如何在给定异步的情况下使用cats-effect的IO生成Traversable

Question

What I'm really trying to do is monitor multiple files and when any of them is modified I'd like to update some state and produce a side effect using this state. I imagine what I want is a scan over a Traversable that produces a Traversable[IO[_]] . But I don't see the path there.

as a minimal attempt to produce this I wrote

package example

import better.files.{File, FileMonitor}
import cats.implicits._
import com.monovore.decline._
import cats.effect.IO
import java.nio.file.{Files, Path}
import scala.concurrent.ExecutionContext.Implicits.global

object Hello extends CommandApp(
  name = "cats-effects-playground",
  header = "welcome",
  main = {
    val filesOpts = Opts.options[Path]("input", help = "input files") 
    filesOpts.map { files =>
      IO.async[File] { cb  =>
        val watchers = files.map { path =>
          new FileMonitor(path, recursive = false) {
            override def onModify(file: File, count: Int) = cb(Right(file))
          }
        }
        watchers.toList.foreach(_.start)
      }
        .flatMap(f => IO { println(f) })
        .unsafeRunSync
    }
  }
)

but this has two major flaws. One it creates a thread for each file I'm watching, which is a little heavy. But more importantly the program finishes as soon as a single file is modified, even though onModify would be called more times if the program stayed running.

I'm not married to using better-files, it just seemed like the path of least resistance. But I do require using Cats IO.

Answer 1

This solution doesn't solve the issue of creating a bunch of threads, and it doesn't strictly produce a Traversable, but it solves the underlying use case. I'm very open to this being critiqued and a better solution provided.

package example

import better.files.{File, FileMonitor}
import cats.implicits._
import com.monovore.decline._
import cats.effect.IO
import java.nio.file.{Files, Path}
import java.util.concurrent.LinkedBlockingQueue
import scala.concurrent.ExecutionContext.Implicits.global

object Hello extends CommandApp(
  name = "cats-effects-playground",
  header = "welcome",
  main = {
    val filesOpts = Opts.options[Path]("input", help = "input files")
    filesOpts.map { files =>
      val bq: LinkedBlockingQueue[IO[File]] = new LinkedBlockingQueue()
      val watchers = files.map { path =>
        new FileMonitor(path, recursive = false) {
          override def onModify(file: File, count: Int) = bq.put(IO(file))
        }
      }
      def ioLoop(): IO[Unit] = bq.take()
          .flatMap(f => IO(println(f)))
          .flatMap(_ => ioLoop())

      watchers.toList.foreach(_.start)
      ioLoop.unsafeRunSync
    }

  }
)