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<h1 class="header-title">A Rustic Re-Podcasting Server (Part 1)</h1>
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<p class="header-date"> <a href="https://bspeice.github.io/author/bradlee-speice.html">Bradlee Speice</a>, Sat 22 October 2016, <a href="https://bspeice.github.io/category/blog.html">Blog</a></p>
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<a href="https://bspeice.github.io/tag/nutone.html">nutone</a>, <a href="https://bspeice.github.io/tag/rust.html">Rust</a> </p>
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<p>I listen to a lot of Drum and Bass music, because it's beautiful music. And
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there's a particular site, <a href="http://bassdrive.com/">Bassdrive.com</a> that hosts
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a lot of great content. Specifically, the
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<a href="http://archives.bassdrivearchive.com/">archives</a> section of the site has a
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list of the past shows that you can download and listen to. The issue is, it's
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just a <a href="http://archives.bassdrivearchive.com/6%20-%20Saturday/Electronic%20Warfare%20-%20The%20Overfiend/">giant list of links to download</a>. I'd really like
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this in a podcast format to take with me on the road, etc.</p>
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<p>So I wrote the <a href="https://github.com/bspeice/elektricity">elektricity</a> web
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application to actually accomplish all that. Whenever you request a feed, it
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goes out to Bassdrive, processes all the links on a page, and serves up some
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fresh, tasty RSS to satisfy your ears. I hosted it on Heroku using the free
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tier because it's really not resource-intensive at all.</p>
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<p><strong>The issue so far</strong> is that I keep running out of free tier hours during a
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month because my podcasting application likes to have a server scan for new
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episodes constantly. Not sure why it's doing that, but I don't have a whole
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lot of control over it. It's a phenomenal application otherwise.</p>
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<p><strong>My (over-engineered) solution</strong>: Re-write the application using the
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<a href="https://www.rust-lang.org/en-US/">Rust</a> programming language. I'd like to run
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this on a small hacker board I own, and doing this in Rust would allow me to
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easily cross-compile it. Plus, I've been very interested in the Rust language
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for a while and this would be a great opportunity to really learn it well.
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The code is available <a href="https://github.com/bspeice/nutone">here</a> as development
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progresses.</p>
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<h1>The Setup</h1>
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<p>We'll be using the <a href="http://ironframework.io/">iron</a> library to handle the
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server, and <a href="http://hyper.rs/">hyper</a> to fetch the data we need from elsewhere
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on the interwebs. <a href="http://doc.servo.org/html5ever/index.html">HTML5Ever</a> allows
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us to ingest the content that will be coming from Bassdrive, and finally,
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output is done with <a href="http://sunng87.github.io/handlebars-rust/handlebars/index.html">handlebars-rust</a>.</p>
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<p>It will ultimately be interesting to see how much more work must be done to
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actually get this working over another language like Python. Coming from a
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dynamic state of mind it's super easy to just chain stuff together, ship it out,
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and call it a day. I think I'm going to end up getting much dirtier trying to
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write all of this out.</p>
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<h1>Issue 1: Strings</h1>
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<p>Strings in Rust are hard. I acknowledge Python can get away with some things
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that make strings super easy (and Python 3 has gotten better at cracking down
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on some bad cases, <code>str <-> bytes</code> specifically), but Rust is hard.</p>
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<p>Let's take for example the <code>404</code> error handler I'm trying to write. The result
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should be incredibly simple: All I want is to echo back
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<code>Didn't find URL: <url></code>. Shouldn't be that hard right? In Python I'd just do
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something like:</p>
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<div class="highlight"><pre><span class="k">def</span> <span class="nf">echo_handler</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
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<span class="k">return</span> <span class="s2">"You're visiting: {}"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">request</span><span class="o">.</span><span class="n">uri</span><span class="p">)</span>
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</pre></div>
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<p>And we'd call it a day. Rust isn't so simple. Let's start with the trivial
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examples people post online:</p>
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<div class="highlight"><pre><span class="k">fn</span><span class="w"> </span><span class="n">hello_world</span><span class="p">(</span><span class="n">req</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">IronResult</span><span class="o"><</span><span class="n">Response</span><span class="o">></span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="s">"You found the server!"</span><span class="p">)))</span><span class="w"></span>
