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); // trueWhy 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); // trueBest Practices
- Use Data.Class for value objects - Automatic Equal/Hash
- Implement both Equal and Hash together - Required for collections
- Use Order.mapInput for simple ordering - Cleaner than manual comparison
- Combine orders for complex sorting - More composable than custom compare
- 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