functional-streams-for-scala/fs2

Compositional, streaming I/O library for Scala

Peter Aaser

@PeterAaser

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

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

Peter Aaser

@PeterAaser

wow, who would have thought

PeterAaser @PeterAaser gathers around the fireplace as the shaman prepares to commune with the spirits of Pull

Fabio Labella

@SystemFw

:joy:

ok enough, I'll better start

although I kind of prefer for the interested people to be there, so they can ask questions if I'm not making any sense...

Peter Aaser

@PeterAaser

Yeah, we should see if he comes back

he must be initiated! He must write his first pipe and be accepted by the tribe as an adult

Gavin Bisesi

@Daenyth

I'm here

sorry

had a $work conversation

Justin Heyes-Jones

@justinhj

@PeterAaser yes that got it! thanks

Fabio Labella

@SystemFw

Right, so, I think a nice way to thinking of effects is as languages

Peter Aaser

@PeterAaser

Glad to hear it, with @SystemFw s blessing the fs2 tribe will now consider you an adult @justinhj

Fabio Labella

@SystemFw

by effects here I mean F[_], like Stream, Pull, IO, but also Option, List or what have you

Gavin Bisesi

@Daenyth

:+1:

Justin Heyes-Jones

@justinhj

@SystemFw yeah the list accumulation was an experiment. I didn't really want to do it that way because it felt wrong

Forgot to delete it from the gist

Fabio Labella

@SystemFw

these languages have some words that are unique to them (e.g. only Stream has uncons), and some that are abstracted in algebras (e.g.flatMap, since they are all monads)

so, what's a Pull

the first thing is that fs2 is a pull based streaming library

Justin Heyes-Jones

@justinhj

If you do want to go into detail on Pull I am all ears

Fabio Labella

@SystemFw

yep, will do, it's not that hard, no worries :)

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