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<h1 id="sum-type"><a class="header" href="#sum-type">Sum Type</a></h1>
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<blockquote>
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<p>This section isn't specific to xplr. However, since xplr configuration makes
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heavy use of this particular data type, even though it isn't available in
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most of the mainstream programming languages (yet), making it a wild or
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unfamiliar concept for many, it's worth doing a quick introduction here.</p>
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<p>If you're already familiar with <a href="https://en.wikipedia.org/wiki/Tagged_union">Sum Type / Tagged Union</a> (e.g. Rust's
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enum), you can skip ahead.</p>
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</blockquote>
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<p>While reading this doc, you'll come across some data types like <a href="layout.html">Layout</a>,
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<a href="style.html#color">Color</a>, <a href="message.html">Message</a> etc. that says something like "x is a sum type that
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can be any of the following", and then you'll see a list of strings and/or lua
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tables just below.</p>
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<p>Yes, they are actually sum types, i.e. they can be any of the given set of
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tagged variants listed there.</p>
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<p>Notice the word "be". Unlike classes or structs (aka product types), they can't
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"have" values, they can only "be" the value, or rather, be one of the possible
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set of values.</p>
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<p>Also notice the word "tagged". Unlike the single variant <code>null</code>, or the dual
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variant <code>boolean</code> types, the variants of sum types are tagged (i.e. named), and
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may further have, or be, value or values of any data type.</p>
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<p>A simple example of a sum type is an enum. Many programming languages have
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them, but only a few modern programming languages allow nesting other types
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into a sum type.</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Color {
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Red,
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Green,
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>Here, <code>Color</code> can be one of two possible set of values: <code>Red</code> and <code>Green</code>, just
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like <code>boolean</code>, but unlike <code>boolean</code>, being tagged allows <code>Color</code> to have more
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than two variants if required, by changing the definition.</p>
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<p>e.g.</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Color {
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Red,
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Green,
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Blue,
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>We'd document it here as:</p>
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<blockquote>
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<p>Result is a sum type that can be one of the following:</p>
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<ul>
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<li>"Red"</li>
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<li>"Green"</li>
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<li>"Blue"</li>
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</ul>
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</blockquote>
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<p>But some languages (like Rust, Haskell, Elm etc.) go even further, allowing us
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to associate each branch of the enum with further nested types like:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Layout {
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Table,
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HelpMenu,
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Horizontal {
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config: LayoutConfig, // A product type (similar to class/struct)
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splits: Vec<Layout> // A list of "Layout"s (i.e. list of sum types)
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},
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>Here, as we can see, unlike the first example, some of <code>Layout</code>'s possible
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variants can have further nested types associated with them. Note that
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<code>Horizontal</code> here can have a sum type (e.g. enum), or a product type (e.g.
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class/struct), or both (any number of them actually) nested in it. But the
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nested values will only exist when <code>Layout</code> is <code>Horizontal</code>.</p>
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<p>We'd document it here as:</p>
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<blockquote>
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<p>Layout is a sum type that can be one of the following:</p>
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<ul>
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<li>"Table"</li>
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<li>"HelpMenu"</li>
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<li>{ Horizontal = { config = Layout Config, splits = { Layout, ... } }</li>
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</ul>
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</blockquote>
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<p>And then we'd go on documenting whatever <code>Layout Config</code> is.</p>
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<p>So, there you go. This is exactly what sum types are - glorified enums that can
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have nested types in each branch.</p>
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<hr />
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<p>If you're still confused about something, or if you found an error in this
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explanation, feel free to <a href="community.html">discuss together</a>.</p>
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