Equivalence

Equivalence works in Effeck exactly as in Effect. It packages a reusable rule for deciding whether two values count as equivalent in a particular operation, without changing their global equality behavior.

Representing a reusable equality relation

const sameLabel: Equivalence.Equivalence<string> = (a, b) =>
  a.trim().toLowerCase() === b.trim().toLowerCase()

sameLabel(" Inbox ", "inbox") // true
sameLabel("inbox", "archive") // false

An Equivalence<A> is a synchronous function from two A values to a boolean. A sound relation is reflexive, symmetric, and transitive. Keep it free of service calls and expected failures so Effect can invoke it as an ordinary JavaScript callback.

Choosing built-in relations

Equivalence.String("cyan", "cyan") // true
Equivalence.Number(12, 13) // false
Equivalence.Boolean(true, true) // true
Equivalence.BigInt(5n, 5n) // true
Equivalence.Date(new Date("2026-04-12"), new Date("2026-04-12")) // true

const sameSymbol = Equivalence.strictEqual<symbol>()
const key = Symbol("entry")
sameSymbol(key, key) // true
sameSymbol(key, Symbol("entry")) // false

The string, boolean, bigint, and strictEqual relations use strict equality. Equivalence.Number also equates NaN with itself. The date relation compares timestamps using that numeric relation instead of comparing object references.

Deriving comparisons through input projections

interface Product {
  sku: string
  displayName: string
}

const bySku = Equivalence.mapInput(
  Equivalence.String,
  (product: Product) => product.sku
)

bySku(
  { sku: "part-4", displayName: "Bolt" },
  { sku: "part-4", displayName: "Steel bolt" }
) // true

mapInput projects each operand before applying the underlying relation. Here the SKU determines equivalence and the display name is ignored. Pass this rule to an API that accepts an equivalence; it does not alter Equal.equals, JavaScript ===, or hash-collection key behavior.