Understanding Batch

[DGolubets]

I understand the reason Batch exists.
I've written quite a few GraphQL services with Caliban and used ZQuery there define data loaders.
That said I've never looked behind the curtain until now.

I've read the docs section about Batch.. and I couldn't grasp it.
Then I went to read about ZQuery, hoping they had a better explanation and I couldn't understand it either..

So, pretty much all examples I saw come down to something like this:

  val source = Batch.sourceSeq(myBatchProcessing)
  val res = for
    a <- Batch.eval(Seq(1, 2, 3))
    b <- source(a)
  yield (a, b)
  Batch.run(res)

This leaves me unsatisfied, because why use Batch when I can call myBatchProcessing(Seq(1,2,3))?
I though that Batch should help me, well, batch stuff.
But in this example I had to batch all ids for it.

Something I tried then, hoping initially that it would work:

  val source = Batch.sourceSeq(myBatchProcessing)
  val res = for
    a <- source(1)
    b <- source(2)
  yield (a, b)
  Batch.run(res)

Which didn't batch. Of course it makes sense, because flatMap allows me to access the previous result..

All that left me puzzled. I must be missing something very obvious.