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An Example With Streaming Input

In this example we will cover:

  • the input channel concept, which streams data in from outside the Hydroflow spec
  • the source_stream operator that brings channel input into Hydroflow
  • Rust syntax to programmatically send data to a (local) channel

In our previous examples, data came from within the Hydroflow spec, via Rust iterators and the source_iter operator. In most cases, however, data comes from outside the Hydroflow spec. In this example, we'll see a simple version of this idea, with data being generated on the same thread and sent into the channel programmatically via Rust.

For discussion, we start with a skeleton much like before:

use hydroflow::hydroflow_syntax;

pub fn main() {
let mut hydroflow = hydroflow_syntax! {
// code will go here


TODO: Make the following less intimidating to users who are not Tokio experts.

To add a new external input channel, we can use the hydroflow::util::unbounded_channel() function in Rust before we declare the Hydroflow spec:

    // Create our channel input
let (input_example, example_recv) = hydroflow::util::unbounded_channel::<usize>();

Under the covers, this is a multiple-producer/single-consumer (mpsc) channel provided by the tokio library for Rust, which is usually the appropriate choice for an inbound Hydroflow stream. Think of it as a high-performance "mailbox" that any sender can fill with well-typed data.

The Rust ::<usize> syntax uses what is affectionately called the "turbofish", which is how type parameters (generic arguments) are supplied to generic types and functions. In this case it specifies that this tokio channel transmits items of type usize. The returned example_recv value can be used via a source_stream to build a Hydroflow subgraph just like before.

Here is the same program as before, but using the input channel. Back in the simple project, replace the contents of src/ with the following:

use hydroflow::hydroflow_syntax;

pub fn main() {
// Create our channel input
let (input_example, example_recv) = hydroflow::util::unbounded_channel::<usize>();

let mut flow = hydroflow_syntax! {
-> filter_map(|n: usize| {
let n2 = n * n;
if n2 > 10 {
else {
-> flat_map(|n| (n..=n+1))
-> for_each(|n| println!("Ahoy, {}", n));


cargo run
<build output>
Ahoy, 16
Ahoy, 17
Ahoy, 25
Ahoy, 26
Ahoy, 36
Ahoy, 37
Ahoy, 49
Ahoy, 50
Ahoy, 64
Ahoy, 65
Ahoy, 81
Ahoy, 82

At the bottom of we can see how to programatically supply usize-typed inputs with the tokio .send() method. We call Rust's .unwrap() method to ignore the error messages from .send() in this simple case. In later examples we'll see how to allow for data coming in over a network.