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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">The Concur UI Framework</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="chapter-item expanded "><a href="ch01-02-first-concur-program.html"><strong aria-hidden="true">1.2.</strong> Your first Concur program!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-quick-tour.html"><strong aria-hidden="true">2.</strong> A Quick Tour of Concur</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch02-01-anatomy-of-a-widget.html"><strong aria-hidden="true">2.1.</strong> Anatomy of a Widget</a></li><li class="chapter-item expanded "><a href="ch02-02-widget-return-types.html"><strong aria-hidden="true">2.2.</strong> Widget Return Types</a></li><li class="chapter-item expanded "><a href="ch02-03-composing-widgets.html"><strong aria-hidden="true">2.3.</strong> Composing Widgets</a></li><li class="chapter-item expanded "><a href="ch02-04-event-handling.html"><strong aria-hidden="true">2.4.</strong> Event Handling</a></li><li class="chapter-item expanded "><a href="ch02-05-modifying-dom-on-events.html"><strong aria-hidden="true">2.5.</strong> Modifying DOM on Events</a></li><li class="chapter-item expanded "><a href="ch02-06-handling-multiple-events.html"><strong aria-hidden="true">2.6.</strong> Handling Multiple Events</a></li><li class="chapter-item expanded "><a href="ch02-07-composing-widgets-with-events.html"><strong aria-hidden="true">2.7.</strong> Composing Widgets with Events</a></li><li class="chapter-item expanded "><a href="ch02-08-input-output.html"><strong aria-hidden="true">2.8.</strong> Input Output</a></li></ol></li><li class="chapter-item expanded "><a href="ch03-00-architectural-notes.html"><strong aria-hidden="true">3.</strong> Architectural Notes</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-replicating-elm-architecture.html"><strong aria-hidden="true">3.1.</strong> Replicating Elm Architecture</a></li><li class="chapter-item expanded "><a href="ch03-02-plan-for-composability-by-breaking-down-components.html"><strong aria-hidden="true">3.2.</strong> Plan for Composability by Breaking Down Components</a></li><li class="chapter-item expanded "><a href="ch03-03-composing-never-ending-widgets.html"><strong aria-hidden="true">3.3.</strong> Composing Never Ending Widgets</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-signals.html"><strong aria-hidden="true">4.</strong> Signals</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-introduction-to-signals.html"><strong aria-hidden="true">4.1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="ch04-02-your-first-signal.html"><strong aria-hidden="true">4.2.</strong> Your First Signal</a></li><li class="chapter-item expanded "><a href="ch04-03-introduction-to-signals.html"><strong aria-hidden="true">4.3.</strong> Converting Display Widgets</a></li><li class="chapter-item expanded "><a href="ch04-04-signal-composition.html"><strong aria-hidden="true">4.4.</strong> Signal Composition</a></li><li class="chapter-item expanded "><a href="ch04-05-signal-example.html"><strong aria-hidden="true">4.5.</strong> Signal Example</a></li><li class="chapter-item expanded "><a href="ch04-06-some-notes-on-signal-usage.html" class="active"><strong aria-hidden="true">4.6.</strong> Some Notes on Signal Usage</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-faqs.html"><strong aria-hidden="true">5.</strong> FAQs</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-why-concur.html"><strong aria-hidden="true">5.1.</strong> Why the Concur Model</a></li><li class="chapter-item expanded "><a href="ch05-02-jumping-into-the-middle-of-widgets.html"><strong aria-hidden="true">5.2.</strong> Jumping Into the Middle of Widgets</a></li></ol></li></ol>
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<h1><a class="header" href="#some-notes-on-signal-usage" id="some-notes-on-signal-usage">Some Notes on Signal Usage</a></h1>
<ol>
<li>
<p>A <code>Signal</code> is basically a <code>Widget</code> + <code>looping</code>. So <code>loopW</code>, which provides stateful looping, is the easiest way to generate individual signals.</p>
</li>
<li>
<p>Signals are "continuous", i.e. they always have a current value. So the notion of "merging" signals does not arise except as a zipping operation, where you provide a continuous function which combines individual values from the constituent signals. This "zipping" is possible using monadic bind like so -</p>
</li>
</ol>
<pre><code class="language-purescript">do
a <- signal1
b <- signal2
c <- signal3
pure (someFunction a b c)
</code></pre>
<ol start="3">
<li>If a signal depends on the value of another <strong>upstream</strong> signal, you can just express that dependency with parameter passing. For example, if <code>signal2</code> depends on the output from <code>signal1</code>, and <code>signal3</code> depends on the output of both <code>signal1</code> and <code>signal2</code> then -</li>
</ol>
<pre><code class="language-purescript">do
a <- signal1
b <- signal2 a
c <- signal3 a b
pure (someFunction a b c)
</code></pre>
<ol start="4">
<li>A slight complication occurs when a signal depends on the value of a <strong>downstream</strong> signal, we need to use the <code>loopS</code> combinator which "loops" the value of the last signal back up to the top so that the upstream signals can use it. Note that you would need the last signal to carry <strong>all</strong> the values that are needed by the upstream signals.</li>
</ol>
<p>Usually you would have some sort of a state that is accessible to all the constituent signals, and loop over that state. In the counter+timer example, the timer and the counter both depended on the current count. So we need to loop the current count over.</p>
<ol start="5">
<li>Note that sometimes this changes the final return value of the signal away from what you want. So you may need to <code>map</code> the final composition function on top of the signal value.</li>
</ol>
<p>To take a pedantic example - if <code>signal1</code> also depended on the values of <code>signal2</code> and <code>signal3</code>, we can have <code>signal3</code> return those values and make them available to <code>signal1</code> with <code>loopS</code>. Let's assume that the initial values for <code>signal2</code> is <code>initialB</code> and for <code>signal3</code> is <code>initialC</code>.</p>
<pre><code class="language-purescript">loopS {b:initialB, c:initialC} \{b, c} -> do
a <- signal1 b c
b' <- signal2 a
c' <- signal3 a b'
pure {b:b', c:c'}
</code></pre>
<p>But now the final value of the signal is <code>Tuple b c</code>. So we need to fix that. We need to add the value of <code>signal1</code> to the output since that's needed in the final output, and then map over with <code>someFunction</code> -</p>
<pre><code class="language-purescript">s = g <$> innerSignal
where
g {a,b,c} = someFunction a b c
innerSignal = loopS {a:initialA, b:initialB, c:initialC} \{b, c} -> do
a' <- signal1 b c
b' <- signal2 a'
c' <- signal3 a' b'
pure {a:a', b:b', c:c'}
</code></pre>
<p>Not pretty, but usually you won't have such complicated state and dependencies.</p>
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