How to Use flat_map in Rust

Use flat_map to transform each item in an iterator into zero or more new items, automatically flattening the result into a single stream. It is ideal when you need to expand or filter elements during iteration without creating intermediate nested collections.

let lines: Vec<_> = input
    .lines()
    .flat_map(|line| {
        if line.starts_with("```") {
            Some(String::from("```"))
        } else {
            let result = regexen.iter().fold(line.to_string(), |result, regex| {
                regex.replace_all(&result, |caps: &Captures<'_>| {
                    caps.get(1).unwrap().as_str().to_string()
                }).to_string()
            });
            Some(result)
        }
    })
    .collect();

Think of flat_map as a two-step process that happens in one go: it takes each item, turns it into a list of new items, and then immediately merges all those lists into one long line. You use it when you want to process data and expand it into multiple pieces without having to manually flatten the result yourself. It saves memory and code by avoiding the creation of temporary nested vectors.