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<span class="p">}</span><span class="w"></span>
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</pre></div>
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<p>Doesn't look too bad right? In fact, it's essentially the same as the Python
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version! All we need to do is just send back a string of some form. So, we
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look up the documentation for <a href="http://ironframework.io/doc/iron/request/struct.Request.html"><code>Request</code></a> and see a <code>url</code> field that will contain
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what we want. Let's try the first iteration:</p>
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<div class="highlight"><pre><span class="k">fn</span><span class="w"> </span><span class="n">hello_world</span><span class="p">(</span><span class="n">req</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">IronResult</span><span class="o"><</span><span class="n">Response</span><span class="o">></span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="s">"You found the URL: "</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">req</span><span class="p">.</span><span class="n">url</span><span class="p">)))</span><span class="w"></span>
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<span class="p">}</span><span class="w"></span>
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</pre></div>
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<p>Which yields the error:</p>
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<div class="highlight"><pre>error[E0369]: binary operation `+` cannot be applied to type `&'static str`
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</pre></div>
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<p>OK, what's going on here? Time to start Googling for <a href="https://www.google.com/#q=concatenate+strings+in+rust">"concatenate strings in Rust"</a>. That's what we
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want to do right? Concatenate a static string and the URL.</p>
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<p>After Googling, we come across a helpful <a href="https://doc.rust-lang.org/std/macro.concat!.html"><code>concat!</code></a> macro that looks really nice! Let's try that one:</p>
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<div class="highlight"><pre><span class="k">fn</span><span class="w"> </span><span class="n">hello_world</span><span class="p">(</span><span class="n">req</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">IronResult</span><span class="o"><</span><span class="n">Response</span><span class="o">></span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="n">concat</span><span class="o">!</span><span class="p">(</span><span class="s">"You found the URL: "</span><span class="p">,</span><span class="w"> </span><span class="n">req</span><span class="p">.</span><span class="n">url</span><span class="p">))))</span><span class="w"></span>
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<span class="p">}</span><span class="w"></span>
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</pre></div>
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<p>And the error:</p>
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<p><code>error: expected a literal</code></p>
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<p>Turns out Rust actually blows up because the <code>concat!</code> macro expects us to know
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at compile time what <code>req.url</code> is. Which, in my outsider opinion, is a bit
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strange. <code>println!</code> and <code>format!</code>, etc., all handle values they don't know at
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compile time. Why can't <code>concat!</code>? By any means, we need a new plan of attack.
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How about we try formatting strings?</p>
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<div class="highlight"><pre><span class="k">fn</span><span class="w"> </span><span class="n">hello_world</span><span class="p">(</span><span class="n">req</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">IronResult</span><span class="o"><</span><span class="n">Response</span><span class="o">></span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="n">format</span><span class="o">!</span><span class="p">(</span><span class="s">"You found the URL: {}"</span><span class="p">,</span><span class="w"> </span><span class="n">req</span><span class="p">.</span><span class="n">url</span><span class="p">))))</span><span class="w"></span>
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<span class="p">}</span><span class="w"></span>
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</pre></div>
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<p>And at long last, it works. Onwards!</p>
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<h1>Issue 2: Fighting with the borrow checker</h1>
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<p>Rust's single coolest feature is how the compiler can guarantee safety in your
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program. As long as you don't use <code>unsafe</code> pointers in Rust, you're guaranteed
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safety. And not having truly manual memory management is really cool; I'm
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totally OK with never having to write <code>malloc()</code> again.</p>
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<p>That said, even <a href="https://doc.rust-lang.org/book/ownership.html">the Rust documentation</a> makes a specific note:</p>
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<blockquote>
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<p>Many new users to Rust experience something we like to call
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‘fighting with the borrow checker’, where the Rust compiler refuses to
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compile a program that the author thinks is valid.</p>
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</blockquote>
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<p>If you have to put it in the documentation, it's not a helpful note:
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it's hazing.</p>
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<p>So now that we have a handler which works with information from the request, we
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want to start making something that looks like an actual web application.
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The router provided by <code>iron</code> isn't terribly difficult so I won't cover it.
