Function core::iter::once_with

pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> 

Creates an iterator that lazily generates a value exactly once by invoking the provided closure.

This is commonly used to adapt a single value generator into a chain() of other kinds of iteration. Maybe you have an iterator that covers almost everything, but you need an extra special case. Maybe you have a function which works on iterators, but you only need to process one value.

Unlike once(), this function will lazily generate the value on request.

Examples

Basic usage:

use std::iter;

// one is the loneliest number
let mut one = iter::once_with(|| 1);

assert_eq!(Some(1), one.next());

// just one, that's all we get
assert_eq!(None, one.next());

Chaining together with another iterator. Let’s say that we want to iterate over each file of the .foo directory, but also a configuration file, .foorc:

use std::iter;
use std::fs;
use std::path::PathBuf;

let dirs = fs::read_dir(".foo").unwrap();

// we need to convert from an iterator of DirEntry-s to an iterator of
// PathBufs, so we use map
let dirs = dirs.map(|file| file.unwrap().path());

// now, our iterator just for our config file
let config = iter::once_with(|| PathBuf::from(".foorc"));

// chain the two iterators together into one big iterator
let files = dirs.chain(config);

// this will give us all of the files in .foo as well as .foorc
for f in files {
    println!("{f:?}");
}