Simple Example

In this example we will cover some additional standard Hydroflow operators:

Lets build on the simplest example to explore some of the operators available in Hydroflow. You may be familiar with operators such as map(...), filter(...), flatten(...), etc. from Rust iterators or from other programming languages, and these are also available in Hydroflow.

In your simple project, replace the contents of src/main.rs with the following:

use hydroflow::hydroflow_syntax;

pub fn main() {
    let mut flow = hydroflow_syntax! {
        source_iter(0..10)
            -> map(|n| n * n)
            -> filter(|&n| n > 10)
            -> map(|n| (n..=n+1))
            -> flatten()
            -> for_each(|n| println!("Howdy {}", n));
    };

    flow.run_available();
}

Let's take this one operator at a time, starting after the source_iter operator we saw in the previous example.

  • -> map(|n| n * n) transforms each element individually as it flows through the subgraph. In this case, we square each number.

  • Next, -> filter(|&n| n > 10) only passes along squared numbers that are greater than 10.

  • The subsequent -> map(|n| (n..=n+1)) uses standard Rust syntax to convert each number n into a RangeInclusive [n, n+1].

  • The -> flatten() operator converts the ranges back into a stream of the individual numbers which they contain.

  • Finally we use the now-familiar for_each operator to print each number.

Now let's run the program:

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

Rewriting with Composite Operators

We can also express the same program with more aggressive use of composite operators like filter_map() and flat_map(). Hydroflow will compile these down to the same machine code.

Replace the contents of src/main.rs with the following:

use hydroflow::hydroflow_syntax;
 pub fn main() {
    let mut flow = hydroflow_syntax! {
        source_iter(0..10)
        -> filter_map(|n| {
            let n2 = n * n;
            if n2 > 10 {
                Some(n2)
            }
            else {
                None
            }
        })
        -> flat_map(|n| (n..=n+1))
        -> for_each(|n| println!("G'day {}", n));
    };

    flow.run_available();
}

Results:

% cargo run
<build output>
G'day 16
G'day 17
G'day 25
G'day 26
G'day 36
G'day 37
G'day 49
G'day 50
G'day 64
G'day 65
G'day 81
G'day 82