# Function itertools::unfold

``pub fn unfold<A, St, F>(initial_state: St, f: F) -> Unfold<St, F>ⓘNotable traits for Unfold<St, F>impl<A, St, F> Iterator for Unfold<St, F> where    F: FnMut(&mut St) -> Option<A>,     type Item = A; where    F: FnMut(&mut St) -> Option<A>, ``
Expand description

Creates a new unfold source with the specified closure as the “iterator function” and an initial state to eventually pass to the closure

`unfold` is a general iterator builder: it has a mutable state value, and a closure with access to the state that produces the next value.

This more or less equivalent to a regular struct with an `Iterator` implementation, and is useful for one-off iterators.

``````// an iterator that yields sequential Fibonacci numbers,
// and stops at the maximum representable value.

use itertools::unfold;

let (mut x1, mut x2) = (1u32, 1u32);
let mut fibonacci = unfold((), move |_| {
// Attempt to get the next Fibonacci number

// Shift left: ret <- x1 <- x2 <- next
let ret = x1;
x1 = x2;
x2 = next;

// If addition has saturated at the maximum, we are finished
if ret == x1 && ret > 1 {
return None;
}

Some(ret)
});

itertools::assert_equal(fibonacci.by_ref().take(8),
vec![1, 1, 2, 3, 5, 8, 13, 21]);
assert_eq!(fibonacci.last(), Some(2_971_215_073))``````