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<ol class="chapter"><li><a href="../00-introduction/00-index.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li><a href="../00-introduction/01-requirements.html"><strong aria-hidden="true">1.1.</strong> Reader Requirements</a></li><li><a href="../00-introduction/02-goals_and_style.html"><strong aria-hidden="true">1.2.</strong> Book Goals and Style</a></li><li><a href="../00-introduction/03-development-setup.html"><strong aria-hidden="true">1.3.</strong> Development Setup</a></li><li><a href="../00-introduction/04-hello-magic.html"><strong aria-hidden="true">1.4.</strong> Hello, Magic</a></li><li><a href="../00-introduction/05-newtype.html" class="active"><strong aria-hidden="true">1.5.</strong> Newtype</a></li><li><a href="../00-introduction/06-help_and_resources.html"><strong aria-hidden="true">1.6.</strong> Help and Resources</a></li></ol></li><li><a href="../01-limitations/00-index.html"><strong aria-hidden="true">2.</strong> Limitations</a></li><li><ol class="section"><li><a href="../01-limitations/01-no_floats.html"><strong aria-hidden="true">2.1.</strong> No Floats</a></li><li><a href="../01-limitations/02-core_only.html"><strong aria-hidden="true">2.2.</strong> Core Only</a></li><li><a href="../01-limitations/03-volatile_destination.html"><strong aria-hidden="true">2.3.</strong> Volatile Destination</a></li></ol></li><li><a href="../02-concepts/00-index.html"><strong aria-hidden="true">3.</strong> Concepts</a></li><li><ol class="section"><li><a href="../02-concepts/01-cpu.html"><strong aria-hidden="true">3.1.</strong> CPU</a></li><li><a href="../02-concepts/02-bios.html"><strong aria-hidden="true">3.2.</strong> BIOS</a></li><li><a href="../02-concepts/03-wram.html"><strong aria-hidden="true">3.3.</strong> Work RAM</a></li><li><a href="../02-concepts/04-io-registers.html"><strong aria-hidden="true">3.4.</strong> IO Registers</a></li><li><a href="../02-concepts/05-palram.html"><strong aria-hidden="true">3.5.</strong> Palette RAM</a></li><li><a href="../02-concepts/06-vram.html"><strong aria-hidden="true">3.6.</strong> Video RAM</a></li><li><a href="../02-concepts/07-oam.html"><strong aria-hidden="true">3.7.</strong> Object Attribute Memory</a></li><li><a href="../02-concepts/08-rom.html"><strong aria-hidden="true">3.8.</strong> Game Pak ROM / Flash ROM</a></li><li><a href="../02-concepts/09-sram.html"><strong aria-hidden="true">3.9.</strong> Save RAM</a></li></ol></li><li><a href="../03-video/00-index.html"><strong aria-hidden="true">4.</strong> Video</a></li><li><ol class="section"><li><a href="../03-video/01-rgb15.html"><strong aria-hidden="true">4.1.</strong> RBG15 Color</a></li><li><a href="../03-video/TODO.html"><strong aria-hidden="true">4.2.</strong> TODO</a></li></ol></li><li><a href="../04-non-video/00-index.html"><strong aria-hidden="true">5.</strong> Non-Video</a></li><li><ol class="section"><li><a href="../04-non-video/01-buttons.html"><strong aria-hidden="true">5.1.</strong> Buttons</a></li><li><a href="../04-non-video/02-timers.html"><strong aria-hidden="true">5.2.</strong> Timers</a></li><li><a href="../04-non-video/03-dma.html"><strong aria-hidden="true">5.3.</strong> Direct Memory Access</a></li><li><a href="../04-non-video/04-sound.html"><strong aria-hidden="true">5.4.</strong> Sound</a></li><li><a href="../04-non-video/05-interrupts.html"><strong aria-hidden="true">5.5.</strong> Interrupts</a></li><li><a href="../04-non-video/06-network.html"><strong aria-hidden="true">5.6.</strong> Network</a></li><li><a href="../04-non-video/07-game_pak.html"><strong aria-hidden="true">5.7.</strong> Game Pak</a></li></ol></li><li><a href="../05-examples/00-index.html"><strong aria-hidden="true">6.</strong> Examples</a></li><li><ol class="section"><li><a href="../05-examples/01-hello_magic.html"><strong aria-hidden="true">6.1.</strong> hello_magic</a></li><li><a href="../05-examples/02-hello_world.html"><strong aria-hidden="true">6.2.</strong> hello_world</a></li><li><a href="../05-examples/03-light_cycle.html"><strong aria-hidden="true">6.3.</strong> light_
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<main>
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<a class="header" href="#newtype" id="newtype"><h1>Newtype</h1></a>
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<p>There's one thing I want to get out of the way near the start of the book and it
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didn't really have a good fit anywhere else in the book so it goes right here.</p>
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<p>We're talking about the "Newtype Pattern"!</p>
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<p>Now, I told you to read the Rust Book before you read this book, and I'm sure
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you're all good students who wouldn't sneak into this book without doing the
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required reading, so I'm sure you all remember exactly what I'm talking about,
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because they touch on the newtype concept in the book twice, in two <em>very</em> long
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named sections:</p>
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<ul>
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<li><a href="https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#using-the-newtype-pattern-to-implement-external-traits-on-external-types">Using the Newtype Pattern to Implement External Traits on External
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Types</a></li>
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<li><a href="https://doc.rust-lang.org/book/ch19-04-advanced-types.html#using-the-newtype-pattern-for-type-safety-and-abstraction">Using the Newtype Pattern for Type Safety and
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Abstraction</a></li>
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</ul>
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<p>...Yeah... The Rust Book doesn't know how to make a short sub-section name to
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save its life. Shame.</p>
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<a class="header" href="#newtype-basics" id="newtype-basics"><h2>Newtype Basics</h2></a>
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<p>So, we have all these pieces of data, and we want to keep them separated, and we
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don't wanna pay the cost for it at runtime. Well, we're in luck, we can pay the
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cost at compile time.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub struct PixelColor(u16);
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#}</code></pre></pre>
