Result Monad - monad represents some result in different states - Initial, Pending, Success and Failure.

Inspired by RemoteData type, but depends not on fp-ts but on smaller @sweet-monads

Explanation

The problem it solving present is a very common one. You are loading a list of things but instead of showing a loading indicator you just see zero items. Same happens with actual data loading - sometimes you just do not think it will fail.

In my mental model, REST requests have one of four states:

• We haven't asked yet.
• We've asked, but we haven't got a response yet.
• We got a response, but it was an error.
• We got a response, and it was the data we wanted.

That is the purpose of this library - allow clean usage over this states.

Better explanation your can read in the artice How Elm Slays a UI Antipattern

Installation

npm install @lonli-lokli/ts-result

Usage

import { Result, success } from "@lonli-lokli/ts-result";

class UserNotFoundError extends Error {
  name: "UserNotFoundError";
}
type User = { email: string; password: string };

function getUser(id: number): Result<UserNotFoundError, User> {
  return success({ email: "test@gmail.com", password: "test" });
}
// Result<UserNotFoundError, string>
const user = getUser(1).map(({ email }) => email);

API

• chain
• merge
• mergeWithConfig
• mergeInOne
• mergeInMany
• initial
• pending
• failure
• success
• from
• fromPromise
• fromTry
• fromMaybe
• fromEither
• fromNullable
• isResult
• Result#isInitial
• Result#isPending
• Result#isFailure
• Result#isSuccess
• Result#equal
• Result#or
• Result#join
• Result#map
• Result#mapSuccess
• Result#mapFailure
• Result#asyncMap
• Result#apply
• Result#asyncApply
• Result#chain
• Result#asyncChain
• Result#toEither
• Result#toMaybe
• Result#toNullable
• Result#toUndefined
• Result#unwrap
• Result#unwrapOr
• Result#unwrapOrElse
• Result#fold
• Result#filter
• Result#filtermap
• Result#tap
• Result#tapFailure
• Result#recover
• Result#recoverWith
• Result#zip
• Result#zipWith
• Result#biMap
• Helpers

chain

function chain<F, S, NF, NS>(fn: (val: S) => Result<NF, NS>): Result<F | NF, NS>;

• Returns a new Result by applying fn to the Success value of this Result
• State handling priority:
  1. If this Result is Initial, returns Initial
  2. If the next Result (returned by fn) is Initial, returns Initial
  3. If this Result is Pending or the next Result is Pending, returns Pending
  4. If this Result is Failure, returns Failure with current value
  5. If the next Result is Failure, returns Failure with next value
  6. Otherwise returns the next Result

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());
const v3 = initial;

// Result<Error | TypeError, string>.Success with value "2"
const newVal1 = v1.chain(a => success<TypeError, string>(a.toString()));

// Result<Error | TypeError, string>.Failure with value new TypeError()
const newVal2 = v1.chain(a => failure<TypeError, string>(new TypeError()));

// Result<Error | TypeError, string>.Failure with value new Error()
const newVal3 = v2.chain(a => success<TypeError, string>(a.toString()));

// Result<Error | TypeError, string>.Failure with value new Error()
const newVal4 = v2.chain(a => failure<TypeError, string>(new TypeError()));

// Result<TypeError, string>.Initial with no value
const newVal5 = v3.chain(a => failure<TypeError, string>(new TypeError()));

The chain method is particularly useful when you need to sequence operations that might fail or be in different states. It handles all possible state combinations according to the priority rules above.

merge

Alias for mergeInOne

function merge<F1, S1>(values: [Result<F1, S1>]): Result<F1, [S1]>;
function merge<F1, S1, F2, S2>(values: [Result<F1, S1>, Result<F2, S2>]): Result<F1 | F2, [S1, S2]>;
function merge<F1, S1, F2, S2, F3, S3>(
  values: [Result<F1, S1>, Result<F2, S2>, Result<F3, S3>]
): Result<F1 | F2 | F3, [S1, S2, S3]>;
// ... until 10 elements

• values: Array<Result<F, S>> - Array of Result values which will be merged into Result of Array
• Returns Result<F, Array<S>> which will contain Success<Array<S>> if all of array elements was Success<R>, Failure<F>if all of array elements was Failure<F>, Initial if at least one Initial, otherwise Pending.

