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: "[email protected]", password: "test" });
}
// Result<UserNotFoundError, string>
const user = getUser(1).map(({ email }) => email);

API

• chain
• merge
• mergeInOne
• mergeInMany
• initial
• pending
• failure
• success
• from
• fromPromise
• fromTry
• fromMaybe
• fromEither
• fromNullable
• isResult
• Result#isInitial
• Result#isPending
• Result#isFailure
• Result#isSuccess
• 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#fold
• Helpers

chain

function chain<F, S, NF, NS>(fn: (v: S) => Promise<Result<NF, NS>>): (m: Result<F, s>) => Promise<Result<F | NF, NS>>;

• fn: (v: S) => Promise<Result<NF, NS>> - function which should be applied asynchronously to Result<F, S> value
• Returns function with Result<F, S> argument and promisied Result with new error or mapped by fn value (could be used inside Promise#then function).

Example:

const getValue = async () => success(1);
// Result<TypeError, number>
const result = await getValue()
  .then(chain(async v => success(v * 2)))
  .then(chain(async g => failure(new TypeError("Unexpected"))));

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

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<R>(promise: Promise<R>): Promise<Result<unknown, R>>;

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<L, R>;

• 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#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<L1, L2, R>(this: Result<L1, Result<L2, R>>): Result<L1 | L2, R>;

• this: Result<F1, Result<F2, S>> - 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<A, B>(this: Result<L, (a: A) => B>, arg: Result<L, A>): Result<L, B>;
function apply<A, B>(this: Result<L, A>, fn: Result<L, (a: A) => B>): Result<L, 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<A, B>(
  this: Result<F, (a: Promise<A> | A) => Promise<B>>,
  arg: Result<F, Promise<A>>): Promise<Result<F, B>>;
asyncApply<A, B>(
  this: Result<F, Promise<A>>,
  fn: Result<F, Promise<(a: Promise<A> | A) => B>>): Promise<Result<F, B>>;
asyncApply<A, B>(
  this: Result<F, Promise<A>> | Result<F, (a: Promise<A> | A) => Promise<B>>,
  argOrFn: Result<F, Promise<A>> | Result<F, (a: Promise<A> | A) => Promise<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, NewF, NewS>(fn: (val: S) => Either<NewF, NewS>): Either<F | NewF, NewS>;

• Returns 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;

// 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()));

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#fold

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

• Extracts value from Result and converts it to D based on the factory

Example:

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

f(initial()) // "it's initial"
f(pending()) // "it's pending"
f(failure(new Error('error text'))) // "it's failure"
f(success(21)) // '22'

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

success(2).unwrap() // number
failure(new TypeError()).unwrap() // throws
failure(2).unwrap() // throws  (don't do this)

failure(2).unwrapOr(3) // returns 3
success(2).unwrapOr(3) // returns 2

failure(2).unwrapOrElse(num => num * 2) // returns 4
success(2).unwrapOrElse(num => num * 2) // returns 2

Development

License

MIT (c)