The Hash trait in Effect is closely tied to the Equal trait and serves a supportive role in optimizing equality checks by providing a mechanism for hashing. Hashing is a crucial step in the efficient determination of equality between two values, particularly when used with data structures like hash tables.

#Role of Hash in Equality Checking

The main function of the Hash trait is to provide a quick and efficient way to determine if two values are definitely not equal, thereby complementing the Equal trait. When two values implement the Equal trait, their hash values (computed using the Hash trait) are compared first:

• Different Hash Values: If the hash values are different, it is guaranteed that the values themselves are different. This quick check allows the system to avoid a potentially expensive equality check.
• Same Hash Values: If the hash values are the same, it does not guarantee that the values are equal, only that they might be. In this case, a more thorough comparison using the Equal trait is performed to determine actual equality.

This method dramatically speeds up the equality checking process, especially in collections where quick look-up and insertion times are crucial, such as in hash sets or hash maps.

#Practical Example and Explanation

Consider a scenario where you have a custom Person class, and you want to check if two instances are equal based on their properties. By implementing both the Equal and Hash traits, you can efficiently manage these checks:

ts
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

ts
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 snippet:

• The [Equal.symbol] method determines equality by comparing the id, name, and age fields of Person instances. This approach ensures that the equality check is comprehensive and considers all relevant attributes.
• The [Hash.symbol] method computes a hash code using the id of the person. This value is used to quickly differentiate between instances in hashing operations, optimizing the performance of data structures that utilize hashing.
• 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.