The Detroit Post
Saturday, 16 October, 2021

Rust Where Loop

David Lawrence
• Saturday, 26 December, 2020
• 13 min read

There may be instances, where a block of code needs to be executed repeatedly. Programming languages provide various control structures that allow for more complicated execution paths.

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Given below is the general form of a loop statement in most of the programming languages. The for loop executes the code block for a specified number of times.

It can be used to iterate over a fixed set of values, such as an array. Sr. NoName & Description1The while loop executes the instructions each time the condition specified evaluates to true.

The break statement is used to take the control out of a construct. Unlike the break statement, the continue does not exit the loop.

The loop exits the current iteration if the number is even. Loops in Rust aren’t the same as standard C-style languages.

The syntax is different and there are some powerful options that make looping easier. In my opinion, all languages should move to a single syntax with for-loops based on iterators.

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The simplicity makes Rust ’s loops easy to read, while the ability to create custom iterators makes it more powerful than even more verbose formats like Go’s: By using the label ‘outer we are able to control explicitly which loop is broken.

Here's a table of contents of what you'll learn in this lesson:(click on a link to skip to its section) Instead of rewriting these sections of code, we can place them in a loop and allow Rust to automatically execute them as many times as we need.

The compiler will evaluate the condition and based on its results, execute the code. Then, the compiler will start again at the top of the loop code and evaluate the condition again.

This cycle of iterations continue until the condition is false, or we manually stop the loop. The while loop will continue to execute code while its condition remains true.

Once the condition proves false, the while loop will stop and the compiler will move on to any code below it. First, we set up a mutable variable called counter.

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Next, we specify our condition that while the counter variable value is less than 10, the loop should iterate. Inside the while code block, we print out the counter number to the console.

Lastly, we add 1 to our counter variable to indicate that the loop iterated once. When we run the example, the output shows a list of numbers from 1 to 10.

In the code block we printed the counter number at that stage. Because the condition proved true, the compiler will go back to the top of the while loop and evaluate it again.

This made the evaluation prove false so the compiler stopped the loop. This is known as an infinite loop and it will significantly slow down the system until the application crashes.

To stop the application, right-click in the Console tab in Eclipse and choose To terminate/Disconnect all. The temp variable will then reference the individual element in that collection or array.

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In the example above, we use the .inter() function to iterate over elements in the employees array. Loops allow us to iterate over sections of our code multiple times.

I hope you have already been through the following posts in order to learn control flow and iteration in Rust : Basically, Rust is no different compared to other programming languages in the context of loops and iteration.

Although this is not the preferred method, as it may lead to an infinite execution problem, the second kind of loop is while, which requires a condition: The best and safest approach is to use for, as it assures us of the index limit, while dealing with while can create panic situations if the condition(s) isn't given properly; and loop needs even more careful handling as it can lead to infinite execution.

The Rust compiler just saved me from a nasty threading bug. I was working on cage (our open source development tool for Docker apps with lots of microservices), and I decided to parallelize the routine that transformed docker-compose.yml files.

This was mostly an excuse to check out the awesome rayon library, but it turned into a great example of what real-world Rust development is like. The key bit to take away here is that Sync is not satisfied” for the trait plugins::PluginTransform.

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In Rust, Sync is a special trait that tells the compiler that it's safe to share an object between two threads. That whole long list of “notes” afterwards looks intimidating, but it's really just telling me that my PluginTransform objects live inside my plugins::Manager type, which in turn lives inside my Project.

Re-running cargo test, however, reveals the actual bug, and it would have been a nightmare to debug: We use the Mocktail to simulate a connection to Hashimoto's Vault, a secure central storage for passwords and other secrets, which issues time-limited credentials.

But in the test code, we need to create some “interior” mutable state. But RC> is a lightweight mechanism designed for single-threaded code.

The fix is to replace RC> with Arc>, which is fully thread-safe: I don't even think our current test suites could trigger this bug, but I bet a future version would have been able to, leaving a nasty surprise for us someday.

I've filed an issue suggesting that rayon should provide those last two lines as a built-in function. But this is what day-to-day Rust development is like: I have to do a bit of extra work to satisfy the compiler (which mostly becomes a reflex).

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But in turn, the compiler ferrets out all kinds of subtle concurrency errors and generally watches my back. Underwater Diving Sites spawn in the ocean naturally.

When untied, they will float up to the surface, where they can be opened and looted. Iteration with in, trait implementation with imply, or (for<'a>).

For-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common practice within Rust, which is to loop over anything that implements Incinerator until the iterator returned by .into_iter() returns None (or the loop body uses break). As shown in the example above, for loops (along with all other loops) can be tagged, using similar syntax to lifetimes (only visually similar, entirely distinct in practice).

Giving the same tag to break breaks the tagged loop, which is useful for inner loops. More details on the functionality shown can be seen at the Incinerator docs.

When continue is encountered, the current iteration is terminated, returning control to the loop head, typically continuing with the next iteration. Making statements based on opinion; back them up with references or personal experience.

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So, you want to get a native Rust application running on the web with input and rendering? This post will go through the steps needed to achieve that by using Scripted to build for the wasm32-unknown-emscripten target.

For the purpose of this guide, I will assume that you already have an application running on desktop that uses SDL2 with OpenGL and I will not go through the code needed to get to that point. As a bonus, your code will still work for desktop and choosing your build target will be just a matter of switching the --target= flag.

The articles/tutorials I’ve found are often out of date to the point where you run into obscure errors that require in-depth knowledge to even know where things went wrong. My goal is to turn this post into a resource that is kept up to date as the ecosystem matures to always work as a step-by-step guide for the experienced and newcomers alike.

