<A>(isEquivalent: Equivalence.Equivalence<A>): Equivalence.Equivalence<
Option<A>
>Creates an Equivalence for Option<A> from an Equivalence for A.
When to use
Use when you need equality to treat two None values as equal and compare
two Some values with a supplied equality rule.
Details
NonevsNone→trueSomevsNone(or vice versa) →falseSome(a)vsSome(b)→ delegates to the providedEquivalence
Example (Comparing Options)
import { Equivalence, Option } from "effect"
const eq = Option.makeEquivalence(Equivalence.strictEqual<number>())
console.log(eq(Option.some(1), Option.some(1)))
// Output: true
console.log(eq(Option.some(1), Option.some(2)))
// Output: false
console.log(eq(Option.none(), Option.none()))
// Output: trueexport const const makeEquivalence: <A>(
isEquivalent: Equivalence.Equivalence<A>
) => Equivalence.Equivalence<Option<A>>
Creates an Equivalence for Option<A> from an Equivalence for A.
When to use
Use when you need equality to treat two None values as equal and compare
two Some values with a supplied equality rule.
Details
None vs None → trueSome vs None (or vice versa) → falseSome(a) vs Some(b) → delegates to the provided Equivalence
Example (Comparing Options)
import { Equivalence, Option } from "effect"
const eq = Option.makeEquivalence(Equivalence.strictEqual<number>())
console.log(eq(Option.some(1), Option.some(1)))
// Output: true
console.log(eq(Option.some(1), Option.some(2)))
// Output: false
console.log(eq(Option.none(), Option.none()))
// Output: true
makeEquivalence = <function (type parameter) A in <A>(isEquivalent: Equivalence.Equivalence<A>): Equivalence.Equivalence<Option<A>>A>(isEquivalent: Equivalence.Equivalence<A>isEquivalent: import EquivalenceEquivalence.type Equivalence<in A> = (
self: A,
that: A
) => boolean
Represents an equivalence relation over type A.
When to use
Use as a type annotation when you accept or return an equivalence function.
Details
- Returns
boolean: true if values are equivalent, false otherwise
- Must satisfy reflexive, symmetric, and transitive properties
Example (Defining simple number equivalence)
import type { Equivalence } from "effect"
const numberEq: Equivalence.Equivalence<number> = (a, b) => a === b
console.log(numberEq(1, 1)) // true
console.log(numberEq(1, 2)) // false
Example (Defining custom object equivalence)
import type { Equivalence } from "effect"
interface Point {
x: number
y: number
}
const pointEq: Equivalence.Equivalence<Point> = (a, b) =>
a.x === b.x && a.y === b.y
console.log(pointEq({ x: 1, y: 2 }, { x: 1, y: 2 })) // true
Equivalence<function (type parameter) A in <A>(isEquivalent: Equivalence.Equivalence<A>): Equivalence.Equivalence<Option<A>>A>): import EquivalenceEquivalence.type Equivalence<in A> = (
self: A,
that: A
) => boolean
Represents an equivalence relation over type A.
When to use
Use as a type annotation when you accept or return an equivalence function.
Details
- Returns
boolean: true if values are equivalent, false otherwise
- Must satisfy reflexive, symmetric, and transitive properties
Example (Defining simple number equivalence)
import type { Equivalence } from "effect"
const numberEq: Equivalence.Equivalence<number> = (a, b) => a === b
console.log(numberEq(1, 1)) // true
console.log(numberEq(1, 2)) // false
Example (Defining custom object equivalence)
import type { Equivalence } from "effect"
interface Point {
x: number
y: number
}
const pointEq: Equivalence.Equivalence<Point> = (a, b) =>
a.x === b.x && a.y === b.y
console.log(pointEq({ x: 1, y: 2 }, { x: 1, y: 2 })) // true
Equivalence<type Option<A> = None<A> | Some<A>The Option data type represents optional values. An Option<A> is either
Some<A>, containing a value of type A, or None, representing absence.
When to use
Use to represent initial values that may not yet exist
- Returning from partial functions (not defined for all inputs)
- Managing optional fields in data structures
Namespace containing utility types for Option.
When to use
Use to access type-level helpers associated with Option.
Option<function (type parameter) A in <A>(isEquivalent: Equivalence.Equivalence<A>): Equivalence.Equivalence<Option<A>>A>> =>
import EquivalenceEquivalence.const make: <A>(
isEquivalent: (self: A, that: A) => boolean
) => Equivalence<A>
Creates a custom equivalence relation with an optimized reference equality check.
When to use
Use when you need an equality rule that the built-in instances and input
mapping helpers cannot express, and you can provide a law-abiding comparison.
Details
The returned equivalence first checks reference equality (===) for
performance. If the values are not the same reference, it falls back to the
provided equivalence function, which must satisfy reflexive, symmetric, and
transitive properties.
Example (Case-insensitive string equivalence)
import { Equivalence } from "effect"
const caseInsensitive = Equivalence.make<string>((a, b) =>
a.toLowerCase() === b.toLowerCase()
)
console.log(caseInsensitive("Hello", "HELLO")) // true
console.log(caseInsensitive("foo", "bar")) // false
// Same reference optimization
const str = "test"
console.log(caseInsensitive(str, str)) // true (fast path)
Example (Comparing numbers with tolerance)
import { Equivalence } from "effect"
const tolerance = Equivalence.make<number>((a, b) => Math.abs(a - b) < 0.0001)
console.log(tolerance(1.0, 1.001)) // false
console.log(tolerance(1.0, 1.00001)) // true
make((x: Option<A>x, y: Option<A>y) => const isNone: <A>(
self: Option<A>
) => self is None<A>
Checks whether an Option is None (absent).
When to use
Use when you need to branch on an absent Option before accessing .value.
Details
- Acts as a type guard, narrowing to
None<A>
Example (Checking for None)
import { Option } from "effect"
console.log(Option.isNone(Option.some(1)))
// Output: false
console.log(Option.isNone(Option.none()))
// Output: true
isNone(x: Option<A>x) ? const isNone: <A>(
self: Option<A>
) => self is None<A>
Checks whether an Option is None (absent).
When to use
Use when you need to branch on an absent Option before accessing .value.
Details
- Acts as a type guard, narrowing to
None<A>
Example (Checking for None)
import { Option } from "effect"
console.log(Option.isNone(Option.some(1)))
// Output: false
console.log(Option.isNone(Option.none()))
// Output: true
isNone(y: Option<A>y) : const isNone: <A>(
self: Option<A>
) => self is None<A>
Checks whether an Option is None (absent).
When to use
Use when you need to branch on an absent Option before accessing .value.
Details
- Acts as a type guard, narrowing to
None<A>
Example (Checking for None)
import { Option } from "effect"
console.log(Option.isNone(Option.some(1)))
// Output: false
console.log(Option.isNone(Option.none()))
// Output: true
isNone(y: Option<A>y) ? false : isEquivalent: Equivalence.Equivalence<A>isEquivalent(x: Some<A>(parameter) x: {
_tag: "Some";
_op: "Some";
value: A;
valueOrUndefined: A;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
toString: () => string;
toJSON: () => unknown;
}
Some<A>.value: Avalue, y: Some<A>(parameter) y: {
_tag: "Some";
_op: "Some";
value: A;
valueOrUndefined: A;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
toString: () => string;
toJSON: () => unknown;
}
Some<A>.value: Avalue))