The things I write

A FizzBuzzy Tour of Traits in Rust

August 13, 2018

Traits are a core part of the Rust programming language, and understanding traits, particularly those which are part of the standard library, is necessary in order to write idiomatic Rust. In this post I’ll write several FizzBuzz implementations, each demonstrating the use of a different trait from the Rust standard library.

While I do think that knowing and using these traits is necessary for writing idiomatic Rust code, I would not claim that any one of these examples is the best way to write FizzBuzz in Rust. This blog post by Tom Dalling provides a great explanation for why ‘optimizing’ FizzBuzz is a bad idea.

One last thing I want to mention before getting started is that there are at least a handful of traits which are more common (and arguably more useful) than the traits I am going to discuss here. Many traits which have obvious implementations can be automatically derived by adding #[derive(TraitName)] annotations to your code (a list of derive-able traits is available here). In this post, I’ve chosen to focus on a set of traits that I think are very useful, but cannot be automatically derived.

A Starting Point

Before beginning the discussion of Traits, I’ll create a baseline implementation of FizzBuzz which will serve as a starting point for all of the examples. It is possible to write FizzBuzz in Rust with a style that would be familiar from many other languages.

for i in 1u32..=100 {

    let divisible_by_three = i % 3 == 0;
    let divisible_by_five = i % 5 == 0;

    if divisible_by_three && divisible_by_five {
        println!("FizzBuzz");
    } else if divisible_by_three {
        println!("Fizz");
    } else if divisible_by_five {
        println!("Buzz");
    } else {
        println!("{}", i);
    }

}

Rust has a feature called pattern matching which can be used to express these kind of if-else statements, while providing a guarantee that you haven’t missed an edge case. Using match, you’d get:

for i in 1u32..=100 {

    let divisible_by_three = i % 3 == 0;
    let divisible_by_five = i % 5 == 0;

    match (divisible_by_three, divisible_by_five) {
        (true, true) => println!("FizzBuzz"),
        (true, false) => println!("Fizz"),
        (false, true) => println!("Buzz"),
        (false, false) => println!("{}", i),
    }

}

Even in this simple example, we are already making use of traits from the standard library. One I’d like to discuss in particular is the Display trait, which we use in the last arm of the match statement when we write println!("{}", i). In this case, i is of type u32, and u32 implements the Display trait. The Display trait is used whenever you use the format/print/println family of macros with the placeholder {}. Later we’ll derive the Display trait ourselves on a new type.

For now let’s move on to the discussion of traits, using this code as a rough starting point.

Trait Bounds and IntoIterator

Lets say you want to write a function which can take a vec of integers and run FizzBuzz on that vec.

fn fizzbuzz_vec(nums: &Vec<u32>) {
    for num in nums {
        let divisible_by_three = num % 3 == 0;
        let divisible_by_five = num % 5 == 0;

        match (divisible_by_three, divisible_by_five) {
            (true, true) => println!("FizzBuzz"),
            (true, false) => println!("Fizz"),
            (false, true) => println!("Buzz"),
            (false, false) => println!("{}", num),
        }
    }
}

But what if you also wanted a function to support FizzBuzzing over an array, rather than a vec, of integers? You could write a second function to handle arrays, but this is the perfect use case for trait bounds.

IntoIterator is a trait from the standard library which says a type can be converted into an iterator. The for-in syntax we are using requires a type which implements either the IntoIterator or the Iterator trait, and the standard library implements the IntoIterator trait for both vectors and arrays.

Using this to write a function which takes any type that implements IntoIterator for items of type u32 (meaning the iterator will return items of type u32) looks like this:

fn fizzbuzz<'a, T>(nums: T)
    where T: IntoIterator<Item = &'a u32>
{
    for num in nums {
        let divisible_by_three = num % 3 == 0;
        let divisible_by_five = num % 5 == 0;

        match (divisible_by_three, divisible_by_five) {
            (true, true) => println!("FizzBuzz"),
            (true, false) => println!("Fizz"),
            (false, true) => println!("Buzz"),
            (false, false) => println!("{}", num),
        }
    }
}

Either of these functions can be called with a vec as the argument, but the second can also be called with an array.

let nums : Vec<u32> = vec![1,2,3,4,5,15];
fizzbuzz_vec(&nums);

let nums : Vec<u32> = vec![1,2,3,4,5,15];
fizzbuzz(&nums);

let nums: [u32; 6] = [1,2,3,4,5,15];
fizzbuzz(&nums);

From/Into and Display

The From trait is used to convert from one type to another. In this case we’ll create a FizzBuzz enum, and implement From so we can easily convert from a u32 to a FizzBuzz.