Example:

const v1 = initial; // Result<never, never>.Initial
const v2 = pending; // Result<never, never>.Pending
const v3 = success<TypeError, number>(2); // Result<TypeError, number>.Success
const v4 = success<ReferenceError, string>("test"); // Result<ReferenceError, string>.Success
const v5 = failure<Error, boolean>(new Error()); // Result<Error, boolean>.Failure

const r1 = merge([v1, v2]); // Result<never, [number, number]>.Initial
const r2 = merge([v2, v5]); // Result<Error, [never, boolean]>.Pending
const r3 = merge([v3, v4]); // Result<TypeError | ReferenceError, [number, string]>.Success
const r4 = merge([v3, v4, v5]); // Result<TypeError | Error | ReferenceError, [number, string, boolean]>.Failure

mergeWithConfig

function mergeWithConfig<F, S>(values: Result<F, S>[], config: { priority: 'failure' | 'pending' }): Result<F, S[]>;

Merges an array of Results with configurable priority between states.

Default behavior (priority: 'pending'):

1. Returns Initial if ANY Result is Initial
2. Returns Pending if ANY Result is Pending (and none are Initial)
3. Returns Failure if ANY Result is Failure (and none are Initial/Pending)
4. Returns Success only if ALL Results are Success

Failure priority (priority: 'failure'):

1. Returns Failure if ANY Result is Failure
2. Returns Initial if ANY Result is Initial
3. Returns Pending if ANY Result is Pending
4. Returns Success only if ALL Results are Success

const v1 = success<string, number>(1);
const v2 = failure('error');
const v3 = pending as Result<string, number>;

// Default behavior - Pending takes precedence
mergeWithConfig([v1, v2, v3], { priority: 'pending' }); // Result.Pending

// Failure priority
mergeWithConfig([v1, v2, v3], { priority: 'failure' }); // Result.Failure('error')

mergeInOne

function merge<F1, S1>(values: [Result<F1, S1>]): Result<F1, [S1]>;
function merge<F1, S1, F2, S2>(values: [Result<F1, S1>, Result<F2, S2>]): Result<F1 | F2, [S1, S2]>;
function merge<F1, S1, F2, S2, F3, S3>(
  values: [Result<F1, S1>, Result<F2, S2>, Result<F3, S3>]
): Result<F1 | F2 | F3, [S1, S2, S3]>;
// ... until 10 elements

• values: Array<Result<F, S>> - Array of Result values which will be merged into Result of Array
• Returns Result<F, Array<S>> which will contain Success<Array<S>> if all of array elements was Success<R>, Failure<F>if all of array elements was Failure<F>, Initial if at least one Initial, otherwise Pending.

Example:

const v1 = initial; // Result<TypeError, number>.Initial
const v2 = pending; // Result<TypeError, number>.Pending
const v3 = success<TypeError, number>(2); // Result<TypeError, number>.Success
const v4 = success<ReferenceError, string>("test"); // Result<ReferenceError, string>.Success
const v5 = failure<Error, boolean>(new Error()); // Result<Error, boolean>.Failure

const r1 = merge([v1, v2]); // Result<TypeError, [number, number]>.Initial
const r2 = merge([v2, v5]); // Result<TypeError | Error, [number, boolean]>.Pending
const r3 = merge([v3, v4]); // Result<TypeError | ReferenceError, [number, string]>.Success
const r4 = merge([v3, v4, v5]); // Result<TypeError | ReferenceError | Error, [number, string, boolean]>.Failure

mergeInMany

static mergeInMany<F1, S1>(values: [Result<F1, S1>]): Result<Array<F1>, [S1]>;
static mergeInMany<F1, S1, F2, S2>(values: [Result<F1, S1>, Result<F2, S2>]): Result<Array<F1 | F2>, [S1, S2]>;
static mergeInMany<F1, S1, F2, S2, F3, S3>(
  values: [Result<F1, S1>, Result<F2, S2>, Result<F3, S3>]
): Result<Array<F1 | F2 | F3>, [S1, S2, S3]>;
// ... until 10 elements

• values: Array<Result<F, S>> - Array of Result values which will be merged into Result of Array
• Returns Result<Array<F>, Array<S>> which will contain Success<Array<S>> if all of array elements was Success<R> otherwise array of all catched Failure<F> values.