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<p>Ah, except that, as I'm sure you remember from <a href="https://doc.rust-lang.org/nomicon/other-reprs.html#reprtransparent">The
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Rustonomicon</a>
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(and from <a href="https://github.com/rust-lang/rfcs/blob/master/text/1758-repr-transparent.md">the
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RFC</a>
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too, of course), if we have a single field struct that's sometimes different
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from having just the bare value, so we should be using <code>#[repr(transparent)]</code>
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with our newtypes.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[repr(transparent)]
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pub struct PixelColor(u16);
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#}</code></pre></pre>
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<p>Ah, and of course we'll need to make it so you can unwrap the value:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[repr(transparent)]
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pub struct PixelColor(u16);
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impl From<PixelColor> for u16 {
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fn from(color: PixelColor) -> u16 {
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color.0
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}
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}
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#}</code></pre></pre>
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<p>And then we'll need to do that same thing for <em>every other newtype we want</em>.</p>
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<p>Except there's only two tiny parts that actually differ between newtype
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declarations: the new name and the base type. All the rest is just the same rote
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code over and over. Generating piles and piles of boilerplate code? Sounds like
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a job for a macro to me!</p>
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<a class="header" href="#making-it-a-macro" id="making-it-a-macro"><h2>Making It A Macro</h2></a>
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<p>The most basic version of the macro we want goes like this:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[macro_export]
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macro_rules! newtype {
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($new_name:ident, $old_name:ident) => {
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#[repr(transparent)]
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pub struct $new_name($old_name);
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};
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}
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#}</code></pre></pre>
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<p>Except we also want to be able to add attributes (which includes doc comments),
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so we upgrade our macro a bit:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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#[macro_export]
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macro_rules! newtype {
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($(#[$attr:meta])* $new_name:ident, $old_name:ident) => {
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$(#[$attr])*
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#[repr(transparent)]
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pub struct $new_name($old_name);
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};
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}
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#}</code></pre></pre>
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<p>And we want to automatically add the ability to turn the wrapper type back into
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the wrapped type.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
|
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#fn main() {
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#[macro_export]
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macro_rules! newtype {
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($(#[$attr:meta])* $new_name:ident, $old_name:ident) => {
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$(#[$attr])*
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#[repr(transparent)]
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pub struct $new_name($old_name);
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impl From<$new_name> for $old_name {
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fn from(x: $new_name) -> $old_name {
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x.0
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}
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}
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};
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}
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#}</code></pre></pre>
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<p>That seems like enough for all of our examples, so we'll stop there. We could
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add more things, such as making the <code>From</code> impl optional (because what if you
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shouldn't unwrap it for some weird reason?), allowing for more precise
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visibility controls (on both the newtype overall and the inner field), and maybe
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even other things I can't think of right now. We won't really need those in our
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example code for this book, so it's probably nicer to just keep the macro
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simpler and quit while we're ahead.</p>
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<p><strong>As a reminder:</strong> remember that macros have to appear <em>before</em> they're invoked in
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your source, so the <code>newtype</code> macro will always have to be at the very top of
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your file, or in a module that's declared before other modules and code.</p>
|
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</main>
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