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Instead, the thing that had me stumped for a couple hours was trying to
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dynamically create routes.</p>
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<p>The unfortunate thing with Rust (in my limited experience at the moment) is that
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there is a severe lack of non-trivial examples. Using the router is easy when
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you want to give an example of a static function. But how do you you start
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working on things that are a bit more complex?</p>
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<p>We're going to cover that here. Our first try: creating a function which returns
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other functions. This is a principle called <a href="http://stackoverflow.com/a/36321/1454178">currying</a>. We set up a function that allows us to keep some data in scope
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for another function to come later.</p>
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<div class="highlight"><pre><span class="k">fn</span><span class="w"> </span><span class="n">build_handler</span><span class="p">(</span><span class="n">message</span><span class="o">:</span><span class="w"> </span><span class="nb">String</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="nb">Fn</span><span class="p">(</span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">IronResult</span><span class="o"><</span><span class="n">Response</span><span class="o">></span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="n">move</span><span class="w"> </span><span class="o">|</span><span class="n">_</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="o">|</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
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<span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="n">message</span><span class="p">)))</span><span class="w"></span>
|
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<span class="w"> </span><span class="p">}</span><span class="w"></span>
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<span class="p">}</span><span class="w"></span>
|
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</pre></div>
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<p>We've simply set up a function that returns another anonymous function with the
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<code>message</code> parameter scoped in. If you compile this, you get not 1, not 2, but 5
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new errors. 4 of them are the same though:</p>
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<div class="highlight"><pre>error[E0277]: the trait bound `for<'r, 'r, 'r> std::ops::Fn(&'r mut iron::Request<'r, 'r>) -> std::result::Result<iron::Response, iron::IronError> + 'static: std::marker::Sized` is not satisfied
|
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</pre></div>
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<p>...oookay. I for one, am not going to spend time trying to figure out what's
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going on there.</p>
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<p>And it is here that I will save the audience many hours of frustrated effort.
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At this point, I decided to switch from <code>iron</code> to pure <code>hyper</code> since using
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<code>hyper</code> would give me a much simpler API. All I would have to do is build a
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function that took two parameters as input, and we're done. That said, it
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ultimately posed many more issues because I started getting into a weird fight
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with the <code>'static</code> <a href="https://doc.rust-lang.org/book/lifetimes.html">lifetime</a>
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and being a Rust newbie I just gave up on trying to understand it.</p>
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<p>Instead, we will abandon (mostly) the curried function attempt, and instead
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take advantage of something Rust actually intends us to use: <code>struct</code> and
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<code>trait</code>.</p>
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<p>Remember when I talked about a lack of non-trivial examples on the Internet?
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This is what I was talking about. I could only find <em>one</em> example of this
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available online, and it was incredibly complex and contained code we honestly
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don't need or care about. There was no documentation of how to build routes that
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didn't use static functions, etc. But, I'm assuming you don't really care about
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my whining, so let's get to it.</p>
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<p>The <code>iron</code> documentation mentions the <a href="http://ironframework.io/doc/iron/middleware/trait.Handler.html"><code>Handler</code></a> trait as being something we can implement.
