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Learning Rust 01: a pwd clone

23 June 2018

I’m learning Rust, and documenting the process. These posts are my notes - they’re not guaranteed to be useful, interesting or correct!

In this post, I’m writing my first Rust program. It’s a simplified clone of the Unix tool pwd, which prints out the current directory. I’m calling it rpwd.

Creating a project

I started a new executable application with:

$ cargo rpwd --bin

Code

The code is simple:

use std::env;

fn main() {
    let path = env::current_dir().unwrap();
    println!("{}", path.display());
}

Importing a module

The first line imports the env module from the standard library, which implements functions to inspect and manipulate the process’s environment.

Variable declaration

let path declares a new variable, path. Variables are immutable in Rust. This variable is set to the result of the function call env::current_dir().unwrap().

Current directory, Results

env::current_dir() calls the current_dir() function in the std::env module. current_dir returns a std::io::Result<PathBuf>. A Result is a type which represents a success or an error. To get at the success value, we must deal with any potential errors. In Rust, ‘functions return Result whenever errors are expected and recoverable’. It’s a way to ensure errors are dealt with.

Unwrap

.unwrap() calls Result’s unwrap method, which returns the success value if the Result is Ok, or panics if there’s an error. According to env::current_dir’s docs, the function can error if:

Printing

Assuming there are no errors, path will hold current_dir’s success value, an object of type std::path::PathBuf. This object represents an owned, mutable path.

Finally, we print out the path. This uses the println! macro. Before printing the path, we must call PathBuf’s display method. This converts the path into a format printable by println!.