sys.process to wrap a process as a function

Question

I have an external process that I would like to treat as a function from String=>String. Given a line of input, it will respond with a single line of output. It seems that I should use scala.sys.process, which is clearly an elegant library that makes many shell operations easily accessible from within scala. However, I can't figure out how to perform this simple use case.

If I write a single line to the process' stdin, it prints the result in a single line. How can I use sys.process to create a wrapper so I can use the process interactively? For example, if I had an implementation for ProcessWrapper, here is a program and it's output:

// abstract definition
class ProcessWrapper(executable: String) {
  def apply(line: String): String
}

// program using an implementation
val process = new ProcessWrapper("cat -b")
println(process("foo"))
println(process("bar"))
println(process("baz"))

Output:

 1  foo
 2  bar
 3  baz

It is important that the process is not reloaded for each call to process because there is a significant initialization step.

Answer 1

So - after my comment - this would be my solution

import java.io.BufferedReader
import java.io.File
import java.io.InputStream
import java.io.InputStreamReader

import scala.annotation.tailrec

class ProcessWrapper(cmdLine: String, lineListenerOut: String => Unit, lineListenerErr: String => Unit,
                     finishHandler: => Unit,
                     lineMode: Boolean = true, envp: Array[String] = null, dir: File = null) {

  class StreamRunnable(val stream: InputStream, listener: String => Unit) extends Runnable {
    def run() {
      try {
        val in = new BufferedReader(new InputStreamReader(this.stream));
        @tailrec
        def readLines {
          val line = in.readLine
          if (line != null) {
            listener(line)
            readLines
          }
        }
        readLines
      }
      finally {
        this.stream.close
        finishHandler
      }
    }
  }
  val process = Runtime.getRuntime().exec(cmdLine, envp, dir);
  val outThread = new Thread(new StreamRunnable(process.getInputStream, lineListenerOut), "StreamHandlerOut")
  val errThread = new Thread(new StreamRunnable(process.getErrorStream, lineListenerErr), "StreamHandlerErr")
  val sendToProcess = process.getOutputStream
  outThread.start
  errThread.start

  def apply(txt: String) {
    sendToProcess.write(txt.getBytes)
    if (lineMode)
      sendToProcess.write('\n')
    sendToProcess.flush
  }

}

object ProcessWrapper {
  def main(args: Array[String]) {
    val process = new ProcessWrapper("python -i", txt => println("py> " + txt),
      err => System.err.println("py err> " + err), System.exit(0))
    while (true) {
      process(readLine)
    }
  }
}

The main part is the StreamRunnable, where the process is read in a thread and the received line is passed on to a "LineListener" (a simple String => Unit - function). The main is just a sample implementation - calling python ;)

Answer 2

I'm not sure, but you want somethings like that ?

  case class ProcessWrapper(executable: String) {
    import java.io.ByteArrayOutputStream
    import scala.concurrent.duration.Duration
    import java.util.concurrent.TimeUnit

    lazy val process = sys.runtime.exec(executable)

    def apply(line: String, blockedRead: Boolean = true): String = {
      process.getOutputStream().write(line.getBytes())
      process.getOutputStream().flush()
      val r = new ByteArrayOutputStream
      if (blockedRead) {
        r.write(process.getInputStream().read())
      }
      while (process.getInputStream().available() > 0) {
        r.write(process.getInputStream().read())
      }
      r.toString()
    }

    def close() = process.destroy()
  }

  val process = ProcessWrapper("cat -b")

  println(process("foo\n"))
  println(process("bar\n"))
  println(process("baz\n"))
  println(process("buz\n"))
  println(process("puz\n"))

  process.close

Result :

 1    foo

 2    bar

 3    baz

 4    buz

 5    puz

I think that PlayCLI is a better way.