Example:

const v1 = success<TypeError, number>(2); // Result<TypeError, number>.Success
const v2 = failure<ReferenceError, string>("test"); // Result<ReferenceError, string>.Success
const v3 = failure<Error, boolean>(new Error()); // Result<Error, boolean>.Failure

merge([v1, v2]); // Result<Array<TypeError | ReferenceError>, [number, string]>.Success
merge([v1, v2, v3]); // Result<Array<TypeError | ReferenceError | Error>, [number, string, boolean]>.Failure

initial

const initial: Result<never, never>;

• Returns Result with Initial state which does not contain value.

Example:

const v1 = initial; // Result<undefined, never>.Initial

pending

const pending: Result<F, S>;

• Returns Result with Pending state which does not contain value.

Example:

const v1 = pending; // Result<never, never>.Initial

failure

function failure<F, S>(value: F): Result<F, S>;

• Returns Result with Failure state which contain value with F type.

Example:

const v1 = failure(new Error()); // Result<Error, never>.Failure
const v2 = failure<Error, number>(new Error()); // Result<Error, number>.Failure

success

function success<F, S>(value: S): Result<F, S>;

• Returns Result with Success state which contain value with S type.

Example:

const v1 = success(2); // Result<never, number>.Success
const v2 = success<Error, number>(2); // Result<Error, number>.Success

from

Alias for success

function from<S>(value: S): Result<never, S>;

• Returns Result with Success state which contain value with S type.

Example:

from(2); // Result<never, number>.Success

fromTry

Returns Success with function result or Failure if function execution throws an error.

function fromTry<L, R>(fn: () => R): Result<L, R>;

fromTry(() => 2); // Result<unknown, number>.Success
fromTry(() => {
  throw new Error("test");
}); // Result<unknown, never>.Failure

fromPromise

Returns promise of Success if the provided promise fulfilled or Failure with the error value if the provided promise rejected.

function fromPromise<F, S>(promise: Promise<S>): Promise<Result<F, S>>;

fromPromise(Promise.resolve(2)); // Promise<Result<unknown, number>.Right>
fromPromise(Promise.reject(new Error("test"))); // Promise<Result<unknown, never>.Left>

fromMaybe

function fromMaybe<never, S>(value: Maybe<S>): Result<never, S>;

• Creates Result from Maybe in Initial or Success state.

Example:

fromMaybe(just(2)); // Result<never, number>.Success
fromMaybe(none()); // Result<never, number>.Initial

fromEither

function fromEither<F, S>(value: Either<F, S>): Result<F, S>;

• Creates Result from Either in Failure or Success state.

Example:

fromEither(right<string, number>(10)); // Result<string, number>.Success

fromNullable

function fromNullable<T>(value: T): Result<unknown, NonNullable<T>>;

• Creates Result with Success state which contain value with T type if value is not null or undefined and initial otherwise.

Example:

fromNullable(10); // Result<unknown, number>.Success
fromNullable(null as Nullable<number>); // Result<unknown, number>.Initial

isResult

function isResult<F, S>(value: unknown | Result<F, S>): value is Result<F, S>;

• Returns boolean if given value is instance of Result constructor.

Example:

const value: unknown = 2;
if (isResult(value)) {
  // ... value is Result<unknown, unknown> at this block
}

Result#isInitial

function isInitial(): boolean;

• Returns true if state of Result is Initial otherwise false

Example:

const v1 = success(2);
const v2 = failure(2);
const v3 = initial();

v1.isInitial(); // false
v2.isInitial(); // false
v3.isInitial(); // true

Result#isPending

function isPending(): boolean;

• Returns true if state of Result is Pending otherwise false

Example:

const v1 = success(2);
const v2 = failure(2);
const v3 = pending();

v1.isPending(); // false
v2.isPending(); // false
v3.isPending(); // true

Result#isFailure

function isFailure(): boolean;

• Returns true if state of Result is Failure otherwise false

Example:

const v1 = success(2);
const v2 = failure(2);

v1.isFailure(); // false
v2.isFailure(); // true

Result#isSuccess

function isSuccess(): boolean;

• Returns true if state of Result is Success otherwise false

Example:

const v1 = success(2);
const v2 = failure(2);

v1.isSuccess(); // true
v2.isSuccess(); // false

Result#equal

function equal<T>(x: Result<F, S>, extractor?: (item: S): T): boolean;

• The equal method compares the current Result instance with another Result instance for equality. It provides flexibility in how equality is determined through an optional field extractor.

