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True Myth / result / Err

Interface: Err<T, E> ​

An Err instance is the failure variant instance of the Result type, representing a failure outcome from an operation which may fail. For a full discussion, see the module docs.

Extends ​

  • Omit<ResultImpl<T, E>, "value" | "cast">

Type Parameters ​

T ​

T

The type which would be wrapped in an Ok variant of Result.

E ​

E

The type wrapped in this Err variant of Result.

Properties ​

error ​

error: E

The wrapped error value.

Overrides ​

Omit.error

isErr ​

isErr: true

Is the Result an Err?

Overrides ​

Omit.isErr

isOk ​

isOk: false

Is the Result an Ok?

Overrides ​

Omit.isOk

variant ​

readonly variant: "Err"

Err is always Variant.Err.

Overrides ​

Omit.variant

Methods ​

and() ​

and<U>(mAnd): Result<U, E>

Method variant for and

Type Parameters ​

U ​

U

Parameters ​

mAnd ​

Result<U, E>

Returns ​

Result<U, E>

Inherited from ​

Omit.and

andThen() ​

Call Signature ​

andThen<U>(andThenFn): Result<U, E>

Method variant for andThen

Type Parameters ​
U ​

U

Parameters ​
andThenFn ​

(t) => Result<U, E>

Returns ​

Result<U, E>

Inherited from ​

Omit.andThen

Call Signature ​

andThen<R>(andThenFn): Result<OkFor<R>, E | ErrFor<R>>

Method variant for andThen

Type Parameters ​
R ​

R extends AnyResult

Parameters ​
andThenFn ​

(t) => R

Returns ​

Result<OkFor<R>, E | ErrFor<R>>

Inherited from ​

Omit.andThen

ap() ​

ap<A, B>(this, r): Result<B, E>

Method variant for ap

Type Parameters ​

A ​

A

B ​

B

Parameters ​

this ​

Result<(a) => B, E>

r ​

Result<A, E>

Returns ​

Result<B, E>

Inherited from ​

Omit.ap

cast() ​

cast<U>(): Result<U, E>

Type Parameters ​

U ​

U

Returns ​

Result<U, E>

equals() ​

equals(comparison): boolean

Method variant for equals

Parameters ​

comparison ​

Result<T, E>

Returns ​

boolean

Inherited from ​

Omit.equals

flatten() ​

flatten<A, F>(this): Result<A, E | F>

Given a nested Result, remove one layer of nesting.

For example, given a Result<Result<string, E2>, E1>, the resulting type after using this method will be Result<string, E1 | E2>.

Note ​

This method only works when the value wrapped in Result is another Result. If you have a Result<string, E> or Result<number, E>, this method won't work. If you have a Result<Result<string, E2>, E1>, then you can call .flatten() to get back a Result<string, E1 | E2>.

Examples ​

ts
import * as result from 'true-myth/result';

const nested = result.ok(result.ok('hello'));
const flattened = nested.flatten(); // Result<string, never>
console.log(flattened); // Ok('hello')

const nestedError = result.ok(result.err('inner error'));
const flattenedError = nestedError.flatten(); // Result<never, string>
console.log(flattenedError); // Err('inner error')

const errorNested = result.err<Result<string, string>, string>('outer error');
const flattenedOuter = errorNested.flatten(); // Result<string, string>
console.log(flattenedOuter); // Err('outer error')

Type Parameters ​

A ​

A

F ​

F

Parameters ​

this ​

Result<Result<A, F>, E>

Returns ​

Result<A, E | F>

Inherited from ​

Omit.flatten

inspect() ​

inspect(fn): Result<T, E>

Run a side effect with the wrapped value without modifying the Result.

This is useful for performing actions like logging, debugging, or other “side effects” external to the wrapped value. (Note: You should never mutate the value in the callback. Doing so will be extremely surprising to callers.) The function is only called if the Result is Ok, and the original Result is returned unchanged for further chaining.

ts
import * as result from 'true-myth/result';

const double = (n: number) => n * 2;
const log = (value: unknown) => console.log(value);

// Logs `42` then `84`, and returns `Ok(84)`.
result.ok<number, string>(42).inspect(log).map(double).inspect(log);

// Does not log anything, and returns `Err('error')`.
result.err<number, string>('error').inspect(log).map(double).inspect(log);

Parameters ​

fn ​

(value) => void

The function to call with the wrapped value, only called for Ok.

