forked from mirror/async-std
59 lines
1.7 KiB
Rust
59 lines
1.7 KiB
Rust
use std::pin::Pin;
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use crate::prelude::*;
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use crate::stream::{Stream, Sum};
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impl<T, U> Sum<Option<U>> for Option<T>
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where
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T: Sum<U>,
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{
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#[doc = r#"
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Takes each element in the `Iterator`: if it is a `None`, no further
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elements are taken, and the `None` is returned. Should no `None` occur,
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the sum of all elements is returned.
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# Examples
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This sums up the position of the character 'a' in a vector of strings,
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if a word did not have the character 'a' the operation returns `None`:
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```
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# fn main() { 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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let words = stream::from_iter(vec!["have", "a", "great", "day"]);
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let total: Option<usize> = words.map(|w| w.find('a')).sum().await;
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assert_eq!(total, Some(5));
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#
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# }) }
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```
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"#]
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fn sum<'a, S>(stream: S) -> Pin<Box<dyn Future<Output = Option<T>> + 'a>>
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where S: Stream<Item = Option<U>> + 'a
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{
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Box::pin(async move {
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// Using `scan` here because it is able to stop the stream early
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// if a failure occurs
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let mut found_none = false;
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let out = <T as Sum<U>>::sum(stream
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.scan((), |_, elem| {
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match elem {
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Some(elem) => Some(elem),
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None => {
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found_none = true;
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// Stop processing the stream on error
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None
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}
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}
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})).await;
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if found_none {
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None
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} else {
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Some(out)
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}
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})
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}
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}
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