Example:

// Comparing entire values
const result1 = success(10);
const result2 = success(10);
const result3 = success(20);
result1.equal(result2);  // true
result1.equal(result3);  // false

// Comparing with a field extractor
interface User { id: number; name: string; }
const user1 = success({ id: 1, name: 'Alice' });
const user2 = success({ id: 1, name: 'Bob' });
user1.equal(user2, user => user.id);  // true

Result#or

function or<F, S>(x: Result<F, S>): Result<F, S>;

• Returns Result<F, S>. If state of this is Success then this will be returned otherwise x argument will be returned

Example:

const v1 = success<string, number>(2);
const v2 = failure<string, number>("Error 1");
const v3 = failure<string, number>("Error 2");
const v4 = success<string, number>(3);
const v5 = initial();

v1.or(v2); // v1 will be returned
v2.or(v1); // v1 will be returned
v2.or(v3); // v3 will be returned
v1.or(v4); // v1 will be returned
v5.or(v1); // v1 will be returned

v2.or(v3).or(v1); // v1 will be returned
v2.or(v1).or(v3); // v1 will be returned
v1.or(v2).or(v3); // v1 will be returned
v2.or(v5).or(v3); // v3 will be returned

Result#join

function join<F1, F2, S>(this: Result<F1, Result<F2, S>>): Result<F1 | F2, S>;

• this: Result<F1, Result<F2, R>> - Result instance which contains other Result instance as Success value.
• Returns unwrapped Result - if current Result has Success state and inner Result has Success state then returns inner Result Success, if inner Result has Failure state then return inner Result Failure otherwise outer Result Failure.

Example:

const v1 = success(success(2));
const v2 = success(failure(new Error()));
const v3 = failure<TypeError, Result<Error, number>>(new TypeError());

v1.join(); // Result.Success with value 2
v2.join(); // Result.Failure with value new Error
v3.join(); // Result.Failure with value new TypeError

Result#map

Alias for Result#mapSuccess

function map<F, S, NewS>(fn: (val: S) => NewS): Result<F, NewS>;

• Returns mapped by fn function value wrapped by Result if Result is Success otherwise Result with current value

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());

const newVal1 = v1.map(a => a.toString()); // Result<Error, string>.Success with value "2"
const newVal2 = v2.map(a => a.toString()); // Result<Error, string>.Failure with value new Error()

Result#mapSuccess

function mapSuccess<F, S, NewS>(fn: (val: S) => NewS): Result<F, NewS>;

• Returns mapped by fn function value wrapped by Result if Result is Success otherwise Result with current value

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());

const newVal1 = v1.mapSuccess(a => a.toString()); // Result<Error, string>.Success with value "2"
const newVal2 = v2.mapSuccess(a => a.toString()); // Result<Error, string>.Failure with value new Error()

Result#mapLeft

function mapFailure<F, S, NewF>(fn: (val: F) => NewF): Result<NewF, S>;

• Returns mapped by fn function value wrapped by Result if Result is Failure otherwise Result with current value

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());

const newVal1 = v1.mapFailure(a => a.toString()); // Result<string, number>.Right with value 2
const newVal2 = v2.mapFailure(a => a.toString()); // Result<string, number>.Left with value "Error"

Result#asyncMap

function asyncMap<F, S, NewS>(fn: (val: S) => Promise<NewS>): Promise<Result<F, NewS>>;

• Returns Promise with mapped by fn function value wrapped by Result if Result is Success otherwise Result with current value

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());

// Promise<Result<Error, string>.Success> with value "2"
const newVal1 = v1.asyncMap(a => Promise.resolve(a.toString()));
// Promise<Result<Error, string>.Failure> with value new Error()
const newVal2 = v2.asyncMap(a => Promise.resolve(a.toString()));

Result#apply

function apply<F, S, B>(this: Result<F, (a: S) => B>, arg: Result<F, S>): Result<F, B>;
function apply<F, S, B>(this: Result<F, S>, fn: Result<F, (a: S) => B>): Result<F, B>;

• this | fn - function wrapped by Result, which should be applied to value arg
• arg | this - value which should be applied to fn
• Returns mapped by fn function value wrapped by Result if Result is Success otherwise Result with current value

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());
const fn1 = success<Error, (a: number) => number>((a: number) => a * 2);
const fn2 = failure<Error, (a: number) => number>(new Error());

const newVal1 = fn1.apply(v1); // Result<Error, number>.Right with value 4
const newVal2 = fn1.apply(v2); // Result<Error, number>.Left with value new Error()
const newVal3 = fn2.apply(v1); // Result<Error, number>.Left with value new Error()
const newVal4 = fn2.apply(v2); // Result<Error, number>.Left with value new Error()