Returns ​

Result<T, E>

The original Result, unchanged

Inherited from ​

Omit.inspect

inspectErr() ​

inspectErr(fn): Result<T, E>

Run a side effect with a wrapped error value without modifying the Result.

This is useful for performing actions like logging, debugging, or other “side effects” external to the wrapped value. (Note: You should never mutate the value in the callback. Doing so will be extremely surprising to callers.) The function is only called if the Result is Err, and the original Result is returned unchanged for further chaining.

ts
import * as result from 'true-myth/result';

const logError = (error: unknown) => console.logError('Got error:', error);

result.err<number, string>('error')
  .inspectErr((error) => console.log('Got error:', error))
  .mapErr((e) => e.toUpperCase());
// Logs: "Got error: error"
// Returns: Err('ERROR')

result.ok<number, string>(42)
  .inspectErr((error) => console.log('Got error:', error))
  .mapErr((e) => e.toUpperCase());
// Logs nothing
// Returns: Ok(42)

Parameters ​

fn ​

(error) => void

The function to call with the error value, only called for Err.

Returns ​

Result<T, E>

The original Result, unchanged

Inherited from ​

Omit.inspectErr

map() ​

map<U>(mapFn): Result<U, E>

Method variant for map

Type Parameters ​

U ​

U

Parameters ​

mapFn ​

(t) => U

Returns ​

Result<U, E>

Inherited from ​

Omit.map

mapErr() ​

mapErr<F>(mapErrFn): Result<T, F>

Method variant for mapErr

Type Parameters ​

F ​

F

Parameters ​

mapErrFn ​

(e) => F

Returns ​

Result<T, F>

Inherited from ​

Omit.mapErr

mapOr() ​

mapOr<U>(orU, mapFn): U

Method variant for mapOr

Type Parameters ​

U ​

U

Parameters ​

orU ​

U

mapFn ​

(t) => U

Returns ​

U

Inherited from ​

Omit.mapOr

mapOrElse() ​

mapOrElse<U>(orElseFn, mapFn): U

Method variant for mapOrElse

Type Parameters ​

U ​

U

Parameters ​

orElseFn ​

(err) => U

mapFn ​

(t) => U

Returns ​

U

Inherited from ​

Omit.mapOrElse

match() ​

match<A>(matcher): A

Method variant for match

Type Parameters ​

A ​

A

Parameters ​

matcher ​

Matcher<T, E, A>

Returns ​

A

Inherited from ​

Omit.match

or() ​

or<F>(orResult): Result<T, F>

Method variant for or

Type Parameters ​

F ​

F

Parameters ​

orResult ​

Result<T, F>

Returns ​

Result<T, F>

Inherited from ​

Omit.or

orElse() ​

Call Signature ​

orElse<F>(orElseFn): Result<T, F>

Method variant for orElse

Type Parameters ​
F ​

F

Parameters ​
orElseFn ​

(err) => Result<T, F>

Returns ​

Result<T, F>

Inherited from ​

Omit.orElse

Call Signature ​

orElse<R>(orElseFn): Result<T | OkFor<R>, ErrFor<R>>

Method variant for orElse

Type Parameters ​
R ​

R extends AnyResult

Parameters ​
orElseFn ​

(err) => R

Returns ​

Result<T | OkFor<R>, ErrFor<R>>

Inherited from ​

Omit.orElse

toJSON() ​

toJSON(): Serialized<T, E>

Method variant for toJSON

Returns ​

Serialized<T, E>

Inherited from ​

Omit.toJSON

toString() ​

toString(): string

Method variant for toString

Returns ​

string

Inherited from ​

Omit.toString

unwrapOr() ​

unwrapOr<U>(defaultValue): T | U

Method variant for unwrapOr

Type Parameters ​

U ​

U = T

Parameters ​

defaultValue ​

U

Returns ​

T | U

Inherited from ​

Omit.unwrapOr

unwrapOrElse() ​

unwrapOrElse<U>(elseFn): T | U

Method variant for unwrapOrElse

Type Parameters ​

U ​

U

Parameters ​

elseFn ​

(error) => U

Returns ​

T | U

Inherited from ​

Omit.unwrapOrElse