Simplest Example

In this example we will cover:

  • Modifying the Hydroflow template project
  • How Hydroflow program specs are embedded inside Rust
  • How to execute a simple Hydroflow program
  • Two Hydroflow operators: source_iter and for_each

Lets start out with the simplest possible Hydroflow program, which prints out the numbers in 0..10.

Create a clean template project:

% cargo generate hydro-project/hydroflow-template
⚠️   Favorite `hydro-project/hydroflow-template` not found in config, using it as a git repository:
🤷   Project Name: simple
🔧   Destination: /Users/jmh/code/sussudio/simple ...
🔧   project-name: simple ...
🔧   Generating template ...
[ 1/11]   Done: .gitignore                                                      [ 2/11]   Done: Cargo.lock                                                      [ 3/11]   Done: Cargo.toml                                                      [ 4/11]   Done:                                                       [ 5/11]   Done: rust-toolchain.toml                                             [ 6/11]   Done: src/                                                   [ 7/11]   Done: src/                                                  [ 8/11]   Done: src/                                                     [ 9/11]   Done: src/                                                 [10/11]   Done: src/                                                   [11/11]   Done: src                                                             🔧   Moving generated files into: `<dir>/simple`...
💡   Initializing a fresh Git repository
✨   Done! New project created <dir>/simple

Change directory into the resulting simple folder or open it in your IDE. Then edit the src/ file, replacing all of its contents with the following code:

use hydroflow::hydroflow_syntax;

pub fn main() {
    let mut flow = hydroflow_syntax! {
        source_iter(0..10) -> for_each(|n| println!("Hello {}", n));


And then run the program:

% cargo run
Hello 0
Hello 1
Hello 2
Hello 3
Hello 4
Hello 5
Hello 6
Hello 7
Hello 8
Hello 9

Understanding the Code

Although this is a trivial program, it's useful to go through it line by line.

use hydroflow::hydroflow_syntax;

This import gives you everything you need from Hydroflow to write code with Hydroflow's surface syntax.

Next, inside the main method we specify a flow by calling the hydroflow_syntax! macro. We assign the resulting Hydroflow instance to a mutable variable flow––mutable because we will be changing its status when we run it.

use hydroflow::hydroflow_syntax;
pub fn main() {
    let mut flow = hydroflow_syntax! {
        source_iter(0..10) -> for_each(|n| println!("Hello {}", n));

Hydroflow surface syntax defines a "flow" consisting of operators connected via -> arrows. This simplest example uses a simple two-step linear flow. It starts with a source_iter operator that takes the Rust iterator 0..10 and iterates it to emit the numbers 0 through 9. That operator then passes those numbers along the -> arrow downstream to a for_each operator that invokes its closure argument to print each item passed in.

The Hydroflow surface syntax is merely a specification; it does not actually do anything until we run it. We run the flow from within Rust via the run_available() method.


Note that run_available() runs the Hydroflow graph until no more work is immediately available. In this example flow, running the graph drains the iterator completely, so no more work will ever be available. In future examples we will use external inputs such as network ingress, in which case more work might appear at any time.

A Note on Project Structure

The template project is intended to be a starting point for your own Hydroflow project, and you can add files and directories as you see fit. The only requirement is that the src/ file exists and contains a main() function.

In this simplest example we did not use a number of the files in the template: notably everything in the src/ subdirectory other than src/ If you'd like to delete those extraneous files you can do so, but it's not necessary, and we'll use them in subsequent examples.