Result#asyncApply

Async variant of Result#apply

asyncApply<F, S, B>(
  this: Result<F, (a: Promise<S> | S) => Promise<B>>,
  arg: Result<F, Promise<S>>): Promise<Result<F, B>>;
asyncApply<F, S, B>(
  this: Result<F, Promise<S>>,
  fn: Result<F, Promise<(a: Promise<S> | S) => B>>): Promise<Result<F, B>>;

• this | fn - function wrapped by Result, which should be applied to value arg
• arg | this - value which should be applied to fn
• Returns Promise with mapped by fn function value wrapped by Result if Result is Success otherwise Result with current value

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());
const fn1 = success<Error, (a: number) => Promise<number>>((a: number) => Promise.resolve(a * 2));
const fn2 = failure<Error, (a: number) => Promise<number>>(new Error());

const newVal1 = fn1.asyncApply(v1); // Promise<Either<Error, number>.Right> with value 4
const newVal2 = fn1.asyncApply(v2); // Promise<Either<Error, number>.Left> with value new Error()
const newVal3 = fn2.asyncApply(v1); // Promise<Either<Error, number>.Left> with value new Error()
const newVal4 = fn2.asyncApply(v2); // Promise<Either<Error, number>.Left> with value new Error()

Result#chain

function chain<F, S, NF, NS>(fn: (val: S) => Result<NF, NS>): Result<F | NF, NS>;

• Returns a new Result by applying fn to the Success value of this Result
• State handling priority:
  1. If this Result is Initial, returns Initial
  2. If the next Result (returned by fn) is Initial, returns Initial
  3. If this Result is Pending or the next Result is Pending, returns Pending
  4. If this Result is Failure, returns Failure with current value
  5. If the next Result is Failure, returns Failure with next value
  6. Otherwise returns the next Result

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());
const v3 = initial;

// Result<Error | TypeError, string>.Success with value "2"
const newVal1 = v1.chain(a => success<TypeError, string>(a.toString()));

// Result<Error | TypeError, string>.Failure with value new TypeError()
const newVal2 = v1.chain(a => failure<TypeError, string>(new TypeError()));

// Result<Error | TypeError, string>.Failure with value new Error()
const newVal3 = v2.chain(a => success<TypeError, string>(a.toString()));

// Result<Error | TypeError, string>.Failure with value new Error()
const newVal4 = v2.chain(a => failure<TypeError, string>(new TypeError()));

// Result<TypeError, string>.Initial with no value
const newVal5 = v3.chain(a => failure<TypeError, string>(new TypeError()));

The chain method is particularly useful when you need to sequence operations that might fail or be in different states. It handles all possible state combinations according to the priority rules above.

Result#asyncChain

function asyncChain<F, S, NewF, NewS>(fn: (val: S) => Promise<Result<NewF, NewS>>): Promise<Result<F | NewF, NewS>>;

• Returns Promise with mapped by fn function value wrapped by Result if Result is Success and returned by fn value is Success too otherwise Result in other state, Initial pwns Pending and Failure.

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());
const v3 = initial;

// Promise<Result<Error | TypeError, string>.Success> with value "2"
const newVal1 = v1.asyncChain(a => Promise.resolve(right<TypeError, string>(a.toString())));
// Promise<Result<Error | TypeError, string>.Failure> with value new TypeError()
const newVal2 = v1.asyncChain(a => Promise.resolve(left<TypeError, string>(new TypeError()));
// Promise<Result<Error | TypeError, string>.Failure> with value new Error()
const newVal3 = v2.asyncChain(a => Promise.resolve(right<TypeError, string>(a.toString())));
// Promise<Result<Error | TypeError, string>.Failure> with value new Error()
const newVal4 = v2.asyncChain(a => Promise.resolve(left<TypeError, string>(new TypeError())));
// Promise<Result<Error | TypeError, string>.Initial> with no value
const newVal5 = v3.asyncChain(a => Promise.resolve(failure<TypeError, string>(new TypeError())));

Result#toEither

function toEither<F, S>(onInitial: () => F, onPending: () => F): Either<F, S>;

• Converts Result into Either in Left or Success state with fallbacks for Initial and Pending states.

