About

fold is a consuming iterator adaptor which applies a function to each element of the iteration, accumulating the result into a new value.

Each iteration of fold takes two arguments. The first is an initial value for the accumulator. The second is a closure which itself takes two arguments: the accumulator and an item. The closure’s return value is the subsequent value of the accumulator. Once the iterator drains, the final value of the accumulator is returned.

fold can be used when it is desired to transform a collection into a single value. For example, if the sum of all even numbers is desired

pub fn main() {
    let even_sum = (1..=10).fold(0, |acc, num| if num % 2 == 0 { acc + num } else { acc });
    println!("{even_sum:?}");
}

Prints 30.

What can be done in fold can often be done by another function or functions. For the above, to filter on (1..=10) and then sum would be less verbose and evaluate the same result, like so

(0..=10).filter(|n| *n % 2 == 0).sum()

Other functions that are more commonly used than fold include product and collect.

An accumulator can be used for other than numeric values. For example

pub fn giant_grunts(initial: char) -> String {
    ["Bee", "Fee", "Gee", "Fi", "Hi", "Fo", "Mo", "Fum", "Tum"].iter().fold(
        String::new(),
        |acc, grunt| if grunt.starts_with(initial) { acc + grunt } else { acc },
    )
}

pub fn main() {
    let song = giant_grunts('F');
    println!("{song:?}");
}

Prints FeeFiFoFum

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Originally from Exercism rust concepts