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Previous sections show how to add and remove an element from a given set. The function Set.update can do both depending on a boolean value: if true, then the given value will be added to the set, otherwise it will be removed (if present).

const nats: set<int> = Set.literal([3, 2, 2, 1]);
const set_with_5 = Set.update(5, true, nats);
const set_without_3 = Set.update(3, false, nats);

The function Set.update implements a one-value update. Sometime we would like to provide a function that is applied in turn to all the elements of the set, and specifies whether the element at hand has to be discarded or replaced by a computed value. This is what Set.filter_map does.

As an example, let us consider a function that removes all the even numbers from a set.

const f = x => x % 2 == 0n ? None() : Some(x);
// odds == Set.literal([3, 1])
const odds = Set.filter_map(f, nats);

Note: See the predefined namespace Set