Example:

success<string, number>(10).toEither(
  () => "initial state",
  () => "pending state"
); // Either<string, number>.Right

Result#toMaybe

function toMaybe<S>(): Maybe<S>;

• Converts Result into Maybe in Just state if Result is in Success state or to Maybe in None state otherwise.

Example:

success<string, number>(10).toMaybe(); // Maybe<number>.Just

Result#toNullable

function toNullable<S>(): S | null;

• Returns S if Result is in Success state and null otherwise

Example:

success<string, number>(10).toNullable(); // number | null

Result#toUndefined

function toUndefined<S>(): S | undefined;

• Returns S if Result is in Success state and undefined otherwise

Example:

success<string, number>(10).toUndefined(); // number | undefined

Result#unwrap

function unwrap<S>(): S;

• Returns S if Result is in Success state and throws otherwise via provided factory or pure Error

Example:

success<string, number>(10).unwrap(); // number
initial.unwrap(); // throws default (Error)
pending.unwrap({ failure: () => new Error('Custom')}); // throws  custom (Error)

Result#unwrapOr

function unwrapOr<S>(s: S): S;

• Returns the success value if Result is Success, otherwise returns the provided default value.

Example:

const v1 = success<Error, number>(2);
const v2 = failure<Error, number>(new Error());
v1.unwrapOr(3); // returns 2
v2.unwrapOr(3); // returns 3

Result#unwrapOrElse

function unwrapOrElse(f: (l: F) => S): S;

• Returns the success value if Result is Success, otherwise returns the result of calling the provided function with the failure value.

Example:

const v1 = success<number, number>(2);
const v2 = failure<number, number>(3);
v1.unwrapOrElse(x => x * 2); // returns 2
v2.unwrapOrElse(x => x *2); // returns 6

Result#fold

function fold<D>(onInitial: () => D, onPending: () => D, onFailure: (failure: F) => D, onSuccess: (success: S) => D): D;

• Transforms the Result value into type D by providing handlers for all possible states
• Parameters:
  • onInitial: () => D - Handler for Initial state
  • onPending: () => D - Handler for Pending state
  • onFailure: (failure: F) => D - Handler for Failure state, receives the failure value
  • onSuccess: (success: S) => D - Handler for Success state, receives the success value
• Returns the result of calling the appropriate handler based on the Result state

Example:

const onInitial = () => "it's initial"
const onPending = () => "it's pending"
const onFailure = (err) => "it's failure"
const onSuccess = (data) => `${data + 1}`

const v1 = initial;
const v2 = pending;
const v3 = failure<string, number>('error');
const v4 = success<string, number>(21);

v1.fold(onInitial, onPending, onFailure, onSuccess) // "it's initial"
v2.fold(onInitial, onPending, onFailure, onSuccess) // "it's pending"
v3.fold(onInitial, onPending, onFailure, onSuccess) // "it's failure"
v4.fold(onInitial, onPending, onFailure, onSuccess) // "22"

The fold method is particularly useful when you need to handle all possible states of a Result and transform them into a single type. This pattern is common when you need to:

• Display different UI states
• Convert Result states into a common format
• Handle all possible outcomes in a type-safe way

Result#filter

function filter(predicate: (value: S) => boolean): Result<F | S, S>;

Validates a Success value using a predicate. If the predicate returns false, converts the Success to a Failure using the success value.

// Age validation
const age = success<string, number>(15);
const isAdult = age.filter(age => age >= 18);
// Result<string | number, number>.Failure with value 15

// Chaining validations
const validAge = success<string, number>(25)
  .filter(age => age >= 0)    // minimum age
  .filter(age => age <= 120); // maximum age
// Result<string | number, number>.Success with value 25

Result#filterMap

function filterMap<NS>(f: (value: S) => Result<S, NS>): Result<F | S, NS>;

Combines filtering and mapping in one operation. Useful for transformations that might fail.

const parseIfPositive = (n: number) => 
  n > 0 ? success(n.toString()) : failure(n);

success(5).filterMap(parseIfPositive)
// Result<number, string>.Success with value "5"

success(-1).filterMap(parseIfPositive)
// Result<number, string>.Failure with value -1

failure('error').filter(x => true)    // stays Failure
initial.filter(x => true)             // stays Initial
pending.filter(x => true)             // stays Pending

failure('error').filterMap(fn)        // stays Failure
initial.filterMap(fn)                 // stays Initial
pending.filterMap(fn)                 // stays Pending

