The Rust Programming Language - Understanding Functions in Rust

The Rust Programming Language - Understanding Functions in Rust

The Rust Programming Language - Understanding Functions in Rust - Functions - Functions are the building blocks of readable, maintainable, and reusable code. Every Rust program has at least one function.

Every Rust program has at least one function, the main function:

fn main() {
}

This is the simplest possible function declaration. As we mentioned before, fn says ‘this is a function’, followed by the name, some parentheses because this function takes no arguments, and then some curly braces to indicate the body. Here’s a function named foo:


# #![allow(unused_variables)]
#fn main() {
fn foo() {
}
#}

So, what about taking arguments? Here’s a function that prints a number:


# #![allow(unused_variables)]
#fn main() {
fn print_number(x: i32) {
    println!("x is: {}", x);
}
#}

Here’s a complete program that uses print_number:

fn main() {
    print_number(5);
}

fn print_number(x: i32) {
    println!("x is: {}", x);
}

As you can see, function arguments work very similar to let declarations: you add a type to the argument name, after a colon.

Here’s a complete program that adds two numbers together and prints them:

fn main() {
    print_sum(5, 6);
}

fn print_sum(x: i32, y: i32) {
    println!("sum is: {}", x + y);
}

You separate arguments with a comma, both when you call the function, as well as when you declare it.

Unlike let, you must declare the types of function arguments. This does not work:

fn print_sum(x, y) {
    println!("sum is: {}", x + y);
}

You get this error:

expected one of `!`, `:`, or `@`, found `)`
fn print_sum(x, y) {

This is a deliberate design decision. While full-program inference is possible, languages which have it, like Haskell, often suggest that documenting your types explicitly is a best-practice. We agree that forcing functions to declare types while allowing for inference inside of function bodies is a wonderful sweet spot between full inference and no inference.

What about returning a value? Here’s a function that adds one to an integer:


# #![allow(unused_variables)]
#fn main() {
fn add_one(x: i32) -> i32 {
    x + 1
}
#}

Rust functions return exactly one value, and you declare the type after an ‘arrow’, which is a dash (-) followed by a greater-than sign (>). The last line of a function determines what it returns. You’ll note the lack of a semicolon here. If we added it in:

fn add_one(x: i32) -> i32 {
    x + 1;
}

We would get an error:

error: not all control paths return a value
fn add_one(x: i32) -> i32 {
     x + 1;
}

help: consider removing this semicolon:
     x + 1;
          ^

This reveals two interesting things about Rust: it is an expression-based language, and semicolons are different from semicolons in other ‘curly brace and semicolon’-based languages. These two things are related.

Expressions vs. Statements

Rust is primarily an expression-based language. There are only two kinds of statements, and everything else is an expression.

So what's the difference? Expressions return a value, and statements do not. That’s why we end up with ‘not all control paths return a value’ here: the statement x + 1; doesn’t return a value. There are two kinds of statements in Rust: ‘declaration statements’ and ‘expression statements’. Everything else is an expression. Let’s talk about declaration statements first.

In some languages, variable bindings can be written as expressions, not statements. Like Ruby:

x = y = 5

In Rust, however, using let to introduce a binding is not an expression. The following will produce a compile-time error:

let x = (let y = 5); // Expected identifier, found keyword `let`.

The compiler is telling us here that it was expecting to see the beginning of an expression, and a let can only begin a statement, not an expression.

Note that assigning to an already-bound variable (e.g. y = 5) is still an expression, although its value is not particularly useful. Unlike other languages where an assignment evaluates to the assigned value (e.g. 5 in the previous example), in Rust the value of an assignment is an empty tuple () because the assigned value can have only one owner, and any other returned value would be too surprising:


# #![allow(unused_variables)]
#fn main() {
let mut y = 5;

let x = (y = 6);  // `x` has the value `()`, not `6`.
#}

The second kind of statement in Rust is the expression statement. Its purpose is to turn any expression into a statement. In practical terms, Rust's grammar expects statements to follow other statements. This means that you use semicolons to separate expressions from each other. This means that Rust looks a lot like most other languages that require you to use semicolons at the end of every line, and you will see semicolons at the end of almost every line of Rust code you see.

What is this exception that makes us say "almost"? You saw it already, in this code:


# #![allow(unused_variables)]
#fn main() {
fn add_one(x: i32) -> i32 {
    x + 1
}
#}

Our function claims to return an i32, but with a semicolon, it would return () instead. Rust realizes this probably isn’t what we want, and suggests removing the semicolon in the error we saw before.

