Scala example source code file (FileBackedHistory.scala)

This example Scala source code file (FileBackedHistory.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

compiler, exception, file, filebackedhistory, int, jpersistenthistory, loading, nsc, string, t, unit, vector

The FileBackedHistory.scala Scala example source code

/* NSC -- new Scala compiler
 * Copyright 2005-2013 LAMP/EPFL
 * @author Paul Phillips
 */

package scala.tools.nsc
package interpreter
package session

import scala.tools.nsc.io._
import FileBackedHistory._

/** TODO: file locking.
 */
trait FileBackedHistory extends JLineHistory with JPersistentHistory {
  def maxSize: Int
  protected lazy val historyFile: File = defaultFile
  private var isPersistent = true

  locally {
    load()
  }

  def withoutSaving[T](op: => T): T = {
    val saved = isPersistent
    isPersistent = false
    try op
    finally isPersistent = saved
  }
  def addLineToFile(item: CharSequence): Unit = {
    if (isPersistent)
      append(item + "\n")
  }

  /** Overwrites the history file with the current memory. */
  protected def sync(): Unit = {
    val lines = asStrings map (_ + "\n")
    historyFile.writeAll(lines: _*)
  }
  /** Append one or more lines to the history file. */
  protected def append(lines: String*): Unit = {
    historyFile.appendAll(lines: _*)
  }

  def load(): Unit = {
    if (!historyFile.canRead)
      historyFile.createFile()

    val lines: IndexedSeq[String] = {
      try historyFile.lines().toIndexedSeq
      catch {
        // It seems that control characters in the history file combined
        // with the default codec can lead to nio spewing exceptions.  Rather
        // than abandon hope we'll try to read it as ISO-8859-1
        case _: Exception =>
          try historyFile.lines("ISO-8859-1").toIndexedSeq
          catch { case _: Exception => Vector() }
      }
    }

    repldbg("Loading " + lines.size + " into history.")

    // avoid writing to the history file
    withoutSaving(lines takeRight maxSize foreach add)
    // truncate the history file if it's too big.
    if (lines.size > maxSize) {
      repldbg("File exceeds maximum size: truncating to " + maxSize + " entries.")
      sync()
    }
    moveToEnd()
  }

  def flush(): Unit = ()
  def purge(): Unit = historyFile.truncate()
}

object FileBackedHistory {
  //   val ContinuationChar = '\003'
  //   val ContinuationNL: String = Array('\003', '\n').mkString
  import Properties.userHome

  def defaultFileName = ".scala_history"
  def defaultFile: File = File(Path(userHome) / defaultFileName)
}