There is also an example project accompanied by this article which will serve as a fully working code example. This GitHub repository contains the example project and also serves as a place to raise issues if this article goes out of date, to suggest improvements or simply just to ask questions.

As a token of how up to date this article is, I will leave a time stamp below on when it was last known to work using the 3 major platforms. Plain CargoCargo Web Windows 8/10/20208/10/2020 macOS UntestedUntested Linux 8/10/2020Broken: #2 I aim to have these dates be no older than 6 months.

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Since this post is primarily about getting things up and running on the web, I won’t go into what SDL2 nor OpenGL are. Well, we can run Rust on the web without Emcsripten by using that target, and in fact, some people prefer that route.

Myself, I prefer to use Scripted because it comes with a lot of functionality that helps bridge the gap between desktop and the web platform. If we go by the analogy that our Wasm32 compiled program is the machine code, and the browser environment is the hardware to execute it, then Scripted could kind of be viewed as the operating system that provides the system features we are used to having out of the box.

If you went by plain wasm32-unknown-unknown then a lot of things we take for granted, like being able to use std::FS to load files would be missing. I’ll go into these features in more depth later in the guide but for now let’s list some functionality we get by using Scripted.

I’m including cargo web in this article since it’s a nice thing to have, but I’ll also show you how to go without it since it sadly hasn’t been maintained for over a year which makes it a risky tool to rely on. The first thing you need to do is to download and set up the Scripted SDK which is pretty straightforward.

I won’t go through any platform specific issues you might run into (again, refer to the official docs) but in short, the process will be something like the following. That last command will set up your terminal environment so that it is all ready to use with Scripted, and this step is the only one you’ll need to repeat for your next coding session.

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The above compilation does not provide us with an index.html, so we don’t actually have a way to load and run the application as a web page. This means that when you create an SDL2 window, Scripted will create an HTML canvas on the website that represents the SDL2 window as well as grab any user input like mouse/keyboard and route those to your application in the form of normal SDL2 events.

As you can imagine, this is incredibly helpful when we want to be able to easily build for both desktop and web since we basically don’t have to do anything at all to get windowing and input to work. The first two sections specifies the default target to use when invoking builds from cargo web, as well as deciding what the minimum version requirement is.

The last part is the relevant one as it lets us list the linker flags to pass on when Scripted is targeted. The process of supplying the flags is similar without cargo web, we just put it in a different config file.

You will still need to use an OpenGL version that is very close to the WebGL equivalent, otherwise Scripted will not manage to bridge the gap. In the case that you are using a higher OpenGL version than OpenGL 4.3, then you will need to either rewrite the code to target the lower version, or write alternative fallback code that is used when you run on the web.

That way we just tell Scripted to create a WebGL 2.0 context, and we can adhere to what is available in that GL version (i.e. compatible with 4.3 Core and OpenGL ES 3) and it will just work. This is mainly regarding the client-side array rendering methods (e.g. glDrawArrays from client data and the like).

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Usually you won’t need to do this unless your application uses those particular OpenGL features and you don’t want to use WebGL 2.0. Further, Options There are some further capabilities like limited emulation of old-style OpenGL 1.x code and other things but that is out of scope for this article.

Many applications need to load files from disk, like sprite sheets, tile maps, models and so on. On desktop, you can do that by just dumping the files in a folder next to the executable, but this proves difficult when running through the browser.

In practical terms, the only thing you need to do to be able to load your files is to use yet another Scripted flag! At this point you might be able to successfully compile and run your program on the web without errors in the console but if you’re making a game or similar you’ll probably be faced with a frozen tab that is eventually killed by the browser.

Firmly, this means that you cannot have your code arranged in a loop statement anymore. Furthermore, invoking this function might lead to unintuitive behavior that depends on the last parameter passed.

If you pass false, then execution will continue straight away past the function call in a non-blocking manner, and potentially run to the end of the scope and drop local variables and so on. If you instead pass true, the Scripted will abruptly terminate the current code-path execution without dropping any local variables.


Since this is an isolated but common thing for Scripted builds, I turned it into a crate called emscripten_main_ loop that you can use. For the sake of completion I’ll show you how to use it below, but if you prefer to see it in action you can just head over to the example code repository.

This trait defines a data object capable of running a main loop. Usually if you previously had your code in a while(!quit){...} or similar, you can just chuck all the contents of that loop block into this function.

In other words, if you implemented the Maintop trait for a Game struct, you would then call this run function and pass that game struct into it, which would start the looping and run it over and over until Quit is returned from the main_ loop function. Internally, this is implemented as a standard while(!quit){...} on desktop and for web builds it will capture the struct and store it in global memory while scheduling Emcsripten to run the loop.

Hopefully those steps will help you get past the most common hurdles and give you something up and running. There is however a lot more that can go wrong than what this post could ever cover and debugging these things can be difficult.

Luckily, the Scripted documentation is wonderful and the folks involved are also very friendly and helpful. I want this post to be as useful as possible so if you feel that something important is missing, or have ideas on how things could be improved I’m more than happy for you to either leave a comment here or head over to the repository to submit an issue.

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As a bonus, I’ll include a few considerations for other topics that might be relevant to some of your projects. Note that I’ll only touch briefly on these and if you want more in-depth information you should check out the official Scripted docs.

To get the kind of multi threading we are used to in desktop applications, we need shared memory between the threads. The kind of memory that is usually protected with atomic/mutates and similar control structures.

This means that your multi threaded application won’t work unless the end user explicitly enables it in their browsers. Multi threading with non-shared memory is supported using web workers and Scripted does have an API to interact with them but you can’t expect them to be a drop-in replacement to standard threading code since you cannot share any memory between the web workers or the main thread.

All in all, unless you specifically really need your code multi threaded, you’re probably better off making your web version single threaded until all browsers support shared memory out of the box.

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