Equal

The Equal module provides a simple and convenient way to define and check for equality between two values in TypeScript.

Here are some key reasons why Effect exports an Equal module:

1. Value-Based Equality: JavaScript’s native equality operators (=== and ==) check for equality by reference, meaning they compare objects based on their memory addresses rather than their content. This behavior can be problematic when you want to compare objects with the same values but different references. The Equal module offers a solution by allowing developers to define custom equality checks based on the values of objects.

2. Custom Equality: The Equal module enables developers to implement custom equality checks for their data types and classes. This is crucial when you have specific requirements for determining when two objects should be considered equal. By implementing the Equal interface, developers can define their own equality logic.

3. Data Integrity: In some applications, maintaining data integrity is crucial. The ability to perform value-based equality checks ensures that identical data is not duplicated within collections like sets or maps. This can lead to more efficient memory usage and more predictable behavior.

4. Predictable Behavior: The Equal module promotes more predictable behavior when comparing objects. By explicitly defining equality criteria, developers can avoid unexpected results that may occur with JavaScript’s default reference-based equality checks.

How to Perform Equality Checking in Effect

In Effect it’s advisable to stop using JavaScript’s === and == operators and instead rely on the Equal.equals function. This function can work with any data type that implements the Equal interface. Some examples of such data types include Option, Either, HashSet, and HashMap.

When you use Equal.equals and your objects do not implement the Equal interface, it defaults to using the === operator for object comparison:

Example (Using Equal.equals with Default Comparison)

import { Equal } from "effect"

// Two objects with identical properties and values
const a = { name: "Alice", age: 30 }
const b = { name: "Alice", age: 30 }

// Equal.equals falls back to the default '===' comparison
console.log(Equal.equals(a, b))
// Output: false

In this example, a and b are two separate objects with the same contents. However, === considers them different because they occupy different memory locations. This behavior can lead to unexpected results when you want to compare values based on their content.

However, you can configure your models to ensure that Equal.equals behaves consistently with your custom equality checks. There are two alternative approaches:

1. Implementing the Equal Interface: This method is useful when you need to define your custom equality check.

2. Using the Data Module: For simple value equality, the Data module provides a more straightforward solution by automatically generating default implementations for Equal.

Let’s explore both.

Implementing the Equal Interface

To create custom equality behavior, you can implement the Equal interface in your models. This interface extends the Hash interface from the Hash module.

Example (Implementing Equal and Hash for a Custom Class)

import { Equal, Hash } from "effect"

class Person implements Equal.Equal {
  constructor(
    readonly id: number, // Unique identifier
    readonly name: string,
    readonly age: number
  ) {}

  // Define equality based on id, name, and age
  [Equal.symbol](that: Equal.Equal): boolean {
    if (that instanceof Person) {
      return (
        Equal.equals(this.id, that.id) &&
        Equal.equals(this.name, that.name) &&
        Equal.equals(this.age, that.age)
      )
    }
    return false
  }

  // Generate a hash code based on the unique id
  [Hash.symbol](): number {
    return Hash.hash(this.id)
  }
}

In the above code, we define a custom equality function [Equal.symbol] and a hash function [Hash.symbol] for the Person class. The Hash interface optimizes equality checks by comparing hash values instead of the objects themselves. When you use the Equal.equals function to compare two objects, it first checks if their hash values are equal. If not, it quickly determines that the objects are not equal, avoiding the need for a detailed property-by-property comparison.

Once you’ve implemented the Equal interface, you can utilize the Equal.equals function to check for equality using your custom logic.

Example (Comparing Person Instances)

import { Equal, Hash } from "effect"

class Person implements Equal.Equal {
  constructor(
    readonly id: number, // Unique identifier for each person
    readonly name: string,
    readonly age: number
  ) {}

  // Defines equality based on id, name, and age
  [Equal.symbol](that: Equal.Equal): boolean {
    if (that instanceof Person) {
      return (
        Equal.equals(this.id, that.id) &&
        Equal.equals(this.name, that.name) &&
        Equal.equals(this.age, that.age)
      )
    }
    return false
  }

  // Generates a hash code based primarily on the unique id
  [Hash.symbol](): number {
    return Hash.hash(this.id)
  }
}

const alice = new Person(1, "Alice", 30)
console.log(Equal.equals(alice, new Person(1, "Alice", 30)))
// Output: true

const bob = new Person(2, "Bob", 40)
console.log(Equal.equals(alice, bob))
// Output: false

In this code, the equality check returns true when comparing alice to a new Person object with identical property values and false when comparing alice to bob due to their differing property values.

