data-hashset.mdx

title	Work with Immutable Sets using HashSet
id	data-hashset
skillLevel	intermediate
applicationPatternId	core-concepts
summary	Use HashSet<A> to model immutable, high-performance sets for efficient membership checks and set operations.
tags	HashSet set collection immutable data-type effect
rule	description Use HashSet to represent sets of unique values with efficient, immutable operations for membership, union, intersection, and difference.
related	use-chunk-for-high-performance-collections data-array
author	PaulJPhilp
lessonOrder	23

Work with Immutable Sets using HashSet

Guideline

Use the HashSet<A> data type to represent sets of unique values with efficient, immutable operations.
HashSet is ideal for membership checks, set algebra, and modeling collections where uniqueness matters.

Rationale

HashSet provides high-performance, immutable set operations that are safe for concurrent and functional programming.
It avoids the pitfalls of mutable JavaScript Set and is optimized for use in Effect workflows.

Good Example

import { HashSet } from "effect";

// Create a HashSet from an array
const setA = HashSet.fromIterable([1, 2, 3]);
const setB = HashSet.fromIterable([3, 4, 5]);

// Membership check
const hasTwo = HashSet.has(setA, 2); // true

// Union, intersection, difference
const union = HashSet.union(setA, setB); // HashSet {1, 2, 3, 4, 5}
const intersection = HashSet.intersection(setA, setB); // HashSet {3}
const difference = HashSet.difference(setA, setB); // HashSet {1, 2}

// Add and remove elements
const withSix = HashSet.add(setA, 6); // HashSet {1, 2, 3, 6}
const withoutOne = HashSet.remove(setA, 1); // HashSet {2, 3}

Explanation:

• HashSet is immutable and supports efficient set operations.
• Use it for membership checks, set algebra, and modeling unique collections.
• Safe for concurrent and functional workflows.

Anti-Pattern

Using mutable JavaScript Set for shared or concurrent data, or for set operations in functional code, which can lead to bugs and unpredictable behavior.