eventual-result

A Result and Option implementation that works with Promise

Why Another Result Implementation?

There are many other libraries that implement these patterns; a few sources of inspiration include ts-results and oxide.ts. These libraries share a problem, however: they do nothing to help you manage asynchronous code.

Since the dawn of ES6, Promise and async/await have become critical parts of working on a modern JavaScript or TypeScript application. While Result provides a great way to model a potential success or failure for a synchronous action, needing to use something like Promise<Result> to model an asynchronous action can get you into trouble:

Example: handling a Promise<Result>

Let's suppose that we want to read a file asynchronously and then validate it to produce a Result. That might look something like this:

import { readFile } from "node:fs/promises";

declare function isValid(content: string): boolean;

function validateFile(content: string): Result<string, string> {
  if (isValid(content)) {
    return Ok(content);
  } else {
    return Err("The file content is not valid");
  }
}

async function readValidFile(path: string): Promise<Result<string, string>> {
  const content = await readFile(path);

  return validateFile(content);
}

// Let's say that `path/to/file.txt` points to a location that does not exist
const potentiallyValidFile = await readValidFile("path/to/file.txt");

What happens here? An exception will be thrown! Even though we want to be using Result to model an error state, readFile doesn't know anything about that; the awaited promise rejects and an exception is thrown.

This example shows how using Promise and Result together can cause errors, because our Result does nothing to help us with the rejected Promise. This isn't impossible to solve though: we can use try/catch to ensure a reject Promise results in an Err:

Example: handling a Promise<Result> and catching exceptions

Let's improve on our last example by ensuring that an error from readFile doesn't cause readValidFile to result in a rejected Promise!

import { readFile } from "node:fs/promises";

declare function isValid(content: string): boolean;

function validateFile(content: string): Result<string, string> {
  if (isValid(content)) {
    return Ok(content);
  } else {
    return Err("The file content is not valid");
  }
}

async function readValidFile(path: string): Promise<Result<string, string>> {
  try {
    const content = await readFile(path);

    return validateFile(content);
  } catch (e: unknown) {
    return Err(String(e));
  }
}

// Let's say that `path/to/file.txt` points to a location that does not exist
const potentiallyValidFile = await readValidFile("path/to/file.txt");

What happens this time? Rather than throwing an exception, we get a resolution to an Err. Success!

But... can we do better? What are some problems with the code above?

  • Having to defensively wrap every asynchronous function in a try/catch doesn't feel good. While we do want to be exhaustive about handling errors, we don't want to have to write defensive code. Additionally, when you are working in a codebase that has adopted the Result pattern, these locations where try/catch are required to wrap third-party code really stand out.
  • We lose the top-to-bottom readability of the readValidFile function. The error handling for readFile is way down at the bottom instead of being anywhere near the actual function call.

What might a solution to these problems look like?

Enter EventualResult! This class provides us an alternative way to represent an eventual value that may result in a success or failure that, rather than competing with Result, is compatible with it.

Example: handling an EventualResult

Let's look at the same example, but this time making use of an EventualResult instead of a Promise<Result>:

import { readFile } from "node:fs/promises";

declare function isValid(content: string): boolean;

function validateFile(content: string): Result<string, string> {
  if (isValid(content)) {
    return Ok(content);
  } else {
    return Err("The file content is not valid");
  }
}

function readValidFile(path: string): EventualResult<string> {
  return new EventualResult(readFile(path)).andThen((content) =>
    validateFile(content)
  );
}

// Let's say that `path/to/file.txt` points to a location that does not exist
const eventualPotentiallyValidFile = readValidFile("path/to/file.txt");

What has changed?

  1. We no longer need specific try/catch wrapping around readFile; by passing it through EventualResult, we no longer end up with a Promise that can reject. If an error during the file read occurs, the EventualResult will resolve to an Err.
  2. We don't need any conditional logic when validating the file that handles what to do when the file read failed. EventualResult implements most of the same methods that Result does, we can lean on our existing knowledge about working with Result to only validate the contents if the file read eventually results in an Ok.

The end result is writing less defensive code that extends the predictability of Result with the eventual resolution of Promise.