The Filter Operator

The filter operator applies a supplied predicate to each element of a collection, and retains only those for which the predicate returns true.

As an example, we might select out those management relation where the manager has greater employee id than the managee, by writing

fn main() {
extern crate timely;
extern crate differential_dataflow;
use timely::dataflow::Scope;
use differential_dataflow::Collection;
use differential_dataflow::lattice::Lattice;
fn example<G: Scope>(manages: &Collection<G, (u64, u64)>)
where G::Timestamp: Lattice
        .filter(|&(m2, m1)| m2 > m1);

Rust makes it very clear when a method is provided with data, or only the ability to look at the data. The filter operator is only allowed to look at the data, which is where the & glyph comes from. This allows us to be more efficient in execution, but it is a subtle concept that further Rust reading may illuminate.