feat(persistentvector): add slice function

[chadoh]

This adds support for slice to PersistentVector, matching the behavior of JavaScript's Array (docs)

[DanielRX]

Just to be clear, this won't return a new PersistentVector, it will output an array?

In rust a slice would defer until used (IIRC) so wouldn't it be best if slice returned a vector and then .toArray returned the array?
It means you could slice(100) Into a vector of length 1mil, loop over 10, without trying to get the last 1mil - 100 elements?

[chadoh]

@DanielRX: Yes, as written, it will return an array.

My goal here was to simplify some code in a NEAR Studio template which looks like this:

export function getMessages(): PostedMessage[] {
  const numMessages = min(MESSAGE_LIMIT, messages.length);
  const startIndex = messages.length - numMessages;
  const result = new Array<PostedMessage>(numMessages);
  for (let i = 0; i < numMessages; i++) {
    result[i] = messages[i + startIndex];
  }
  return result;
}

If PersistentArray supported slice, this function could be simplified to:

export function getMessages(): PostedMessage[] {
  return messages.slice(-MESSAGE_LIMIT)
}

Which feels much more natural to me, coming from JavaScript.

As to whether myPersistentVector.slice() ought to return an array or a new PersistentVector, I don't yet feel confident enough with the design goals of this library to feel equipped to judge.

A third option: I think @willemneal may be working on a new pattern for serializing arbitrary data structures to contract storage, which could eventually obsolesce all Persistent* classes. If we think that might be the direction we go in, I would want to avoid merging any new improvements to PersistentVector, so people don't start depending on a feature just to have it deprecated soon after.

I'd definitely love input from others. @potatodepaulo @evgenykuzyakov, what do y'all think?

[DanielRX]

If you added start and end to PersistentVector and did @op("[]") as get(index + start) then

myPersistentVector.slice(0, 10) => new PV which then can be cast to an array
So slice would be like a data view, rather than a cast.

With this slice, there would be no way to do things like "Get a slice of the messages from x onwards, and loop over them until you find an item" because as the vector grows, the sliced array would grow and run OOG eventually.