Traits in Effect

Overview

Effect provides trait-based abstractions for:

• Equal – Structural equality comparison
• Hash – Hash code generation
• Equivalence – Custom equality relations
• Order – Ordering and comparison

These enable consistent behavior across Effect’s data structures.

Equal – Structural Equality

The Equal Interface

import { Equal } from "effect"

Equal.equals(a, b)

import { Data } from "effect"

class Person extends Data.Class<{
  name: string
  age: number
}> {}

const p1 = new Person({ name: "Alice", age: 30 })
const p2 = new Person({ name: "Alice", age: 30 })

Equal.equals(p1, p2)
p1 === p2

Implementing Equal (Recommended: Use Data.Class)

PREFER Data.Class which provides Equal and Hash automatically:

import { Data, Equal } from "effect"

// Data.Class provides Equal and Hash automatically
class Point extends Data.Class<{ readonly x: number; readonly y: number }> {}

const p1 = new Point({ x: 1, y: 2 })
const p2 = new Point({ x: 1, y: 2 })

Equal.equals(p1, p2)

For Schema types, use Schema.Class which also provides Equal:

import { Schema, Equal } from "effect"

class Point extends Schema.Class<Point>("Point")({
  x: Schema.Number,
  y: Schema.Number
}) {}

const p1 = new Point({ x: 1, y: 2 })
const p2 = new Point({ x: 1, y: 2 })

Equal.equals(p1, p2) // true

Why Equal Matters

Effect’s collections (HashMap, HashSet) use Equal for lookups:

import { HashMap, Data } from "effect"

class UserId extends Data.Class<{ id: string }> {}

const map = HashMap.make([
  [new UserId({ id: "1" }), "Alice"],
  [new UserId({ id: "2" }), "Bob"]
])

// Works because UserId implements Equal
HashMap.get(map, new UserId({ id: "1" })) // Option.some("Alice")

Hash – Hash Code Generation

The Hash Interface

import { Hash } from "effect"

Hash.hash(value)

Hash.string("hello")
Hash.number(42)
Hash.combine(h1)(h2)
Hash.array([1, 2, 3])

Hash Contract

Objects that are Equal MUST have the same hash:

// If Equal.equals(a, b) === true
// Then Hash.hash(a) === Hash.hash(b)

// The reverse is NOT required:
// Same hash does NOT mean equal (hash collisions are allowed)

Hash with Data.Class (Recommended)

Data.Class provides both Equal and Hash automatically:

import { Data, Hash, Equal } from "effect"

class Rectangle extends Data.Class<{
  readonly width: number
  readonly height: number
}> {}

const r1 = new Rectangle({ width: 10, height: 5 })
const r2 = new Rectangle({ width: 10, height: 5 })

Equal.equals(r1, r2)
Hash.hash(r1) === Hash.hash(r2)

Equivalence – Custom Equality Relations

Creating Equivalences

import { Equivalence } from "effect"

const caseInsensitive = Equivalence.make<string>(
  (a, b) => a.toLowerCase() === b.toLowerCase()
)

caseInsensitive("Hello", "hello")
caseInsensitive("Hello", "HELLO")

Equivalence.string
Equivalence.number
Equivalence.boolean
Equivalence.strict

Transforming Equivalences

// Map to compare by derived value
const byLength = Equivalence.string.pipe(
  Equivalence.mapInput((s: string) => s.length.toString())
)

// Actually, better approach:
const byLength = Equivalence.make<string>(
  (a, b) => a.length === b.length
)

// Combine equivalences
const userEquivalence = Equivalence.make<User>(
  (a, b) =>
    Equivalence.string(a.name, b.name) &&
    Equivalence.number(a.age, b.age)
)

Using with Collections

import { Array as Arr } from "effect"

const users = [
  { name: "Alice", age: 30 },
  { name: "Bob", age: 25 },
  { name: "alice", age: 30 } // Same as Alice?
]

// Dedupe with custom equivalence
const byNameIgnoreCase = Equivalence.make<User>(
  (a, b) => a.name.toLowerCase() === b.name.toLowerCase()
)

const unique = Arr.dedupe(users) // Uses default equality
const uniqueByName = Arr.dedupeWith(users, byNameIgnoreCase)

Order – Comparison and Sorting

The Order Interface

import { Order } from "effect"

Order.string("a", "b")
Order.string("b", "a")
Order.string("a", "a")

Order.lessThan(Order.number)(5, 10)
Order.greaterThan(Order.number)(5, 10)
Order.lessThanOrEqualTo(Order.number)(5, 5)
Order.between(Order.number)({ minimum: 0, maximum: 10 })(5)

Order.min(Order.number)(3, 7)
Order.max(Order.number)(3, 7)

Order.clamp(Order.number)({ minimum: 0, maximum: 100 })(150)

Built-in Orders

Order.string
Order.number
Order.boolean
Order.bigint
Order.Date

Creating Custom Orders

const byAge = Order.make<Person>((a, b) =>
  a.age < b.age ? -1 : a.age > b.age ? 1 : 0
)

const byName = Order.mapInput(Order.string, (p: Person) => p.name)
const byAge = Order.mapInput(Order.number, (p: Person) => p.age)

Combining Orders

const byAgeThenName = Order.combine(
  byAge,
  byName
)

const byAgeDesc = Order.reverse(byAge)

const sortUsers = Order.combine(
  Order.mapInput(Order.string, (u: User) => u.department),
  Order.combine(
    Order.reverse(Order.mapInput(Order.number, (u: User) => u.salary)),
    Order.mapInput(Order.string, (u: User) => u.name)
  )
)

Sorting with Order

import { Array as Arr } from "effect"

const people = [
  { name: "Charlie", age: 35 },
  { name: "Alice", age: 30 },
  { name: "Bob", age: 30 }
]

const byAge = Arr.sort(people, Order.mapInput(Order.number, (p) => p.age))

const sorted = Arr.sort(people, Order.combine(
  Order.mapInput(Order.number, (p) => p.age),
  Order.mapInput(Order.string, (p) => p.name)
))

Data.Class – Automatic Implementation

Using Data.Class automatically implements Equal and Hash:

import { Data, Equal, Hash } from "effect"

class User extends Data.Class<{
  id: string
  name: string
  email: string
}> {}

const u1 = new User({ id: "1", name: "Alice", email: "a@example.com" })
const u2 = new User({ id: "1", name: "Alice", email: "a@example.com" })

Equal.equals(u1, u2) // true
Hash.hash(u1) === Hash.hash(u2) // true

Best Practices

1. Use Data.Class for value objects – Automatic Equal/Hash
2. Implement both Equal and Hash together – Required for collections
3. Use Order.mapInput for simple ordering – Cleaner than manual comparison
4. Combine orders for complex sorting – More composable than custom compare
5. Consider Equivalence for custom equality – When default equality isn’t enough

Additional Resources

For comprehensive trait documentation, consult ${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt.

Search for these sections:

• “Equal” for equality comparison
• “Hash” for hash code generation
• “Equivalence” for custom equality relations
• “Order” for ordering and comparison