Early returns

But what about early returns? Rust does have a keyword for that, return:


# #![allow(unused_variables)]
#fn main() {
fn foo(x: i32) -> i32 {
    return x;

    // We never run this code!
    x + 1
}
#}

Using a return as the last line of a function works, but is considered poor style:


# #![allow(unused_variables)]
#fn main() {
fn foo(x: i32) -> i32 {
    return x + 1;
}
#}

The previous definition without return may look a bit strange if you haven’t worked in an expression-based language before, but it becomes intuitive over time.

Diverging functions

Rust has some special syntax for ‘diverging functions’, which are functions that do not return:


# #![allow(unused_variables)]
#fn main() {
fn diverges() -> ! {
    panic!("This function never returns!");
}
#}

panic! is a macro, similar to println!() that we’ve already seen. Unlike println!(), panic!() causes the current thread of execution to crash with the given message. Because this function will cause a crash, it will never return, and so it has the type ‘!’, which is read ‘diverges’.

If you add a main function that calls diverges() and run it, you’ll get some output that looks like this:

thread ‘main’ panicked at ‘This function never returns!’, hello.rs:2

If you want more information, you can get a backtrace by setting the RUST_BACKTRACE environment variable:

$ RUST_BACKTRACE=1 ./diverges
thread 'main' panicked at 'This function never returns!', hello.rs:2
Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
  hello::diverges
        at ./hello.rs:2
  hello::main
        at ./hello.rs:6

If you want the complete backtrace and filenames:

$ RUST_BACKTRACE=full ./diverges
thread 'main' panicked at 'This function never returns!', hello.rs:2
stack backtrace:
   1:     0x7f402773a829 - sys::backtrace::write::h0942de78b6c02817K8r
   2:     0x7f402773d7fc - panicking::on_panic::h3f23f9d0b5f4c91bu9w
   3:     0x7f402773960e - rt::unwind::begin_unwind_inner::h2844b8c5e81e79558Bw
   4:     0x7f4027738893 - rt::unwind::begin_unwind::h4375279447423903650
   5:     0x7f4027738809 - diverges::h2266b4c4b850236beaa
   6:     0x7f40277389e5 - main::h19bb1149c2f00ecfBaa
   7:     0x7f402773f514 - rt::unwind::try::try_fn::h13186883479104382231
   8:     0x7f402773d1d8 - __rust_try
   9:     0x7f402773f201 - rt::lang_start::ha172a3ce74bb453aK5w
  10:     0x7f4027738a19 - main
  11:     0x7f402694ab44 - __libc_start_main
  12:     0x7f40277386c8 - <unknown>
  13:                0x0 - <unknown>

If you need to override an already set RUST_BACKTRACE, in cases when you cannot just unset the variable, then set it to 0 to avoid getting a backtrace. Any other value (even no value at all) turns on backtrace.

$ export RUST_BACKTRACE=1
...
$ RUST_BACKTRACE=0 ./diverges 
thread 'main' panicked at 'This function never returns!', hello.rs:2
note: Run with `RUST_BACKTRACE=1` for a backtrace.

RUST_BACKTRACE also works with Cargo’s run command:

$ RUST_BACKTRACE=full cargo run
     Running `target/debug/diverges`
thread 'main' panicked at 'This function never returns!', hello.rs:2
stack backtrace:
   1:     0x7f402773a829 - sys::backtrace::write::h0942de78b6c02817K8r
   2:     0x7f402773d7fc - panicking::on_panic::h3f23f9d0b5f4c91bu9w
   3:     0x7f402773960e - rt::unwind::begin_unwind_inner::h2844b8c5e81e79558Bw
   4:     0x7f4027738893 - rt::unwind::begin_unwind::h4375279447423903650
   5:     0x7f4027738809 - diverges::h2266b4c4b850236beaa
   6:     0x7f40277389e5 - main::h19bb1149c2f00ecfBaa
   7:     0x7f402773f514 - rt::unwind::try::try_fn::h13186883479104382231
   8:     0x7f402773d1d8 - __rust_try
   9:     0x7f402773f201 - rt::lang_start::ha172a3ce74bb453aK5w
  10:     0x7f4027738a19 - main
  11:     0x7f402694ab44 - __libc_start_main
  12:     0x7f40277386c8 - <unknown>
  13:                0x0 - <unknown>