Result#tap

function tap(f: (value: S) => void): Result<F, S>;

Executes a side effect function if the Result is Success, then returns the original Result unchanged. Useful for logging, debugging, or other side effects without modifying the Result chain.

success(5)
  .tap(x => console.log('Value:', x)) // logs "Value: 5"
  .map(x => x * 2);                   // Result<never, number>.Success(10)

failure('error')
  .tap(x => console.log('Value:', x)) // nothing logged
  .map(x => x * 2);                   // Result<string, number>.Failure('error')

Result#tapFailure

function tapFailure(f: (value: F) => void): Result<F, S>;

Executes a side effect function if the Result is Failure, then returns the original Result unchanged. Useful for error logging or debugging without modifying the Result chain.

success<Error, number>(5)
  .tapFailure(e => console.error(e)) // nothing logged
  .map(x => x * 2);                  // Result<Error, number>.Success(10)

failure(new Error('oops'))
  .tapFailure(e => console.error(e)) // logs Error: oops
  .map(x => x * 2);                  // Result<Error, number>.Failure(Error: oops)

Result#recover

function recover(value: S): Result<F, S>;

Recovers from a Failure state by providing a default success value.

const v1 = failure<string, number>('error');
const v2 = success<string, number>(5);

v1.recover(42);  // Result<string, number>.Success with value 42
v2.recover(42);  // Result<string, number>.Success with value 5 (unchanged)

Result#recoverWith

function recoverWith<NF, NS>(f: (error: F) => Result<NF, NS>): Result<NF, NS | S>;

Recovers from a Failure state by applying a function that returns a new Result. Useful for handling specific error cases differently or transforming errors.

const handler = (error: string): Result<Error, number> => 
  error === 'known' ? success(42) : failure(new Error('still failed'));

failure('known').recoverWith(handler);     // Result<Error, number>.Success(42)
failure('unknown').recoverWith(handler);    // Result<Error, number>.Failure(Error: still failed)
success<string, number>(5).recoverWith(handler); // Result<Error, number>.Success(5)

// Initial and Pending states pass through
initial.recoverWith(handler);              // Result.Initial
pending.recoverWith(handler);              // Result.Pending

Result#zip

function zip<F2, S2>(other: Result<F2, S2>): Result<F | F2, [S, S2]>;

Combines two Results into a Result containing a tuple of their success values. Returns Failure if either Result is a Failure.

const num = success<string, number>(2);
const str = success<Error, string>('test');

num.zip(str) // Result<string | Error, [number, string]>.Success([2, 'test'])
num.zip(failure('error')) // Result<string | Error, [number, string]>.Failure('error')

Result#zipWith

function zipWith<F2, S2, R>(
  other: Result<F2, S2>,
  f: (a: S, b: S2) => R
): Result<F | F2, R>;

Combines two Results using a function. Returns Failure if either Result is a Failure.

const num1 = success<string, number>(2);
const num2 = success<Error, number>(3);

num1.zipWith(num2, (a, b) => a + b) // Result<string | Error, number>.Success(5)
num1.zipWith(failure('error'), (a, b) => a + b) // Result<string | Error, number>.Failure('error')

Result#bimap

function bimap<NF, NS>(
  failureMap: (f: F) => NF,
  successMap: (s: S) => NS
): Result<NF, NS>;

Maps both the Failure and Success values of a Result simultaneously. Useful for transforming both possible outcomes in one operation.

const result = success<string, number>(42);

// Transform both success and failure values
const transformed = result.bimap(
  (error: string) => new Error(error), // transform failure
  (value: number) => value.toString()  // transform success
);
// Result<Error, string>.Success('42')

const failed = failure<string, number>('oops');
const transformedFailure = failed.bimap(
  (error: string) => new Error(error),
  (value: number) => value.toString()
);
// Result<Error, string>.Failure(Error: oops)

// Initial and Pending states pass through unchanged
initial.bimap(f, s)  // Result.Initial
pending.bimap(f, s)  // Result.Pending

Helpers

// Value from Result instance
const { value } = success<Error, number>(2); // number | Error | undefined
const { value } = success(2); // number | undefined
const { value } = failure<Error, number>(new Error()); // number | Error | undefined
const { value } = failure(new Error()); // Error | undefined

Development

License

MIT (c)