Hey folks. I have a sequence and I want to create ...

# getting-started

[Gus]

Hey folks. I have a sequence and I want to create a function to group the elements in that sequence on the fly (i.e., without waiting until the last element on the original sequence). Can I do that with a purely functional approach? I can get it to work with sequence {} but it feels like an overkill to use a couroutine for this.

Consider the code below:

data class MessageWithExpiry(
    val messageSerialized: ByteArray,
    val expiryDate: ZonedDateTime
)

data class Batch(
    val messages: Array<ByteArray>,
    val latestMessageExpiryDate: ZonedDateTime
)

fun batchMessages(messages: Sequence<MessageWithExpiry>): Sequence<Batch> = TODO()

Each Batch should contain an array of messages (taken from MessageWithExpiry.messageSerialized) which collectively don't exceed 8 MiB. If messages is empty, the result should be empty too. If all the messages fit in one batch (i.e., they don't exceed the 8 MiB limit collectively), then only one batch should be output -- otherwise, multiple `Batch`es should be output. Additionally, each batch should have the latest expiry date of any message in the batch.

Intuitively, I think I should split this in two parts: One for the batching itself (using something like Sequence.scan but not that per se), and another for the computation of each Batch.latestMessageExpiryDate (using Sequence.map). But I don't think Sequence.scan, Sequence.reduce , Sequence.fold or any of their variations would help me do the first part.

Any pointers on whether/how I can do this using a functional approach?

[Tobias Berger]

So if I understood correctly, you don't want to group by a certain property, but you just want to build batches of multiple messages (up to a certain size).
The only way I see to achieve this is implementing a custom Sequence that combines multiple elements of the original sequence into one when the next element is fetched.

[Gus]

So if I understood correctly, you don't want to group by a certain property, but you just want to build batches of multiple messages (up to a certain size).

That's right

The only way I see to achieve this is implementing a custom Sequence that combines multiple elements of the original sequence into one when the next element is fetched.

Sorry, what do you mean by custom sequence? Using sequence {} and `yield`ing the values?

[Tobias Berger]

that's one way to do it - and actually that way uses coroutines.
Sequence in kotlin is actually just an interface that requires you to provide an interator. You can implement it as a class without any predefined kotlin functions. But that's rarely necessary.

[Gus]

I see! I didn't realise I could just implement that interface. Thanks Tobias!

[Tobias Berger]

just for fun I wrote a possible implementation down (using sequence{} and yield). I have not tested this at all but it looks like it should work
https://gist.github.com/tobiberger/178f436d23fa40f388f4c891921cf0f7

[Gus]

LGTM. Thank you so much!