Why is there no ResultAsync.fromThrowable?

[ehaynes99]

Result.fromThrowable takes a function as an argument, and returns a new function that returns a Result. There is no corollary for ResultAsync, which really diminishes the utility of the library. There have been a lot of issues in here mentioning how people want to have an async function that returns a ResultAsync. This is just fundamentally wrong. It defeats the entire purpose of using a library like this, because the whole point is that the errors are moved from thrown, untyped values into the return type of the function.

For example:

// doesn't compile because it's not returning a `Promise`, and thats a GOOD THING
const getOrders = async (username: string): ResultAsync<Order[], AxiosError> => {
  // This is bad, because it can throw
  const { data } = await axios.get(`/users/username/${username}`)
  const { id } = data
  return ResultAsync.fromPromise(axios.get(`orders/customer-id/${id}`).then(({ data }) => data), (cause) => cause as AxiosError)
}

To use such a function, you would have to handle the potential error in the result, but you would also still need to wrap it in a try/catch

try {
  await getOrders('someUser').match(
    (orders: Order[]) => console.log(orders),
    // typesafe error type
    (error: AxiosError) => res.send(500, { message: error.message }),
  )
} catch (error: unknown) {
  // no type safety
  res.send(500, { message: error instanceof Error ? error.message : 'unknown error' })
}

fromPromise does not provide sufficient type safety, because it's possible to have synchronous error thrown while creating said promise:

const updateUser = (user: Partial<User>): Promise<User> => {
  const { id, ...updates } = user
  if (!id || Object.keys(updates).length === 0) {
    // since this is not an `async` function, this throws in a synchronous context
    throw new TypeError(`invalid user value: ${user}`)
  }
  return axios.put(`/users/$id`, updates).then(({ data }) => data)
}
// `TypeError` will be thrown here, not captured in the result
const userResult = ResultAsync.fromPromise(updateUser({}), (err: unknown) => err as TypeError | AxiosError)

For ResultAsync to actually provide safety, it's rather critical IMO to be able to convert an arbitrary function that returns a Promise to a wrapper enclosed with ResultAsync that is able to handle both types of errors, not just the rejected Promise

const updateUser = ResultAsync.fromThrowable(
  (user: Partial<User>): Promise<User> => {
    const { id, ...updates } = user
    if (!id || Object.keys(updates).length === 0) {
      // wrapper could capture sync errors
      throw new TypeError(`invalid user value: ${user}`)
    }
    // and also async errors from a rejected `Promise`
    return axios.put(`/users/$id`, updates).then(({ data }) => data)
  },
  (err: unknown) => err as AxiosError | TypeError,
)

[ehaynes99]

An implementation might look like this, which would "hoist" any synchronous errors into the async context.

export const fromThrowable = <Fn extends (...args: readonly any[]) => Promise<any>, E>(
  fn: Fn,
  errorFn: (err: unknown) => E,
): ((...args: Parameters<Fn>) => ResultAsync<ReturnType<Fn> extends Promise<infer R> ? R : never, E>) => {
  return (...args) => {
    try {
      return ResultAsync.fromPromise(fn(...args), errorFn)
    } catch (error) {
      return ResultAsync.fromPromise(Promise.reject(error), errorFn)
    }
  }
}

[paduc]

@ehaynes99 Thanks for this ! I think this proposal merits a PR ! Could you do it ?

[supermacro]

#496

[ehaynes99]

Better late than never. I wasn't using this library for a while, but I added a PR for this.

[danawoodman]

It doesn't seem ResultAsync.fromThrowable actually exists in v6.1.0 (latest) package, am I missing something?

[supermacro]

Just published now under v6.2.0:

https://github.com/supermacro/neverthrow/releases/tag/v6.2.0

[borolgs]

Broken build on npm. There is no dist directory.

[supermacro]

Fixed in v6.2.1 .. sorry about that!