Async/Promise support – gathering information

[jstasiak]

Relevant issue in the upstream repository: vultix#33

Basically it's really inconvenient to mix Option and Result with promises right now because a Promise<Option<...>> or Promise<Result<...>> needs to be awaited first in order to use any of the composition/combination methods (like map(), andThen() etc.)

Some code I wrote today:

// readFileChecked() returns Promise<Result<Buffer, ...>>
// parseCsvChecked() takes Buffer and returns Result<object[], ...>

const rawContent = await readFileChecked(path)
if (rawContent.isErr()) {
    return rawContent
}
return rawContent.andThen(parseCsvChecked)

What I wish I could write is:

return readFileChecked(path).andThen(parseCsvChecked)

Any ideas about the API or the design of this feature are welcome.

[jstasiak]

Some ideas off the top of my head:

(In this context an async function is something that returns Promise<Result<>> or AsyncResult<>).

• We need something like AsyncOption and AsyncResult and they need to support most of the existing Option and Result API
• Given a Result I should be able to compose it with an async function and get an AsyncResult, for example:

syncResult: Result<number, Error>
asyncFunction: (number) -> Promise<Result<string, Error>>

syncResult.andThen(asyncFunction) // -> AsyncResult<string, Error>

• Given an AsyncResult I should be able to compose with with sync function and async function and in both cases get AsyncResult, for example:

asyncResult: AsyncResult<number, Error>
asyncFunction: (number) -> Promise<Result<string, Error>>
syncFunction: (string) -> Result<number[], Error>>

asyncResult.andThen(asyncFunction).andThen(syncFunction) // -> AsyncResult<number[], Error>

• Awaiting AsyncResult<T, E> should yield Result<T, E> I think?

[jstasiak]

#87 adds some initial async support. Once it's merged we'll use it for a bit in the Lune backend to see if it's satisfactory.

[jstasiak]

We've implemented some more stuff since then:

• Implement AsyncResult or and orElse #109
• Implement AsyncResult.mapErr #113
• Implement AsyncOption #114

I'll close this ticket, the feature exists, we can iterate on it.