Function lending_iterator::from_fn

pub fn from_fn<Item, State, Next>(
    state: State,
    next: Next
) -> FromFn<Item, State, Next> where
    Item: HKT,
    Next: FnMut(&mut State) -> Option<Feed<'_, Item>>,

Main ad-hoc / closure-based constructor of LendingIterators.

It expects the both necessary and sufficient elements for such an impl:

• a State, which will play a role akin to that of Self in a manual impl;

• a fn next “method” on it. There is actually a certain level of flexibility gained from this being a closure rather than a stateless associated function.

  For instance, non-lent state (such as an internal mutable counter) can be implicity captured by such a closure, without having to funnel it through the lendable State.

Example

• use ::lending_iterator::prelude::*;
  
  struct Person {
      name: String,
      age: u8,
  }
  
  fn example (person: &mut Person)
    -> impl '_ + LendingIterator
  /* or:
    -> impl '_ + LendingIteratorDyn<Item = HKT!(&str)>
   */
  {
      lending_iterator::from_fn::<HKT!(&str), _, _>(
          person,
          |p| if p.age > 0 {
              Some(&p.name)
          } else {
              None
          },
      )
  }

Remarks

It can also be viewed as a convenience layer over:

::lending_iterator::repeat_mut(state)
.filter_map::<Item, _>(move |[], it| next(it))

• use ::lending_iterator::prelude::*;
  
  struct Person {
      name: String,
      age: u8,
  }
  
  fn example (person: &mut Person)
    -> impl '_ + LendingIterator
  /* or:
    -> impl '_ + LendingIteratorDyn<Item = HKT!(&str)>
   */
  {
      lending_iterator::repeat_mut(person)
          .filter_map::<HKT!(&str), _>(
              |[], p| if p.age > 0 {
                  Some(&p.name)
              } else {
                  None
              },
          )
  }

FromFn

The returned struct —FromFn— can also be used directly, to benefit from “named arguments”, at the cost of having to provide a PhantomData parameter.