<E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>
<A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>Transforms the failure channel of a Result, leaving the success channel unchanged.
When to use
Use to transform only the failure channel while preserving success values.
Details
- If the result is a
Failure, appliesfto the error and returns a newFailure - If the result is a
Success, returns it as-is
Example (Adding context to an error)
import { pipe, Result } from "effect"
const result = pipe(
Result.fail("not found"),
Result.mapError((e) => `Error: ${e}`)
)
console.log(result)
// Output: { _tag: "Failure", failure: "Error: not found", ... }export const const mapError: {
<E, E2>(f: (err: E) => E2): <A>(
self: Result<A, E>
) => Result<A, E2>
<A, E, E2>(
self: Result<A, E>,
f: (err: E) => E2
): Result<A, E2>
}
Transforms the failure channel of a Result, leaving the success channel unchanged.
When to use
Use to transform only the failure channel while preserving success values.
Details
- If the result is a
Failure, applies f to the error and returns a new Failure
- If the result is a
Success, returns it as-is
Example (Adding context to an error)
import { pipe, Result } from "effect"
const result = pipe(
Result.fail("not found"),
Result.mapError((e) => `Error: ${e}`)
)
console.log(result)
// Output: { _tag: "Failure", failure: "Error: not found", ... }
mapError: {
<function (type parameter) E in <E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>E, function (type parameter) E2 in <E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>E2>(f: (err: E) => E2f: (err: Eerr: function (type parameter) E in <E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>E) => function (type parameter) E2 in <E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>E2): <function (type parameter) A in <A>(self: Result<A, E>): Result<A, E2>A>(self: Result<A, E>self: 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) A in <A>(self: Result<A, E>): Result<A, E2>A, function (type parameter) E in <E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>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<function (type parameter) A in <A>(self: Result<A, E>): Result<A, E2>A, function (type parameter) E2 in <E, E2>(f: (err: E) => E2): <A>(self: Result<A, E>) => Result<A, E2>E2>
<function (type parameter) A in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E, function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E2>(self: Result<A, E>self: 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) A in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E>, f: (err: E) => E2f: (err: Eerr: function (type parameter) E in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E) => function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E2): 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) A in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>A, function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E2>
} = import dualdual(
2,
<function (type parameter) A in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E, function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E2>(self: Result<A, E>self: 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) A in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E>, f: (err: E) => E2f: (err: Eerr: function (type parameter) E in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E) => function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E2): 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) A in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>A, function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, f: (err: E) => E2): Result<A, E2>E2> =>
const isFailure: <A, E>(
self: Result<A, E>
) => self is Failure<A, E>
Checks whether a Result is a Failure.
When to use
Use to narrow a known Result to the Failure variant.
Details
- Acts as a TypeScript type guard, narrowing to
Failure<A, E>
- After narrowing, you can access
.failure to read the error value
Example (Narrowing to failure)
import { Result } from "effect"
const result = Result.fail("oops")
if (Result.isFailure(result)) {
console.log(result.failure)
// Output: "oops"
}
isFailure(self: Result<A, E>self) ? const fail: <E>(
left: E
) => Result<never, E>
Creates a Result holding a Failure value.
When to use
Use to represent a failed Result with a typed failure value.
Details
- The success type
A defaults to never
Example (Creating a failure)
import { Result } from "effect"
const result = Result.fail("Something went wrong")
console.log(Result.isFailure(result))
// Output: true
fail(f: (err: E) => E2f(self: Result<A, E>(parameter) self: {
_tag: "Failure";
_op: "Failure";
failure: E;
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;
}
Failure<A, E>.failure: Efailure)) : const succeed: <A>(right: A) => Result<A>Creates a Result holding a Success value.
Details
- Use when you have a value and want to lift it into the
Result type
- The error type
E defaults to never
Example (Wrapping a value)
import { Result } from "effect"
const result = Result.succeed(42)
console.log(Result.isSuccess(result))
// Output: true
succeed(self: Result<A, E>(parameter) self: {
_tag: "Success";
_op: "Success";
success: 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;
}
Success<A, E>.success: Asuccess)
)