Result

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Using Result

The library is designed to be used with a functional style, allowing you to compose operations easily. Thus, standalone pure function versions of every operation are supplied. However, the same operations also exist on the Ok and Err types directly, so you may also write them in a more traditional "fluent" object style.

Examples: functional style

import { ok, err, map, toString } from 'true-myth/result';

const length = (s: string) => s.length;

const anOk = ok('some string');
const okLength = map(length, anOk);
console.log(toString(okLength)); // Ok(11)

const anErr = err('gah!');
const errLength = map(length, anErr);
console.log(toString(errLength)); // Err(gah!)

Examples: fluent object invocation

You can also use a "fluent" method chaining style to apply the various helper functions to a Result instance:

import { ok, err } from 'true-myth/result';

const length = (s: string) => s.length;

const hooray = ok('yay');
const okLen = hooray.map(length).unwrapOr(0); // okLen = 3

const muySad = err('oh no');
const errLen = muySad.map(length).unwrapOr(0); // errLen = 0

Writing type constraints

When creating a Result instance, you'll nearly always be using either the Ok or the Err type, so the type system won't necessarily be able to infer the other type parameter.

import { ok, err } from 'true-myth/result';

const anOk = ok(12); // TypeScript infers: `Result<number, {}>`
const anErr = err('sad'); // TypeScript infers: `Result<{}, string>`

In TypeScript, because of the direction type inference happens, you will need to specify the type at declaration to make it type check when returning values from a function with a specified type. Note that this only applies when the instance is declared in its own statement and returned separately, not when it is the expression value of a single-expression arrow function or the explicit return value of any function.

import Result, { ok } from 'true-myth/result';

// ERROR: Type 'Result<number, {}>' is not assignable to type 'Result<number, string>'.
const getAResultNotAssignable = (): Result<number, string> => {
  const theResult = ok(12);
  return theResult;
};

// Succeeds
const getAResultExpression = (): Result<number, string> => ok(12);

// Succeeds
const getAResultReturn = (): Result<number, string> => {
  return ok(12);
};