FS2 stream to unread InputStream

Question

I'd like to convert fs2.Stream to java.io.InputStream so I can pass that input stream to an http framework (Finch and Akka Http).

I found a fs2.io.toInputStream , but this doesn't work (it prints nothing):

import java.io.{ByteArrayInputStream, InputStream}

import cats.effect.IO
import scala.concurrent.ExecutionContext.Implicits.global

object IOTest {

  def main(args: Array[String]): Unit = {
    val is: InputStream = new ByteArrayInputStream("test".getBytes)
    val stream: fs2.Stream[IO, Byte] = fs2.io.readInputStream(IO(is), 128)

    val test: Seq[InputStream] = stream.through(fs2.io.toInputStream).compile.toList.unsafeRunSync()

    println(scala.io.Source.fromInputStream(test.head).mkString)
  }
}

As far as I understand when I run .unsafeRunSync() it's consuming the whole stream, so even though it returns a Seq[InputStream] the under-laying input stream is already consumed.

Is there any way I can convert fs2.Stream[IO, Byte] to java.io.InputStream without it being consumed?

Thnaks!

Answer 1

The problem is that compile is being invoked prematurely. I'm sure that under the hood fs2.io.toInputStream does the correct thing and brackets the created InputStream . Which means that the InputStream must be accessed inside the Stream itself (eg, in a map / flatMap call):

val wire: fs2.Stream[IO, Byte] = ???

val result: fs2.Stream[IO, String] = for {
  is <- wire.through(fs2.io.toInputStream)
  str = scala.io.Source.fromInputStream(is).mkString //<--- use the InputStream here
} yield str

println( result.compile.lastOrError.unsafeRunSync() ) //<--- compile at the _very_ end

Outputs:

test

Answer 2

It looks that Finch has fs2 support https://github.com/finagle/finch/tree/master/fs2 and Akka also has it's stream implementation and there are fs2 - Akka Stream interop libraries like https://github.com/krasserm/streamz/tree/master/streamz-converter

So i recommend you to take a look to the implementations because they take care of the resources life cycle. Probably you don't need the whole library but it serves as guideline.

And if you are starting at the "safe zone" with fs2, why moving out of there :)