<A, B, E = never>(f: (self: A) => Result<B, E>): (
self: Option<A>
) => Result<Option<B>, E>
<A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<
Option<B>,
E
>Maps an Option value with a Result-producing function, then transposes
the structure from Option<Result<B, E>> to Result<Option<B>, E>.
When to use
Use when an optional value should be validated only when present, preserving
absence as a successful None.
Details
NonebecomesSuccess(None)(the function is never called)Some(a)wheref(a)isSuccess(b)becomesSuccess(Some(b))Some(a)wheref(a)isFailure(e)becomesFailure(e)
Example (Mapping and transposing in one step)
import { Option, Result } from "effect"
const parse = (s: string) =>
isNaN(Number(s))
? Result.fail("not a number" as const)
: Result.succeed(Number(s))
console.log(Result.transposeMapOption(Option.some("42"), parse))
// Output: { _tag: "Success", success: { _tag: "Some", value: 42 }, ... }
console.log(Result.transposeMapOption(Option.none(), parse))
// Output: { _tag: "Success", success: { _tag: "None" }, ... }export const const transposeMapOption: (<
A,
B,
E = never
>(
f: (self: A) => Result<B, E>
) => (self: Option<A>) => Result<Option<B>, E>) &
(<A, B, E = never>(
self: Option<A>,
f: (self: A) => Result<B, E>
) => Result<Option<B>, E>)
Maps an Option value with a Result-producing function, then transposes
the structure from Option<Result<B, E>> to Result<Option<B>, E>.
When to use
Use when an optional value should be validated only when present, preserving
absence as a successful None.
Details
None becomes Success(None) (the function is never called)Some(a) where f(a) is Success(b) becomes Success(Some(b))Some(a) where f(a) is Failure(e) becomes Failure(e)
Example (Mapping and transposing in one step)
import { Option, Result } from "effect"
const parse = (s: string) =>
isNaN(Number(s))
? Result.fail("not a number" as const)
: Result.succeed(Number(s))
console.log(Result.transposeMapOption(Option.some("42"), parse))
// Output: { _tag: "Success", success: { _tag: "Some", value: 42 }, ... }
console.log(Result.transposeMapOption(Option.none(), parse))
// Output: { _tag: "Success", success: { _tag: "None" }, ... }
transposeMapOption = dual<<A, B, E = never>(f: (self: A) => Result<B, E>) => (self: Option<A>) => Result<Option<B>, E>, <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>) => Result<Option<B>, E>>(arity: 2, body: <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>) => Result<Option<B>, E>): (<A, B, E = never>(f: (self: A) => Result<B, E>) => (self: Option<A>) => Result<Option<B>, E>) & (<A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>) => Result<Option<B>, E>) (+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<
<function (type parameter) A in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>A, function (type parameter) B in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>B, function (type parameter) E in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>E = never>(
f: (self: A) => Result<B, E>f: (self: Aself: function (type parameter) A in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>A) => type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) B in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>B, function (type parameter) E in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>E>
) => (self: Option<A>self: 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, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>A>) => type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<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) B in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>B>, function (type parameter) E in <A, B, E = never>(f: (self: A) => Result<B, E>): (self: Option<A>) => Result<Option<B>, E>E>,
<function (type parameter) A in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>A, function (type parameter) B in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>B, function (type parameter) E in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>E = never>(
self: Option<A>self: 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, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>A>,
f: (self: A) => Result<B, E>f: (self: Aself: function (type parameter) A in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>A) => type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) B in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>B, function (type parameter) E in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>E>
) => type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<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) B in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>B>, function (type parameter) E in <A, B, E = never>(self: Option<A>, f: (self: A) => Result<B, E>): Result<Option<B>, E>E>
>(2, (self: Option<A>self, f: (self: A) => Result<B, E>f) => import option_option_.const isNone: <A>(
fa: Option.Option<A>
) => fa is Option.None<A>
self: Option<A>self) ? const succeedNone: Result<
Option<never>,
never
>
Provides a pre-built Result<Option<never>> that succeeds with None.
When to use
Use when an optional success should be absent, such as the None branch of
transposeOption or transposeMapOption.
Details
This is equivalent to Result.succeed(Option.none()), but reuses a shared
Success wrapper instead of allocating one each time.
Example (Succeeding with None)
import { Result } from "effect"
console.log(Result.isSuccess(Result.succeedNone))
// Output: true
succeedNone : const map: {
<A, A2>(f: (ok: A) => A2): <E>(
self: Result<A, E>
) => Result<A2, E>
<A, E, A2>(
self: Result<A, E>,
f: (ok: A) => A2
): Result<A2, E>
}
f: (self: A) => Result<B, E>f(self: Some<A>(parameter) self: {
_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), import option_option_.const some: <A>(
value: A
) => Option.Option<A>