<A>(predicate: (a: NoInfer<A>, i: number) => boolean): (
self: Iterable<A>
) => Option.Option<number>
<A>(
self: Iterable<A>,
predicate: (a: A, i: number) => boolean
): Option.Option<number>Returns the index of the last element matching the predicate, wrapped in an
Option.
When to use
Use to find the index of the last matching element from the end of an array.
Example (Finding the last matching index)
import { Array } from "effect"
console.log(Array.findLastIndex([1, 3, 8, 9], (x) => x < 5)) // Option.some(1)export const const findLastIndex: {
<A>(
predicate: (
a: NoInfer<A>,
i: number
) => boolean
): (self: Iterable<A>) => Option.Option<number>
<A>(
self: Iterable<A>,
predicate: (a: A, i: number) => boolean
): Option.Option<number>
}
Returns the index of the last element matching the predicate, wrapped in an
Option.
When to use
Use to find the index of the last matching element from the end of an array.
Example (Finding the last matching index)
import { Array } from "effect"
console.log(Array.findLastIndex([1, 3, 8, 9], (x) => x < 5)) // Option.some(1)
findLastIndex: {
<function (type parameter) A in <A>(predicate: (a: NoInfer<A>, i: number) => boolean): (self: Iterable<A>) => Option.Option<number>A>(predicate: (a: NoInfer<A>, i: number) => booleanpredicate: (a: NoInfer<A>a: type NoInfer<A> = [A][A extends any ? 0 : never]Prevents TypeScript from inferring a type parameter from a specific
position.
When to use
Use when a function parameter must match an inferred type without becoming
an inference source.
Details
The parameter using NoInfer must still match the inferred type.
Example (Controlling inference)
import type { Types } from "effect"
declare function withDefault<T>(value: T, fallback: Types.NoInfer<T>): T
// T is inferred as "a" | "b" from the first argument only
const result = withDefault<"a" | "b">("a", "b")
NoInfer<function (type parameter) A in <A>(predicate: (a: NoInfer<A>, i: number) => boolean): (self: Iterable<A>) => Option.Option<number>A>, i: numberi: number) => boolean): (self: Iterable<A>self: interface Iterable<T, TReturn = any, TNext = any>Iterable<function (type parameter) A in <A>(predicate: (a: NoInfer<A>, i: number) => boolean): (self: Iterable<A>) => Option.Option<number>A>) => import OptionOption.type Option<A> = Option.None<A> | Option.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<number>
<function (type parameter) A in <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean): Option.Option<number>A>(self: Iterable<A>self: interface Iterable<T, TReturn = any, TNext = any>Iterable<function (type parameter) A in <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean): Option.Option<number>A>, predicate: (a: A, i: number) => booleanpredicate: (a: Aa: function (type parameter) A in <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean): Option.Option<number>A, i: numberi: number) => boolean): import OptionOption.type Option<A> = Option.None<A> | Option.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<number>
} = dual<(...args: Array<any>) => any, <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean) => Option.Option<number>>(arity: 2, body: <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean) => Option.Option<number>): ((...args: Array<any>) => any) & (<A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean) => Option.Option<number>) (+1 overload)Creates a function that can be called in data-first style or data-last
(pipe-friendly) style.
When to use
Use to expose one implementation through both direct and pipe-friendly
call styles.
Details
Pass either the arity of the uncurried function or a predicate that decides
whether the current call is data-first. Arity is the common case. Use a
predicate when optional arguments make arity ambiguous.
Example (Selecting data-first or data-last style by arity)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(2, (self, that) => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Defining overloads with call signatures)
import { Function, pipe } from "effect"
const sum: {
(that: number): (self: number) => number
(self: number, that: number): number
} = Function.dual(2, (self: number, that: number): number => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Selecting data-first or data-last style with a predicate)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(
(args) => args.length === 2,
(self, that) => self + that
)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
dual(2, <function (type parameter) A in <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean): Option.Option<number>A>(self: Iterable<A>self: interface Iterable<T, TReturn = any, TNext = any>Iterable<function (type parameter) A in <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean): Option.Option<number>A>, predicate: (a: A, i: number) => booleanpredicate: (a: Aa: function (type parameter) A in <A>(self: Iterable<A>, predicate: (a: A, i: number) => boolean): Option.Option<number>A, i: numberi: number) => boolean): import OptionOption.type Option<A> = Option.None<A> | Option.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<number> => {
const const input: A[]input = const fromIterable: <A>(
collection: Iterable<A>
) => A[]
Converts an Iterable to an Array.
When to use
Use to convert any Iterable (Set, Generator, etc.) into an array.
Details
If the input is already an array, this returns it by reference without
copying. Otherwise, it creates a new array from the iterable. Use copy if
you need a fresh array even when the input is already an array.
Example (Converting a Set to an array)
import { Array } from "effect"
const result = Array.fromIterable(new Set([1, 2, 3]))
console.log(result) // [1, 2, 3]
fromIterable(self: Iterable<A>self)
for (let let i: numberi = const input: A[]input.Array<T>.length: numberGets or sets the length of the array. This is a number one higher than the highest index in the array.
length - 1; let i: numberi >= 0; let i: numberi--) {
if (predicate: (a: A, i: number) => booleanpredicate(const input: A[]input[let i: numberi], let i: numberi)) {
return import OptionOption.const some: <A>(value: A) => Option<A>Wraps the given value into an Option to represent its presence.
When to use
Use to wrap a known present value as Option
- Returning a successful result from a partial function
Details
- Always returns
Some<A>
- Does not filter
null or undefined; use
fromNullishOr
for that
Example (Wrapping a value)
import { Option } from "effect"
// ┌─── Option<number>
// ▼
const value = Option.some(1)
console.log(value)
// Output: { _id: 'Option', _tag: 'Some', value: 1 }
some(let i: numberi)
}
}
return import OptionOption.const none: <A = never>() => Option<A>Creates an Option representing the absence of a value.
When to use
Use to represent a missing or uninitialized value, such as returning "no
result" from a function.
Details
- Returns
Option<never>, which is a subtype of Option<A> for any A
- Always returns the same singleton instance
Example (Creating an empty Option)
import { Option } from "effect"
// ┌─── Option<never>
// ▼
const noValue = Option.none()
console.log(noValue)
// Output: { _id: 'Option', _tag: 'None' }
none()
})