```
pub fn unfold<A, St, F>(initial_state: St, f: F) -> Unfold<St, F> ⓘ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
let next = x1.saturating_add(x2);
// 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))
```