Either

The Either data type encapsulates the idea of a computation that may fail.

An Either value can either be Right some value or Left some error.

type Either<T> = Right<T> | Left;

Install

npm install ts.data.either --save

Example

import { Either, tryCatch, andThen, withDefault } from './either';

interface UserJson {
  id: number;
  nickname: string;
  email: string;
}

// throws if file does not exists
const readFile = (filename: string): string => {
  const fileSystem: { [key: string]: string } = {
    'something.json': `
    [
      {
        "id": 1,
        "nickname": "rick",
        "email": "[email protected]"
      },
      {
        "id": 2,
        "nickname": "morty",
        "email": "[email protected]"
      }
    ]`
  };
  const fileContents = fileSystem[filename];
  if (fileContents === undefined) {
    throw new Error(`${filename} does not exists.`);
  }
  return fileContents;
};

// Wraps the read file operation in an Either
const readFileContent = (filename: string): Either<string> =>
  tryCatch(
    () => readFile(filename),
    err => err
  );

// Wraps the json parsing operation in an Either
const parseJson = (json: string): Either<UserJson[]> =>
  tryCatch(
    () => JSON.parse(json),
    err => new Error(`There was an error parsing this Json.`)
  );

// The pipeline function just makes function invocations flow
const pipeline = (initialValue: any, ...fns: Function[]) =>
  fns.reduce((acc, fn) => fn(acc), initialValue);

const usersFull: UserJson[] = pipeline(
  'something.json',
  (fname: string) => readFileContent(fname),
  (json: Either<string>) => andThen(parseJson, json),
  (users: Either<UserJson[]>) => withDefault(users, [])
); // returns the Array of users because all intermediate operations have succeeded

const usersEmpty: UserJson[] = pipeline(
  'nothing.json',
  (fname: string) => readFileContent(fname),
  (json: Either<string>) => andThen(parseJson, json),
  (users: Either<UserJson[]>) => withDefault(users, [])
); // returns an empty Array because the readFile operations failed

Api

(Inspired by elm-lang)

right

right<T>(value: T): Either<T>

Wraps a value in an instance of Right.

right(5); // Right<number>(5)

left

left<T>(error: Error): Either<T>

Creates an instance of Left.

left(new Error('Something bad happened')); // Left<unknown>(Error('Something bad happened'))
left<number>(new Error('The calculation failed')); // Left<number>(Error('The calculation failed'))

isRight

isRight<T>(value: Either<T>): boolean

Returns true if a value is an instance of Right.

isRight(left(new Error('Wrong!'))); // false

isLeft

isLeft<T>(value: Either<T>): boolean

Returns true if a value is not an instance of Right.

isLeft(right(5)); // false
isLeft(left('Hi!')); // true
isLeft(null); // true

withDefault

withDefault<T>(value: Either<T>, defaultValue: T): T

If value is an instance of Right it returns its wrapped value, if it's an instance of Left it returns the defaultValue.

withDefault(right(5), 0); // 5
withDefault(left(new Error('Wrong!')), 0); // 0

caseOf

caseOf<A, B>(caseof: {Right: (v: A) => B; Left: (err: Error) => B;}, value: Either<A>): B

Run different computations depending on whether an Either is Right or Left and returns the result.

caseOf(
  {
    Left: err => `Error: ${err.message}`,
    Right: n => `Launch ${n} missiles`
  },
  right('5')
); // 'Launch 5 missiles'

map

map<A, B>(f: (a: A) => B, value: Either<A>): Either<B>

Transforms an Either value with a given function.

const add1 = (n: number) => n + 1;
map(add1, right(4)); // Right<number>(5)
map(add1, left(new Error('Something bad happened'))); // Left('Something bad happened')

tryCatch

tryCatch<A>(f: () => A, onError: (e: Error) => Error): Either<A>

Transforms a function (that might throw an exception) that produces A to a function that produces Either<A>.

tryCatch(
  () => JSON.parse(''),
  err => err
); // Left('Unexpected end of JSON input')

andThen

andThen<A, B>(f: (a: A) => Either<B>, value: Either<A>): Either<B>

Chains together computations that may fail.

const removeFirstElement = <T>(arr: T[]): T[] => {
  if (arr.length === 0) {
    throw new Error('Array is empty');
  }
  return arr.slice(1);
};

const safeRemoveFirst = <T>(arr: T[]): Either<T[]> => {
  try {
    return right(removeFirstElement(arr));
  } catch (error) {
    return left(error);
  }
};

// The pipeline function just makes function invocations flow
const pipeline = (initialValue: any, ...fns: Function[]) =>
  fns.reduce((acc, fn) => fn(acc), initialValue);

const result: string[] = pipeline(
  ['a', 'b', 'c'],
  safeRemoveFirst, // Right(['b', 'c'])
  (arr: Either<string[]>) => andThen(safeRemoveFirst, arr), // Right(['b'])
  (arr: Either<string[]>) => andThen(safeRemoveFirst, arr), // Right([])
  (arr: Either<string[]>) => andThen(safeRemoveFirst, arr), // Left(Error('Array is empty'))
  (arr: Either<string[]>) => andThen(safeRemoveFirst, arr), // Left(Error('Array is empty'))
  (arr: Either<string[]>) => withDefault(arr, [])
);

console.log(result); // []