Simplifying Equality with the Data Module

Implementing both Equal and Hash can become cumbersome when all you need is straightforward value equality checks. Luckily, the Data module provides a simpler solution. It offers APIs that automatically generate default implementations for both Equal and Hash.

Example (Using Data.struct for Equality Checks)

import { Equal, Data } from "effect"

const alice = Data.struct({ id: 1, name: "Alice", age: 30 })

const bob = Data.struct({ id: 2, name: "Bob", age: 40 })

console.log(
  Equal.equals(alice, Data.struct({ id: 1, name: "Alice", age: 30 }))
)
// Output: true

console.log(Equal.equals(alice, { id: 1, name: "Alice", age: 30 }))
// Output: false

console.log(Equal.equals(alice, bob))
// Output: false

In this example, we use the Data.struct function to create structured data objects and check their equality using Equal.equals. The Data module simplifies the process by providing a default implementation for both Equal and Hash, allowing you to focus on comparing values without the need for explicit implementations.

The Data module isn’t limited to just structs. It can handle various data types, including tuples, arrays, and records. If you’re curious about how to leverage its full range of features, you can explore the Data module documentation.

Working with Collections

JavaScript’s built-in Set and Map can be a bit tricky when it comes to checking equality:

Example (Native Set with Reference-Based Equality)

const set = new Set()

// Adding two objects with the same content to the set
set.add({ name: "Alice", age: 30 })
set.add({ name: "Alice", age: 30 })

// Even though the objects have identical values, they are treated
// as different elements because JavaScript compares objects by reference,
// not by value.
console.log(set.size)
// Output: 2

Even though the two elements in the set have the same values, the set contains two elements. Why? JavaScript’s Set checks for equality by reference, not by values.

To perform value-based equality checks, you’ll need to use the Hash* collection types available in the effect package. These collection types, such as HashSet and HashMap, provide support for the Equal interface.

HashSet

When you use the HashSet, it correctly handles value-based equality checks. In the following example, even though you’re adding two objects with the same values, the HashSet treats them as a single element.

Example (Using HashSet for Value-Based Equality)

import { HashSet, Data } from "effect"

// Creating a HashSet with objects that implement the Equal interface
const set = HashSet.empty().pipe(
  HashSet.add(Data.struct({ name: "Alice", age: 30 })),
  HashSet.add(Data.struct({ name: "Alice", age: 30 }))
)

// HashSet recognizes them as equal, so only one element is stored
console.log(HashSet.size(set))
// Output: 1

Note: It’s crucial to use elements that implement the Equal interface, either by implementing custom equality checks or by using the Data module. This ensures proper functionality when working with HashSet. Without this, you’ll encounter the same behavior as the native Set data type:

Example (Reference-Based Equality in HashSet)

import { HashSet } from "effect"

// Creating a HashSet with objects that do NOT implement
// the Equal interface
const set = HashSet.empty().pipe(
  HashSet.add({ name: "Alice", age: 30 }),
  HashSet.add({ name: "Alice", age: 30 })
)

// Since these objects are compared by reference,
// HashSet considers them different
console.log(HashSet.size(set))
// Output: 2

In this case, without using the Data module alongside HashSet, you’ll experience the same behavior as the native Set data type. The set contains two elements because it checks for equality by reference, not by values.

HashMap

When working with the HashMap, you have the advantage of comparing keys by their values instead of their references. This is particularly helpful in scenarios where you want to associate values with keys based on their content.

Example (Value-Based Key Comparisons with HashMap)

import { HashMap, Data } from "effect"

// Adding two objects with identical values as keys
const map = HashMap.empty().pipe(
  HashMap.set(Data.struct({ name: "Alice", age: 30 }), 1),
  HashMap.set(Data.struct({ name: "Alice", age: 30 }), 2)
)

console.log(HashMap.size(map))
// Output: 1

// Retrieve the value associated with a key
console.log(HashMap.get(map, Data.struct({ name: "Alice", age: 30 })))
/*
Output:
{ _id: 'Option', _tag: 'Some', value: 2 }
*/

In this code snippet, HashMap is used to create a map where the keys are objects constructed with Data.struct. These objects contain identical values, which would usually create separate entries in a regular JavaScript Map because the default comparison is reference-based.

HashMap, however, uses value-based comparison, meaning the two objects with identical content are treated as the same key. Thus, when we add both objects, the second key-value pair overrides the first, resulting in a single entry in the map.