functional-streams-for-scala/fs2

Most discussion is on Typelevel Discord: https://discord.gg/bSQBZA3Ced

Jose C

@jmcardon

@justinhj this is just for the sake of an exercise correct?

because the actual implementation of take is in terms of uncons

Justin Heyes-Jones

@justinhj

I'm just extending the exercise in a different way for the learning experience. I was taking the first implementation of tk (that uses scanSegmentsOpt) as a starting point rather than the second one that uses uncons

Jose C

@jmcardon

:+1: cool. yeah that's a good way to learn

Denis Mikhaylov

@notxcain

Hi! How do I change the effect type? I have a FunctionK[F, G] and a Stream[F, A], and I need Stream[G, A]

epifania villamor

@fgalaban_twitter

Denis Mikhaylov

@notxcain

Ah, nevermind, found a way around

Fabio Labella

@SystemFw

@notxcain generally you can do so with translate, or in the case of Pure ~> F, simply use covary

Denis Mikhaylov

@notxcain

@SystemFw nice, thank you!

Paulius Imbrasas

@CremboC

hey @SystemFw , we're running a service reading from Google PubSub, and we occassionaly this beautiful stack trace: https://gist.github.com/CremboC/6f3f158bd32720890e47519211a1a71d (sorry it's in one line, but you'll see a pattern quickly!). any ideas?

Fabio Labella

@SystemFw

@CremboC what version are you on?

this was fixed in 0.10.3

Paulius Imbrasas

@CremboC

ah, on 0.10.1

I assume they're backwards compatible?

Fabio Labella

@SystemFw

yep

Paulius Imbrasas

@CremboC

coolio, will update, danke!

Justin Heyes-Jones

@justinhj

I've been trying to write a function that streams until it has seen N objects equal to a provided instance of O. The last piece of the puzzle is how to emit the stream. I think I need to pull output but somehow need to join that to the tail of the stream in the recursive call? https://gist.github.com/justinhj/10fd8dc2a364a1fdbade36352b15abe0

Peter Aaser

@PeterAaser

I'm not sure what you are trying to do here

maintaining a list, but with Pipe[F,O,O] as type seems incompatible

Pull.output1 might be what you want to use

typically you compose Pulls with >>

how about

s.pull.uncons1.flatMap {

            case Some((o, tl)) =>

              if(o == thing) {
                output1(o) >> go(tl, n - 1)
              }
              else {
                output1(o) >> go(tl,n)
              }

            case None =>
              Pull.done

does that seem like what you are after @justinhj

Peter Aaser

@PeterAaser

Pull forms a monad in R btw, so >> is just myPull.flatMap{ _ => ... } if that helps any (but this is in no way necessary to understand for using pulls)

Jose C

@jmcardon

@PeterAaser sort of useful when you need to evaluate effects in a Pull though

since eval returns it in R not O

Fabio Labella

@SystemFw

@justinhj you need to Pull.output/output1 as @PeterAaser says, before recursing (using >> to tie things)

another problem is that you are accumulating a List[O] as your output

actually that's the confusion

you are used to the typical accumulator based thing, where you first accumulate the whole result and then you "return it"

but here you want to do this in a streaming fashion, so as soon as you find something that needs to be emitted, you just emit it

and Pull.output/output1 is the command that lets you do so

Fabio Labella

@SystemFw

I can explain the idea behind Pull a bit more in depth, if you want

Peter Aaser

@PeterAaser

To make it very clear, imagine if the stream never reaches enough elements to halt

in that case it would never emit!

When I started out I wish I'd ask what @SystemFw is offering @justinhj

so you should say yes!

Gavin Bisesi

@Daenyth

@SystemFw I'd be interested in hearing about it

Peter Aaser

@PeterAaser

What being pull based entailed did not fully dawn on me for a long while

it was a huge missing piece for me in my internal model of how fs2 worked

Fabio Labella

@SystemFw

@Daenyth sure

I was going on to add a little interlude on how to view effects as languages, but I can skip that if you want

and only focus on Pull

wdyt?

jmcardon @jmcardon gathers for grand wizard Fabio's mini-talk

Fabio Labella

@SystemFw

Peter Aaser

@PeterAaser

how do I do that

the purple text thingy

PeterAaser @PeterAaser attempts to read documentation

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