data-array.mdx

title	Working with Immutable Arrays using Data.array
id	data-array
skillLevel	beginner
applicationPatternId	core-concepts
summary	Use Data.array to create immutable, type-safe arrays that support value-based equality and safe functional operations.
tags	Data.array array structural-equality immutable data-type effect
rule	description Use Data.array to define arrays whose equality is based on their contents, enabling safe, predictable comparisons and functional operations.
related	use-chunk-for-high-performance-collections data-struct data-tuple
author	PaulJPhilp
lessonOrder	24

Working with Immutable Arrays using Data.array

Guideline

Use Data.array to create immutable, type-safe arrays that support value-based equality and safe functional operations.
This is useful for modeling ordered collections where immutability and structural equality are important.

Rationale

JavaScript arrays are mutable and compared by reference, which can lead to bugs in value-based logic and concurrent code.
Data.array provides immutable arrays with structural equality, making them ideal for functional programming and safe domain modeling.

Good Example

import { Data, Equal } from "effect";

// Create two structurally equal arrays
const arr1 = Data.array([1, 2, 3]);
const arr2 = Data.array([1, 2, 3]);

// Compare by value, not reference
const areEqual = Equal.equals(arr1, arr2); // true

// Use arrays as keys in a HashSet or Map
import { HashSet } from "effect";
const set = HashSet.make(arr1);
console.log(HashSet.has(set, arr2)); // true

// Functional operations (map, filter, etc.)
const doubled = arr1.map((n) => n * 2); // Data.array([2, 4, 6])

Explanation:

• Data.array creates immutable arrays with value-based equality.
• Useful for modeling ordered collections in a safe, functional way.
• Supports all standard array operations, but with immutability and structural equality.

Anti-Pattern

Using plain JavaScript arrays for value-based logic, as keys in sets/maps, or in concurrent code, which can lead to bugs due to mutability and reference-based comparison.