Unexpected Unit in Result-Monad after a function returns null

[GregOnNet]

Describe the bug When an asynchronous function returns null I expect either null to be the value or that the Result turns into a failure, but Unit is returned.

To Reproduce Steps to reproduce the behaviour:

1. Go to https://stackblitz.com/edit/typescript-tp3pbw?file=index.ts
2. Check the code
3. Have a look at the preview website, where Unit is printed.

Expected behaviour I expect either null to be the value or that the Result turns into a failure

Screenshots

1. Function resolves null
2. Expecting Value to be null and checking for it in ensure
3. Since Unit is returned, .ensure has no effect.

[seangwright]

This issue is in ResultAsync.from (https://github.com/seangwright/typescript-functional-extensions/blob/main/src/resultAsync.ts#L57)

If the value in the Promise isn't a Result<T>, then ResultAsync.from initializes its internal Result with Result.success which guards against null or undefined values for success with an isSome(value) check. In this specific case Result.success can actually replace the success value with Unit for null or undefined.

new Result<Unit, TError>({
  value: Unit.Instance,
  error: undefined,
  isSuccess: true,
}

This means value cannot be null or undefined or void when using Result.success (which ResultAsync.from uses in this case).

Result.success should be updated to store null and undefined and void as success values.

ResultAsync.from is also strange because it treats all values as success, however this is stated in the comment of the method

/**
 * Creates a new ResultAsync from the given Promise
 * @param value a Promise which will be converted into a successful Result if it resolves
 * and a failed Result if it rejects
 */
from(...)

Finally, there are checks inside Result which prevent null/undefined/void values from being stored as the success value. Namely the constructor and getValueOrThrow() which returns a value of Some<TValue>. If we want Result to be typeable as Result<null, string> then a lot of work will need to be done to allow it to succeed with null/undefined/void.

[seangwright]

@GregOnNet Coming back to this after looking at the types for #24 ... maybe the better approach is to do this:

function serviceReturningNothing: Promise<Maybe<string>> {
	return Promise.resolve(Maybe.none<string>());
}