24 days of Rust - itertools

Important note: this article is outdated! Go to http://zsiciarz.github.io/24daysofrust/ for a recent version of all of 24 days of Rust articles. The blogpost here is kept as it is for historical reasons.

The itertools crate contains several utility functions and macros inspired by Haskell and Python itertools. As you can guess from the name, these have to do with iteration and iterators. Rust has recently split the Iterator trait into Iterator and IteratorExt for so called object safety reasons (see the RFC for an explanation). This is mostly irrelevant for today's episode of 24 days of Rust, but worth keeping in mind.

To use itertools, add the following dependency declaration to Cargo.toml:

itertools = "~0.0.4"

We'll start from the helper functions and cover the macros later.


This and a few other functions live in the Itertools trait, so we need to bring them into scope by placing use itertools::Itertools in our module.

apply() is very simple conceptually. It consumes the (mutable) iterator, calling a closure witch each of the elements. The return type is () (unit), meaning that apply() usually should be at the end of a call chain, like below:

let mut words = "hello supercalifragilisticexpialidocious programmer".words();
words.apply(|word| println!("{} is {} characters long", word, word.len()));

As you can see, apply() is similar to the map() method from the standard library, however map returns another iterator. Therefore it's lazy and allows for further method chaining, while apply is eager and has the final word.

interleave and intersperse

interleave() is somewhat similar to zip(). But when zip builds tuples from two iterators, interleave yields the values alternating between both iterators.

let even = range(1, 10u).map(|x| x * 2);
let odd = range(1, 5u).map(|x| x * 2 + 1);
println!("{}", even.interleave(odd).collect::<Vec<_>>());

The result:

$ cargo run
[2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18]

As you can see, the iterators don't have to contain the same number of elements. In that case interleave continues to emit values from the other iterator, after it consumes the "shorter" one.

In a manner similar to its Haskell counterpart, intersperse takes a single value and an iterator (implicitly as the self argument) and emits the given value between every element of the wrapped iterator. For example:

let it = range(1, 10u);
println!("{}", it.intersperse(15u).collect::<Vec<_>>());


$ cargo run
[1, 15, 2, 15, 3, 15, 4, 15, 5, 15, 6, 15, 7, 15, 8, 15, 9]


Let's now turn our attention to macros provided by itertools. Sometimes there is a need to iterate over a cartesian product of some lists/arrays/vectors. Usually it involves two nested loops; however we can use the iproduct() macro to simplify it to a single for loop.

let numbers = range(1, 4u);
let chars = vec!['a', 'b', 'c'];
for (i, c) in iproduct!(numbers, chars.iter()) {
    println!("{}: {}", i, c);


This is possibly the tastiest bit in itertools. Those of you coming to Rust from Python will recognize it immediately. Generator expressions allow creating lazy iterators with a very simple, convenient syntax. The icompr!() (iterator comprehension) macro brings that syntax to Rust. The following example borrows from David Beazley's Generators for System Programmers tutorial.

use std::iter::AdditiveIterator;

let log = "GET / 4096\nGET /home/ 16301\nPOST /home/ 49\nGET / 4096\n";
let lines = log.lines();
let rows = icompr!(line.words().collect::<Vec<_>>() for line in lines);
let bytes = icompr!(row[2] for row in rows if row[0] != "POST");
let total = icompr!(from_str::<uint>(b).unwrap() for b in bytes).sum();
println!("Total GET throughput: {} bytes", total);

We parse a very simplified server log, counting a total number of bytes sent in response to GET requests. Just like in Python, we can use conditionals (`if row[0] != "POST") to filter the values directly in the generator expression.

Code examples in this article were built with rustc 0.13.0-nightly and itertools 0.0.4.

Photo by Charles Hutchins and shared under the Creative Commons Attribution2.0 Generic License. See https://www.flickr.com/photos/celesteh/3266906452