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Does the function signature for that <code>handle()</code> method look familiar? It's what
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we've been working with so far.</p>
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<p>The principle is that we need to define a new <code>struct</code> to hold our data, then
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implement that <code>handle()</code> method to return the result. Something that looks
|
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like this might do:</p>
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<div class="highlight"><pre><span class="k">struct</span><span class="w"> </span><span class="n">EchoHandler</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="n">message</span><span class="o">:</span><span class="w"> </span><span class="nb">String</span><span class="w"></span>
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<span class="p">}</span><span class="w"></span>
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<span class="k">impl</span><span class="w"> </span><span class="n">Handler</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="n">EchoHandler</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
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<span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">handle</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">,</span><span class="w"> </span><span class="n">_</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">IronResult</span><span class="o"><</span><span class="n">Response</span><span class="o">></span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">message</span><span class="p">)))</span><span class="w"></span>
|
||
<span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="p">}</span><span class="w"></span>
|
||
|
||
<span class="c1">// Later in the code when we set up the router...</span>
|
||
<span class="kd">let</span><span class="w"> </span><span class="n">echo</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">EchoHandler</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="n">message</span><span class="o">:</span><span class="w"> </span><span class="s">"Is it working yet?"</span><span class="w"></span>
|
||
<span class="p">}</span><span class="w"></span>
|
||
<span class="n">router</span><span class="p">.</span><span class="n">get</span><span class="p">(</span><span class="s">"/"</span><span class="p">,</span><span class="w"> </span><span class="n">echo</span><span class="p">.</span><span class="n">handle</span><span class="p">,</span><span class="w"> </span><span class="s">"index"</span><span class="p">);</span><span class="w"></span>
|
||
</pre></div>
|
||
|
||
|
||
<p>We attempt to build a struct, and give its <code>handle</code> method off to the router
|
||
so the router knows what to do.</p>
|
||
<p>You guessed it, more errors:</p>
|
||
<div class="highlight"><pre><span class="n">error</span><span class="o">:</span> <span class="n">attempted</span> <span class="n">to</span> <span class="n">take</span> <span class="n">value</span> <span class="n">of</span> <span class="n">method</span> <span class="err">`</span><span class="n">handle</span><span class="err">`</span> <span class="n">on</span> <span class="n">type</span> <span class="err">`</span><span class="n">EchoHandler</span><span class="err">`</span>
|
||
</pre></div>
|
||
|
||
|
||
<p>Now, the Rust compiler is actually a really nice fellow, and offers us help:</p>
|
||
<div class="highlight"><pre><span class="n">help</span><span class="o">:</span> <span class="n">maybe</span> <span class="n">a</span> <span class="err">`</span><span class="o">()</span><span class="err">`</span> <span class="n">to</span> <span class="n">call</span> <span class="n">it</span> <span class="k">is</span> <span class="n">missing</span><span class="o">?</span> <span class="n">If</span> <span class="n">not</span><span class="o">,</span> <span class="k">try</span> <span class="n">an</span> <span class="n">anonymous</span> <span class="kd">function</span>
|
||
</pre></div>
|
||
|
||
|
||
<p>We definitely don't want to call that function, so maybe try an anonymous
|
||
function as it recommends?</p>
|
||
<div class="highlight"><pre><span class="n">router</span><span class="p">.</span><span class="n">get</span><span class="p">(</span><span class="s">"/"</span><span class="p">,</span><span class="w"> </span><span class="o">|</span><span class="n">req</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="o">|</span><span class="w"> </span><span class="n">echo</span><span class="p">.</span><span class="n">handle</span><span class="p">(</span><span class="n">req</span><span class="p">),</span><span class="w"> </span><span class="s">"index"</span><span class="p">);</span><span class="w"></span>
|
||
</pre></div>
|
||
|
||
|
||
<p>Another error:</p>
|
||
<div class="highlight"><pre>error[E0373]: closure may outlive the current function, but it borrows `echo`, which is owned by the current function
|
||
</pre></div>
|
||
|
||
|
||
<p>Another helpful message:</p>
|
||
<div class="highlight"><pre><span class="n">help</span><span class="o">:</span> <span class="n">to</span> <span class="n">force</span> <span class="n">the</span> <span class="n">closure</span> <span class="n">to</span> <span class="n">take</span> <span class="n">ownership</span> <span class="n">of</span> <span class="err">`</span><span class="n">echo</span><span class="err">`</span> <span class="o">(</span><span class="n">and</span> <span class="n">any</span> <span class="n">other</span> <span class="n">referenced</span> <span class="n">variables</span><span class="o">),</span> <span class="n">use</span> <span class="n">the</span> <span class="err">`</span><span class="n">move</span><span class="err">`</span> <span class="n">keyword</span>
|
||
</pre></div>
|
||
|
||
|
||
<p>We're getting closer though! Let's implement this change:</p>
|
||
<div class="highlight"><pre><span class="n">router</span><span class="p">.</span><span class="n">get</span><span class="p">(</span><span class="s">"/"</span><span class="p">,</span><span class="w"> </span><span class="n">move</span><span class="w"> </span><span class="o">|</span><span class="n">req</span><span class="o">:</span><span class="w"> </span><span class="o">&</span><span class="k">mut</span><span class="w"> </span><span class="n">Request</span><span class="o">|</span><span class="w"> </span><span class="n">echo</span><span class="p">.</span><span class="n">handle</span><span class="p">(</span><span class="n">req</span><span class="p">),</span><span class="w"> </span><span class="s">"index"</span><span class="p">);</span><span class="w"></span>
|
||
</pre></div>
|
||
|
||
|
||
<p>And here's where things get strange:</p>
|
||
<div class="highlight"><pre>error[E0507]: cannot move out of borrowed content
|
||
--> src/main.rs:18:40
|
||
|
|
||
18 | Ok(Response::with((status::Ok, self.message)))
|
||
| ^^^^ cannot move out of borrowed content
|
||
</pre></div>
|
||
|
||
|
||
<p>Now, this took me another couple hours to figure out. I'm going to explain it,
|
||
but <strong>keep this in mind: Rust only allows one reference at a time</strong> (exceptions
|
||
apply of course).</p>
|
||
<p>When we attempt to use <code>self.message</code> as it has been created in the earlier
|
||
<code>struct</code>, we essentially are trying to give it away to another piece of code.
