https://www.snoyman.com/blog/2018/11/rust-crash-course-03-iterators-and-errors

I also blog frequently on the Yesod Web Framework blog, as well as the FP Complete blog.

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Last time, we finished off with a bouncy ball implementation with some downsides: lackluster error handling and ugly buffering. It also had another limitation: a static frame size. Today, we’re going to address all of these problems, starting with that last one: let’s get some command line arguments to control the frame size.

This post is part of a series based on teaching Rust at FP Complete. If you’re reading this post outside of the blog, you can find links to all posts in the series at the top of the introduction post. You can also subscribe to the RSS feed.

Like last time, I’m going to expect you, the reader, to be making changes to the source code along with me. Make sure to actually type in the code while reading!

Command line arguments

We’re going to modify our application as follows:

• Accept two command line arguments: the width and the height
• Both must be valid u32s
• Too many or too few command line arguments will result in an error message

Sounds easy enough. In a real application, we would use a proper argument-handling library, like clap. But for now, we’re going lower level. Like we did for the sleep function, let’s start by searching the standard library docs for the word args. The first two entries both look relevant.

• std::env::Args An iterator over the arguments of a process, yielding a String value for each argument.
• std::env::args Returns the arguments which this program was started with (normally passed via the command line).

Now’s a good time to mention that, by strong convention:

• Module names (like std and env) and function names (like args) are snake_cased
• Types (like Args) are PascalCased
  • Exception: primitives like u32 and str are lower case

The std module has an env module. The env module has both an Args type and a args function. Why do we need both? Even more strangely, let’s look at the type signature for the args function:

pub fn args() -> Args