use Pin;
use crate Stream;
use crate;
/// A stream that yields elements by calling a closure.
///
/// This stream is created by the [`from_fn`] function. See its
/// documentation for more.
///
/// [`from_fn`]: fn.from_fn.html
/// Creates a new stream where to produce each new element a provided closure is called.
///
/// This allows creating a custom stream with any behaviour without using the more verbose
/// syntax of creating a dedicated type and implementing a `Stream` trait for it.
///
/// # Examples
///
/// ```
/// # async_std::task::block_on(async {
/// #
/// use async_std::prelude::*;
/// use async_std::stream;
///
/// let mut count = 0u8;
/// let s = stream::from_fn(|| {
/// count += 1;
/// if count > 3 {
/// None
/// } else {
/// Some(count)
/// }
/// });
///
/// pin_utils::pin_mut!(s);
///
/// assert_eq!(s.next().await, Some(1));
/// assert_eq!(s.next().await, Some(2));
/// assert_eq!(s.next().await, Some(3));
/// assert_eq!(s.next().await, None);
/// #
/// # })
/// ```