal</var></em></dt>
-<dd><p>Create a new signal that takes on the value of <var>value-signal</var>
-whenever <var>sample-signal</var> receives a new value.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dnegate"></a>Scheme Procedure: <strong>signal-negate</strong> <em><var>signal</var></em></dt>
-<dd><p>Create a new signal whose value is the negation of <var>signal</var> by
-applying <code>not</code> to each value received.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dfold"></a>Scheme Procedure: <strong>signal-fold</strong> <em><var>proc</var> <var>init</var> <var>signal</var> . <var>rest</var></em></dt>
-<dd><p>Create a new signal that applies <var>proc</var> with the value received
-from <var>signal</var> and the past value of itself, starting with
-<var>init</var>. Like <code>signal-map</code>, more than one input signal may be
-given.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dfilter"></a>Scheme Procedure: <strong>signal-filter</strong> <em><var>predicate</var> <var>default</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal that takes on the value received from <var>signal</var>
-when it satisfies the procedure <var>predicate</var>. The value of the
-signal is <var>default</var> in the case that the predicate is never
-satisfied.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002ddrop"></a>Scheme Procedure: <strong>signal-drop</strong> <em><var>predicate</var> <var>default</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal that takes on the value received from <var>signal</var>
-when it does <em>not</em> satisfy the procedure <var>predicate</var>. The
-value of the signal is <var>default</var> in the case that the predicate is
-never satisfied.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002ddrop_002drepeats"></a>Scheme Procedure: <strong>signal-drop-repeats</strong> <em><var>signal</var> [<var>equal?</var>]</em></dt>
-<dd><p>Create a new signal that drops the value received from <var>signal</var>
-when it is equivalent to the current value. By default, <code>equal?</code>
-is used for testing equivalence.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dswitch"></a>Scheme Procedure: <strong>signal-switch</strong> <em><var>predicate</var> <var>on</var> <var>off</var></em></dt>
-<dd><p>Create a new signal whose value is that of the signal <var>on</var> when
-the signal <var>predicate</var> is true, or the value of the signal
-<var>off</var> otherwise.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dconstant"></a>Scheme Procedure: <strong>signal-constant</strong> <em><var>constant</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal whose value is always <var>constant</var> no matter the
-value received from <var>signal</var>.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dcount"></a>Scheme Procedure: <strong>signal-count</strong> <em><var>signal</var> [<var>start</var>] [<var>step</var>]</em></dt>
-<dd><p>Create a new signal that increments a counter by <var>step</var> when a
-value from <var>signal</var> is received, starting from <var>start</var>. By
-default, <var>start</var> is 0 and <var>step</var> is 1.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dtap"></a>Scheme Procedure: <strong>signal-tap</strong> <em><var>proc</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal that applies <var>proc</var> for side-effects when a
-value from <var>signal</var> is received. The value of the new signal will
-always be the value of <var>signal</var>. This signal is a convenient way
-to sneak in a procedure that with a side-effect into a signal graph.
-Such a signal might write text to a file, or play a sound.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dtimestamp"></a>Scheme Procedure: <strong>signal-timestamp</strong> <em><var>signal</var></em></dt>
-<dd><p>Create a new signal whose value is a pair, the car of which is the
-time that the value of <var>signal</var> was received and the cdr of which
-is the received value.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dtime"></a>Scheme Procedure: <strong>signal-time</strong> <em><var>signal</var></em></dt>
-<dd><p>Create a new signal whose value is the time that the value of
-<var>signal</var> was received.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dsample"></a>Scheme Procedure: <strong>signal-sample</strong> <em><var>step</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal that takes on the value of <var>signal</var> every
-<var>step</var> ticks.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002devery"></a>Scheme Procedure: <strong>signal-every</strong> <em><var>step</var></em></dt>
-<dd><p>Create a new signal that emits <var>step</var> every <var>step</var> ticks.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dsince"></a>Scheme Procedure: <strong>signal-since</strong> <em><var>step</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal that emits the time since <var>signal</var> was updated
-ever <var>step</var> ticks.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002ddelay"></a>Scheme Procedure: <strong>signal-delay</strong> <em><var>delay</var> <var>signal</var></em></dt>
-<dd><p>Create a new signal that delays propagation of <var>signal</var> by
-<var>delay</var> ticks..
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dthrottle"></a>Scheme Procedure: <strong>signal-throttle</strong> <em>delay signal</em></dt>
-<dd><p>Create a new signal that propagates <var>signal</var> at most once every
-<var>delay</var> ticks.
-</p></dd></dl>
-
-<dl>
-<dt><a name="index-signal_002dgenerator"></a>Scheme Syntax: <strong>signal-generator</strong> <em><var>body</var> &hellip;</em></dt>
-<dd><p>Create a new signal whose value is the most recently yielded value of
-the coroutine defined by <var>body</var>. A special <code>yield</code> syntax is
-available within <var>body</var> to specify which values are passed to the
-signal.
-</p></dd></dl>
-
-<hr>
-<div class="header">
-<p>
-Next: <a href="Coroutines.html#Coroutines" accesskey="n" rel="next">Coroutines</a>, Up: <a href="Time.html#Time" accesskey="u" rel="up">Time</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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