#[derive(Debug)]
enum FizzBuzz {
    Fizz,
    Buzz,
    FizzBuzz,
    Other(u32),
}

impl From<u32> for FizzBuzz {
    fn from(item: u32) -> Self {
        match (item % 3 == 0, item % 5 == 0) {
            (false, false) => FizzBuzz::Other(item),
            (true, false) => FizzBuzz::Fizz,
            (false, true) => FizzBuzz::Buzz,
            (true, true) => FizzBuzz::FizzBuzz,
        }
    }
}

for i in 1..=100 {
    println!("{:?}", FizzBuzz::from(i));
}

An additional benefit of implementing the From trait is the Into trait is automatically implemented. In most cases, using into requires specifying the type you want to convert to because there will not be enough information for the compiler to infer it. An example of this is below:

for i in 1..=100 {
    let fizzbuzz : FizzBuzz = i.into();
    println!("{:?}", fizzbuzz);
}

Note that while we derive Debug on our enumeration and rely on that to print the variants ({:?} is used to debug print), we could implement our own display logic with the Display trait. While the Fizz, Buzz, and FizzBuzz variants are handled like we’d want already, the Other variant should be changed so that it only prints the number (the derived Debug implementation prints Other(x)).

impl Display for FizzBuzz {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            FizzBuzz::Other(n) => write!(f, "{}", n),
            _ => write!(f, "{:?}", self),
        }
    }
}

Fn

The Fn family of traits, Fn/FnMut/FnOnce, is used to mark that a type can be called like a function. Using this, we can create a struct which takes four functions as constructor arguments, and then exposes an eval method which takes a u32 as input and runs one of the four functions depending on whether the input is divisible by three, divisible by five, divisible by both three and five, or divisible by neither three nor five.

struct FizzBuzzer<FnFizz, FnBuzz, FnFizzBuzz, FnOther> {
    fn_fizz: FnFizz,
    fn_buzz: FnBuzz,
    fn_fizzbuzz: FnFizzBuzz,
    fn_other: FnOther,
}

impl<FnFizz, FnBuzz, FnFizzBuzz, FnOther> FizzBuzzer<FnFizz, FnBuzz, FnFizzBuzz, FnOther>
    where
        FnFizz: Fn(),
        FnBuzz: Fn(),
        FnFizzBuzz: Fn(),
        FnOther: Fn(u32),
{
    fn new(fn_fizz: FnFizz, fn_buzz: FnBuzz, fn_fizzbuzz: FnFizzBuzz, fn_other: FnOther) -> Self {
        Self { fn_fizz, fn_buzz, fn_fizzbuzz, fn_other }
    }

    fn eval(&self, num: u32) {
        match (num % 3 == 0, num % 5 == 0) {
            (false, false) => (self.fn_other)(num),
            (true, false) => (self.fn_fizz)(),
            (false, true) => (self.fn_buzz)(),
            (true, true) => (self.fn_fizzbuzz)(),
        }
    }
}

Notice in the where clause on the impl statement, we specify that FnFizz/FnBuzz/FnFizzBuzz are of type Fn(), a function which takes no arguments and provides no return value, and FnOther is of type Fn(u32), a function which takes a single u32 argument and provides no return.

The struct can be instantiated by passing four closures to the new method, and used by passing u32 types to the eval method, as shown below.

let fizzbuzzer = FizzBuzzer::new(
    || println!("Fizz"),
    || println!("Buzz"),
    || println!("FizzBuzz"),
    |num| println!("{}", num),
);

for i in 1..=100 {
    fizzbuzzer.eval(i);
}

Iterator

As mentioned in the section on trait bounds, implementing either the Iterator or IntoIterator trait in Rust allows for your type to be used in for loops.

To implement Iterator on a type only requires implementing a single method, fn next(&mut self) -> Option<Self::Item> where Self::Item is the type that the iterator will return. To start, we’ll create a struct to contain the state of the iterator.

struct FizzBuzzer {
    next: u32,
    max: u32,
}

impl FizzBuzzer {
    fn new(starting_value: u32, length: u32) -> Self {
        let max = if length > 0 { starting_value + length - 1 } else { 0 }; // protect from underflow
        FizzBuzzer { next: starting_value, max }
    }
}

Then we implement Iterator for our new type by creating a next method which will return an Option<String>.

impl Iterator for FizzBuzzer {
    type Item = String;

    fn next(&mut self) -> Option<Self::Item> {
        if self.next > self.max { return None }

        let s = match (self.next % 3 == 0, self.next % 5 == 0) {
            (false, false) => format!("{}", self.next),
            (true, false) => String::from("Fizz"),
            (false, true) => String::from("Buzz"),
            (true, true) => String::from("FizzBuzz"),
        };

        self.next += 1;

        Some(String::from(s))
    }
}

Note, this code could be improved by using the clone on write type, as discussed in this Rust-based FizzBuzz deep-dive by Chris Morgan.

With Iterator implemented on the FizzBuzzer type, it can be used as follows.

for text in FizzBuzzer::new(1, 100) {
    println!("{}", text);
}

Summary

Writing idiomatic Rust code requires not only understanding traits on a conceptual level, but also having a familiarity with the traits which are part of the Rust standard library. In typical Rust fashion, many of the common standard library traits can be automatically derived if they have an obvious implementation. In this post, I’ve covered some of the traits which integrate very tightly with the language, but which cannot be automatically derived. Using these traits, and implementing them on your own types where appropriate, will significantly improve the quality of the Rust code you produce.

Edits

  1. Modified code examples to use u32 rather than usize based on this discussion

Josh Mcguigan
Written by Josh Mcguigan