|
||
Rust's semantics then state that <em>we may no longer access it</em> unless it is
|
||
returned to us (which <code>iron</code>'s code does not do). There are two ways to fix
|
||
this:</p>
|
||
<ol>
|
||
<li>Only give away references (i.e. <code>&self.message</code> instead of <code>self.message</code>)
|
||
instead of transferring ownership</li>
|
||
<li>Make a copy of the underlying value which will be safe to give away</li>
|
||
</ol>
|
||
<p>I didn't know these were the two options originally, so I hope this helps the
|
||
audience out. Because <code>iron</code> won't accept a reference, we are forced into the
|
||
second option: making a copy. To do so, we just need to change the function
|
||
to look like this:</p>
|
||
<div class="highlight"><pre><span class="nb">Ok</span><span class="p">(</span><span class="n">Response</span><span class="o">::</span><span class="n">with</span><span class="p">((</span><span class="n">status</span><span class="o">::</span><span class="nb">Ok</span><span class="p">,</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">message</span><span class="p">.</span><span class="n">clone</span><span class="p">())))</span><span class="w"></span>
|
||
</pre></div>
|
||
|
||
|
||
<p>Not so bad, huh? My only complaint is that it took so long to figure out exactly
|
||
what was going on.</p>
|
||
<p>And now we have a small server that we can configure dynamically. At long last.</p>
|
||
<blockquote>
|
||
<p>Final sidenote: You can actually do this without anonymous functions. Just
|
||
change the router line to:
|
||
<code>router.get("/", echo, "index");</code></p>
|
||
<p>Rust's type system seems to figure out that we want to use the <code>handle()</code> method.</p>
|
||
</blockquote>
|
||
<h1>Conclusion</h1>
|
||
<p>After a good long days' work, we now have the routing functionality set up on
|
||
our application. We should be able to scale this pretty well in the future:
|
||
the RSS content we need to deliver in the future can be treated as a string, so
|
||
the building blocks are in place.</p>
|
||
<p>There are two important things I learned starting with Rust today:</p>
|
||
<ol>
|
||
<li>Rust is a new language, and while the code is high-quality, the mindshare is coming.</li>
|
||
<li>I'm a terrible programmer.</li>
|
||
</ol>
|
||
<p>Number 1 is pretty obvious and not surprising to anyone. Number two caught me
|
||
off guard. I've gotten used to having either a garbage collector (Java, Python,
|
||
etc.) or playing a little fast and loose with scoping rules (C, C++). You don't
|
||
have to worry about object lifetime there. With Rust, it's forcing me to fully
|
||
understand and use well the memory in my applications. In the final mistake I
|
||
fixed (using <code>.clone()</code>) I would have been fine in C++ to just give away that
|
||
reference and never use it again. I wouldn't have run into a "use-after-free"
|
||
error, but I would have potentially been leaking memory. Rust forced me to be
|
||
incredibly precise about how I use it.</p>
|
||
<p>All said I'm excited for using Rust more. I think it's super cool, it's just
|
||
going to take me a lot longer to do this than I originally thought.</p>
|
||
|
||
|
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