better_collect

Provides a composable, declarative way to consume an iterator.

If Iterator is the "source half" of data pipeline, Collector is the "sink half" of the pipeline.

In order words, Iterator describes how to produce data, and Collector describes how to consume it.

Motivation

Suppose we are given an array of i32 and we are asked to find its sum and maximum value. What would be our approach?

• Approach 1: Two-pass

let nums = [1, 3, 2];
let sum: i32 = nums.into_iter().sum();
let max = nums.into_iter().max().unwrap();

assert_eq!(sum, 6);
assert_eq!(max, 3);

Cons: This performs two passes over the data, which is worse than one-pass in performance. That is fine for arrays, but can be much worse for HashSet, LinkedList, or... data from an IO stream.

• Approach 2: Iterator::fold()

let nums = [1, 3, 2];
let (sum, max) = nums
    .into_iter()
    .fold((0, i32::MIN), |(sum, max), num| {
        (sum + num, max.max(num))
    });

assert_eq!(sum, 6);
assert_eq!(max, 3);

Cons: Not very declarative. The main logic is still kind of procedural. (Doing sum and max by ourselves)

• Approach 3: Iterator::inspect()

let nums = [1, 3, 2];
let mut sum = 0;
let max = nums
    .into_iter()
    .inspect(|i| sum += i)
    .max()
    .unwrap();

assert_eq!(sum, 6);
assert_eq!(max, 3);

Cons: This approach has multiple drawbacks:

• If the requirement changes to "calculate sum and find any negative value," this approach may produce incorrect results. The "any" logic may short-circuit on finding the desired value, preventing the "sum" logic from summing every value. It is possible that we can rearrange so that the "any" logic goes first, but if the requirement changes to "find any negative value and even value," we cannot escape.

• The state is kept outside. Now the iterator cannot go anywhere else (e.g. sending to another thread, sending through a channel).

• Very unintuitive and hack-y (hard to reason about).

• And most importantly, not declarative enough.

This crate proposes a one-pass, declarative approach:

use better_collect::{prelude::*, num::Sum, cmp::Max};

let nums = [1, 3, 2];
let (sum, max) = nums
    .into_iter()
    .better_collect(Sum::<i32>::new().combine(Max::new()));

assert_eq!(sum, 6);
assert_eq!(max.unwrap(), 3);

This approach achieves both one-pass and declarative, while is also composable (more of this later).

This is only with integers. How about with a non-Copy type?

// Suppose we open a connection...
fn socket_stream() -> impl Iterator<Item = String> {
    ["the", "noble", "and", "the", "singer"]
        .into_iter()
        .map(String::from)
}

// Task: Returns:
// - An array of data from the stream.
// - How many bytes were read.
// - The last-seen data.

// Usually, we're pretty much stuck with for-loop (tradition, `(try_)fold`, `(try_)for_each`).
// No common existing tools can help us here:
let mut received = vec![];
let mut byte_read = 0_usize;
let mut last_seen = None;

for data in socket_stream() {
    received.push(data.clone());
    byte_read += data.len();
    last_seen = Some(data);
}

let expected = (received, byte_read, last_seen);

// This crate's way:
use better_collect::{prelude::*, Last, num::Sum};

let ((received, byte_read), last_seen) = socket_stream()
    .better_collect(
        vec![]
            .into_collector()
            .cloning()
            // Use `map_ref` so that our collector is a `RefCollector`
            // (only a `RefCollector` is `combine`-able)
            .combine(Sum::<usize>::new().map_ref(|data: &mut String| data.len()))
            .combine(Last::new())
    );

assert_eq!((received, byte_read, last_seen), expected);

Very declarative! We describe what we want to collect.

You might think this is just like Iterator::unzip(), but this crate does a bit better: it can split the data and feed separately WITHOUT additional allocation.

To demonstrate the difference, take this example:

use std::collections::HashSet;
use better_collect::prelude::*;

// Suppose we open a connection...
fn socket_stream() -> impl Iterator<Item = String> {
    ["the", "noble", "and", "the", "singer"]
        .into_iter()
        .map(String::from)
}

// Task: Collect UNIQUE chunks of data and concatenate them.

// `Iterator::unzip`
let (chunks, concatenated_data): (HashSet<_>, String) = socket_stream()
    // Sad. We have to clone.
    // We can't take a reference, since the referenced data is returned too.
    .map(|chunk| (chunk.clone(), chunk))
    .unzip();

let unzip_way = (concatenated_data, chunks);

// Another approach is do two passes (collect to `Vec`, then iterate),
// which is still another allocation,
// or `Iterator::fold`, which's procedural.

// `Collector`
let collector_way = socket_stream()
    // No clone. The data flows smoothly.
    .better_collect(ConcatString::new().combine(HashSet::new()));

assert_eq!(unzip_way, collector_way);

Traits

Main traits

Unlike std::iter, this crate defines two main traits instead. Roughly:

use std::ops::ControlFlow;

pub trait Collector {
    type Item;

    type Output
    where
        Self: Sized;

    fn collect(&mut self, item: Self::Item) -> ControlFlow<()>;

    fn finish(self) -> Self::Output
    where
        Self: Sized;
}

pub trait RefCollector: Collector {
    fn collect_ref(&mut self, item: &mut Self::Item) -> ControlFlow<()>;
}

Collector is similar to Extend, but it also returns a ControlFlow value to indicate whether it should stop accumulating items after a call to collect(). This serves as a hint for adaptors like combine() or chain() to "vectorize" the remaining items to another collector. In short, it is like a composable Extend.

RefCollector is a collector that does not require ownership of an item to process it. This allows items to flow through multiple collectors without being consumed, avoiding unnecessary cloning. It powers combine(), which creates a pipeline of collectors, letting each item pass through safely by reference until the final collector takes ownership.

Other traits

BetterCollect extends Iterator with the better_collect() method, which feeds all items from an iterator into a Collector and returns the collector’s result. To use this method, the BetterCollect trait must be imported.

IntoCollector is a conversion trait that converts a type into a Collector.

More types, traits and functions can be found in this crate's documentation.

Features

• alloc — Enables collectors and implementations for types in the alloc crate (e.g., Vec, VecDeque, BTreeSet).

• std (default) — Enables the alloc feature and implementations for std-only types (e.g., HashMap). When this feature is disabled, the crate builds in no_std mode.

• unstable — Enables experimental and unstable features. Items gated behind this feature do not follow normal semver guarantees and may change or be removed at any time.

  Although the crate as a whole is technically still experimental, the items under unstable are even more experimental, and it is generally discouraged to use them until their designs are finalized and not under this flag anymore.