use std::iter;

/// Iterate over a sequence with the first element the function returns
/// Some() for updated, but only if there is such an element.
pub fn replace_first<
  T: Clone,
  F: FnMut(&T) -> Option<T>
>(
  slice: &[T], mut f: F
) -> Option<impl Iterator<Item = T> + '_> {
  for i in 0..slice.len() {
    if let Some(new) = f(&slice[i]) {
      let subbed_iter = slice[0..i].iter().cloned()
        .chain(iter::once(new))
        .chain(slice[i+1..].iter().cloned());
      return Some(subbed_iter)
    }
  }
  None
}