A diverging function can be used as any type:


# #![allow(unused_variables)]
#fn main() {
# fn diverges() -> ! {
#    panic!("This function never returns!");
# }
let x: i32 = diverges();
let x: String = diverges();
#}
Function pointers

We can also create variable bindings which point to functions:


# #![allow(unused_variables)]
#fn main() {
let f: fn(i32) -> i32;
#}

f is a variable binding which points to a function that takes an i32 as an argument and returns an i32. For example:


# #![allow(unused_variables)]
#fn main() {
fn plus_one(i: i32) -> i32 {
    i + 1
}

// Without type inference:
let f: fn(i32) -> i32 = plus_one;

// With type inference:
let f = plus_one;
#}

We can then use f to call the function:


# #![allow(unused_variables)]
#fn main() {
# fn plus_one(i: i32) -> i32 { i + 1 }
# let f = plus_one;
let six = f(5);
#}

Rust & WebAssembly para JavaScripters

Rust & WebAssembly para JavaScripters

A lo largo de la charla descubriremos las características más destacables de Rust, sus similitudes y diferencias con JavaScript y veremos qué aporta Rust al futuro de la Web gracias a WebAssembly. Rust es un lenguaje tipado, rápido y seguro, que ha sido diseñado por Mozilla como lenguaje de sistemas, aunque en los últimos tiempos ha ganado mucha popularidad en el terreno del desarrollo Web gracias a WebAssembly, su amplio ecosistema y gran comunidad

Rust es un lenguaje tipado, rápido y seguro, que ha sido diseñado por Mozilla como lenguaje de sistemas, aunque en los últimos tiempos ha ganado mucha popularidad en el terreno del desarrollo Web gracias a WebAssembly, su amplio ecosistema y gran comunidad. A lo largo de la charla descubriremos las características más destacables de Rust, sus similitudes y diferencias con JavaScript y veremos qué aporta Rust al futuro de la Web gracias a WebAssembly.

Rust vs. Go: Should I Rust, or Should I Go

Rust vs. Go: Should I Rust, or Should I Go

Well both Rust and Go provide amazing performance. Should you write you’re next big thing with Rust or with Go? Go is fast and powerful, but it avoids bogging the developer down, focusing instead on simplicity and uniformity. Rust. If on the other hand, wringing out every last ounce of performance is a necessity, then Rust should be your choice. Rust is more of a competitor to C++ than it is with Go.

Should I stay, or should I go?” Great song by the band The Clash. I’m listening to it, right now, while I’m writing this article. The song debuted back in 1982, a long time ago. Back then, I was just a kid exploring a new hobby — programming my Atari 2600. The first video game I ever wrote was written using 6502 Assembly for that console. The compiler for it cost about $65, if I recall, which at the time equated to mowing ~13 or so lawns.

The game was simple: using the joystick, maneuver your spaceship through a randomly generated scrolling cave. The cave walls were sinusoidal, scrolling vertically on the left and right sides of the screen, and you had to make sure your craft didn’t crash into them. I know, I know: Not that sophisticated. But I was only ten or eleven years old at the time.

Despite the “power” of the processor, computing sine values at run-time was simply too much for it. So, using my handy Texas Instruments calculator, I pre-calculated a bunch of the sine values, carefully writing them down on paper, and then entering them in as constants for the game. This greatly enhanced the performance of the game, and made it usable.

So what’s my point? What’s any of this got to do with Rust or Go?

Today’s languages are far more advanced than 6502 Assembly, which make it easier to write complex programs. It took a lot of my time to write that game, and I could do it much faster today, with less code than I did back then. But which language today provides that magic combination of simplicity and power?

Well both Rust and Go provide amazing performance. They both compile to machine code, the Holy Grail of performance. And with today’s processing power, developers can do amazing things with either of these languages. So the question is: Should you write you’re next big thing with Rust or with Go?

With a quick search, you can easily find several articles that go into detail about the differences between the two languages. But the focus of this article is the bang for the buck, that magic combination of performance per line of code.

To put it another way, where is that sweet spot of simple code and top-end performance? And in this case, is it Rust, or is it Go?
There really isn’t any argument: Rust is faster than Go. In the benchmarks above, Rust was faster, and in some cases, an order of magnitude faster.

