forked from mirror/async-std
66 lines
1.5 KiB
Rust
66 lines
1.5 KiB
Rust
use std::pin::Pin;
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use crate::stream::Stream;
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use crate::task::{Context, Poll};
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/// A stream that yields elements by calling a closure.
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///
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/// This stream is created by the [`from_fn`] function. See its
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/// documentation for more.
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///
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/// [`from_fn`]: fn.from_fn.html
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#[derive(Clone, Debug)]
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pub struct FromFn<F> {
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f: F,
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}
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impl<F> Unpin for FromFn<F> {}
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/// Creates a new stream where to produce each new element a provided closure is called.
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///
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/// This allows creating a custom stream with any behaviour without using the more verbose
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/// syntax of creating a dedicated type and implementing a `Stream` trait for it.
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///
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/// # Examples
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///
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/// ```
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/// # async_std::task::block_on(async {
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/// #
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/// use async_std::prelude::*;
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/// use async_std::stream;
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///
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/// let mut count = 0u8;
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/// let mut s = stream::from_fn(|| {
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/// count += 1;
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/// if count > 3 {
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/// None
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/// } else {
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/// Some(count)
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/// }
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/// });
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///
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/// assert_eq!(s.next().await, Some(1));
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/// assert_eq!(s.next().await, Some(2));
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/// assert_eq!(s.next().await, Some(3));
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/// assert_eq!(s.next().await, None);
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/// #
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/// # })
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/// ```
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pub fn from_fn<T, F>(f: F) -> FromFn<F>
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where
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F: FnMut() -> Option<T>,
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{
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FromFn { f }
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}
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impl<T, F> Stream for FromFn<F>
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where
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F: FnMut() -> Option<T>,
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{
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type Item = T;
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fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
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let item = (&mut self.f)();
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Poll::Ready(item)
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}
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}
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