Defer in Scala

In Scala, we can use a try-finally block to ensure that certain code is executed after a block of code, regardless of whether an exception was thrown. This is similar to the defer keyword in other languages.

Here’s how we could create a file, write to it, and then close it when we’re done:

import java.io._
import scala.util.Using

object DeferExample {
  def main(args: Array[String]): Unit = {
    // We use the `Using` construct to automatically close the resource
    Using(createFile("/tmp/defer.txt")) { file =>
      writeFile(file)
    }
  }

  def createFile(path: String): PrintWriter = {
    println("creating")
    new PrintWriter(new File(path))
  }

  def writeFile(writer: PrintWriter): Unit = {
    println("writing")
    writer.println("data")
  }
}

In this example, we’re using Scala’s Using construct, which is part of the standard library since Scala 2.13. It ensures that the resource (in this case, our file) is properly closed after we’re done with it, even if an exception occurs.

The Using construct takes care of closing the resource for us, so we don’t need a separate closeFile function. However, if we needed to perform additional cleanup, we could do it like this:

def closeFile(writer: PrintWriter): Unit = {
  println("closing")
  writer.close()
}

// In the main function:
Using(createFile("/tmp/defer.txt")) { file =>
  try {
    writeFile(file)
  } finally {
    closeFile(file)
  }
}

Running the program confirms that the file is closed after being written:

$ scala DeferExample.scala
creating
writing
closing

This approach ensures that resources are properly managed and cleaned up, which is crucial for maintaining the integrity and efficiency of your program.