But before you run off choosing to write everything in Rust, consider that Go wasn’t that far behind it in many of those benchmarks, and it’s still much faster than the likes of Java, C#, JavaScript, Python and so on. So in other words, it’s almost a wash between Rust and Go on the axis of performance. Now, if what you’re building needs to wring out every last ounce of performance, then by all means, choose Rust. But if what you need is top-of-the-line performance, then you’ll be ahead of the game choosing either of these two languages.

So then we’re down to the complexity of the code. This is where things can be muddy since this can be more subjective than performance benchmarks. Let’s look at a simple exercise: building a small web server that prints out “Hello World” when it receives an HTTP request. To do this in Rust, it looks something like this:

use std::net::{TcpStream, TcpListener};
use std::io::{Read, Write};
use std::thread;


fn handle_read(mut stream: &TcpStream) {
    let mut buf = [0u8; 4096];
    match stream.read(&mut buf) {
        Ok(_) => {
            let req_str = String::from_utf8_lossy(&buf);
            println!("{}", req_str);
            },
        Err(e) => println!("Unable to read stream: {}", e),
    }
}

fn handle_write(mut stream: TcpStream) {
    let response = b"HTTP/1.1 200 OK\r\nContent-Type: text/html; charset=UTF-8\r\n\r\n<html><body>Hello world</body></html>\r\n";
    match stream.write(response) {
        Ok(n) => println!("Response sent: {} bytes", n),
        Err(e) => println!("Failed sending response: {}", e),
    }
}

fn handle_client(stream: TcpStream) {
    handle_read(&stream);
    handle_write(stream);
}

fn main() {
    let port = "8080";
    let listener = TcpListener::bind(format!("127.0.0.1:{}", port)).unwrap();
    println!("Listening for connections on port {}", port);

    for stream in listener.incoming() {
        match stream {
            Ok(stream) => {
                thread::spawn(|| {
                    handle_client(stream)
                });
            }
            Err(e) => {
                println!("Unable to connect: {}", e);
            }
        }
    }
}

Something pretty similar in Go looks like this:

package main

import (
	"fmt"
	"io"
	"log"
	"net/http"
)

type handler struct{}

func (theHandler *handler) ServeHTTP(writer http.ResponseWriter, request *http.Request) {
	log.Printf("Received request: %s\n", request.URL)
	log.Printf("%v\n", request)
	io.WriteString(writer, "Hello world!")
}

const port = "8080"

func main() {
	server := http.Server{
		Addr:    fmt.Sprintf(":%s", port),
		Handler: &handler{},
	}

	server.ListenAndServe()
}

Now, they are not 100% exactly the same, but they are close enough. The difference between them is ~20 lines of code. Rust definitely forces the developer to consider more, and thus write more code than Go.

Another example: Consider one of the more difficult aspects of software development: multi-threading. When tackling something like this, as you undoubtedly would when building an HTTP server, there’s a lot to think about:

  • You need to ensure everything you design is thread safe (locks)
  • You need to handle communication between threads (channels)
  • You have to design with concurrency and parallelism in mind (threads and routines)

Both Rust and Go handle these hurdles really efficiently, but Go requires less effort. With Rust, you have way more options, and thus more power, when spawning threads. Just look at some of the documentation on this. Here’s just one way to spawn a thread in Rust:

use std::thread;

let handler = thread::spawn(|| {
    // thread code
});

handler.join().unwrap();

On the other hand, here’s how to create something similar using Go:

go someFunction(args)

Another crucial part of writing code is handling errors. Here I think Rust and Go are quite similar. Rust enables the developer to handle errors cases through the use of the enum return types: Option<T>and Result<T, E>. The Option<T> will return either None or Some(T) whereas Result<T, E> will return either Ok(T) or Err(T). Given that most of Rust’s own libraries, as well as other third-party libraries, return one of these types, the developer will usually have to handle the case where nothing is returned, or where an error is returned.

Here’s a simple example of the Result type being returned by a function in Rust:

fn foo_divide(a: f32, b: f32) -> Result<f32, &'static str> {
    if b == 0.0 {
        Err("divide by zero error!")
    } else {
        Ok(a / b)
    }
}fn main() {
    match foo_divide(5.0, 4.0) {
        Err(err) => println!("{}", err),
        Ok(result) => println!("5 / 4 = {}", result),
    }
}

Notice that the Err case must be handled within the match statement.

Go, on the other hand, leaves this more up to the developer, since errors can be ignored using the _. However, idiomatic Go strongly recommends returning an error, especially since functions in Go can return multiple values. Therefore, it’s easy to have functions return their intended value along with an error, if there is one.

Here is the corresponding example from above done in Go:

func fooDivide(a float32, b float32) (float32, error) {
    if b == 0 {
        return 0, errors.New("divide by zero error!")
    }    return a / b, nil
}func main() {
    result, err := fooDivide(5, 4)
    if err != nil {
       log.Printf("an error occurred: %v", err)
    } else {
       log.Printf("The answer is: 5 / 4 = %f", result)
    }
}

Notice that this line:

result, err := fooDivide(5, 4)

could have been written as

result, _ := fooDivide(5, 4)

In the latter case, the error returned would have been ignored.

Honestly, they’re both pretty similar, except for Rust forcing error checking. Otherwise, there’s little difference, and it’s difficult to find an advantage one has over the other. To my eyes, this is a draw.

I could keep going, digging deeper into other language differences. But the bottom line, from threads, to channels, to generics, Rust provides the developer with more options. In this respect, Rust is closer to C++ than Go. Does this make Rust inherently more complex?

I think so, yes.

So here are my recommendations:

  • Either. If you’re building a web service that handles high load, that you want to be able to scale both vertically and horizontally, either language will suit you perfectly.
  • Go. But if you want to write it faster, perhaps because you have many different services to write, or you have a large team of developers, then Go is your language of choice. Go gives you concurrency as a first-class citizen, and does not tolerate unsafe memory access (neither does Rust), but without forcing you to manage every last detail. Go is fast and powerful, but it avoids bogging the developer down, focusing instead on simplicity and uniformity.
  • Rust. If on the other hand, wringing out every last ounce of performance is a necessity, then Rust should be your choice. Rust is more of a competitor to C++ than it is with Go. Having battled with C++, Rust feels just as powerful but with many happy improvements. Rust empowers developers to have control over every last detail of how their threads behave with the rest of the system, how errors should be handled, and even the lifetime of their variables!
  • Rust. Rust was designed to interoperate with C. Go can as well, but gives up a lot to achieve this goal, and it’s not really its focus.
  • Go. If readability is a requirement, go with Go. It’s far too easy to make your code hard for others to grok with Rust.

I hope you enjoyed reading this!

The Rust Programming Language - Understanding If in Rust

The Rust Programming Language - Understanding If in Rust

The Rust Programming Language - Understanding If in Rust. Rust’s take on if is not particularly complex, but it’s much more like the if you’ll find in a dynamically typed language than in a more traditional systems language. if is a specific form of a more general concept, the ‘branch’, whose name comes from a branch in a tree: a decision point, where depending on a choice, multiple paths can be taken.

Rust’s take on if is not particularly complex, but it’s much more like the if you’ll find in a dynamically typed language than in a more traditional systems language. So let’s talk about it, to make sure you grasp the nuances.

if is a specific form of a more general concept, the ‘branch’, whose name comes from a branch in a tree: a decision point, where depending on a choice, multiple paths can be taken.

In the case of if, there is one choice that leads down two paths:


# #![allow(unused_variables)]
#fn main() {
let x = 5;

if x == 5 {
    println!("x is five!");
}
#}

If we changed the value of x to something else, this line would not print. More specifically, if the expression after the if evaluates to true, then the block is executed. If it’s false, then it is not.

If you want something to happen in the false case, use an else:


# #![allow(unused_variables)]
#fn main() {
let x = 5;

if x == 5 {
    println!("x is five!");
} else {
    println!("x is not five :(");
}
#}

If there is more than one case, use an else if:


# #![allow(unused_variables)]
#fn main() {
let x = 5;

if x == 5 {
    println!("x is five!");
} else if x == 6 {
    println!("x is six!");
} else {
    println!("x is not five or six :(");
}
#}

This is all pretty standard. However, you can also do this:


# #![allow(unused_variables)]
#fn main() {
let x = 5;

let y = if x == 5 {
    10
} else {
    15
}; // y: i32
#}

Which we can (and probably should) write like this:


# #![allow(unused_variables)]
#fn main() {
let x = 5;

let y = if x == 5 { 10 } else { 15 }; // y: i32
#}

This works because if is an expression. The value of the expression is the value of the last expression in whichever branch was chosen. An if without an else always results in () as the value.