How to write to a file in Scala?

Edit 2019 (8 years later), Scala-IO being not very active, if any, Li Haoyi suggests his own library lihaoyi/os-lib, that he presents below.

June 2019, Xavier Guihot mentions in his answer the library Using, a utility for performing automatic resource management.

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Edit (September 2011): since Eduardo Costa asks about Scala2.9, and since Rick-777 comments that scalax.IO commit history is pretty much non-existent since mid-2009...

Scala-IO has changed place: see its GitHub repo, from Jesse Eichar (also on SO):

The Scala IO umbrella project consists of a few sub projects for different aspects and extensions of IO.
There are two main components of Scala IO:

• Core - Core primarily deals with Reading and writing data to and from arbitrary sources and sinks. The corner stone traits are Input, Output and Seekable which provide the core API.
  Other classes of importance are Resource, ReadChars and WriteChars.
• File - File is a File (called Path) API that is based on a combination of Java 7 NIO filesystem and SBT PathFinder APIs.
  Path and FileSystem are the main entry points into the Scala IO File API.

import scalax.io._

val output:Output = Resource.fromFile("someFile")

// Note: each write will open a new connection to file and 
//       each write is executed at the begining of the file,
//       so in this case the last write will be the contents of the file.
// See Seekable for append and patching files
// Also See openOutput for performing several writes with a single connection

output.writeIntsAsBytes(1,2,3)
output.write("hello")(Codec.UTF8)
output.writeStrings(List("hello","world")," ")(Codec.UTF8)

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Original answer (January 2011), with the old place for scala-io:

If you don't want to wait for Scala2.9, you can use the scala-incubator / scala-io library.
(as mentioned in "Why doesn't Scala Source close the underlying InputStream?")

See the samples

{ // several examples of writing data
    import scalax.io.{
      FileOps, Path, Codec, OpenOption}
    // the codec must be defined either as a parameter of ops methods or as an implicit
    implicit val codec = scalax.io.Codec.UTF8


    val file: FileOps = Path ("file")

    // write bytes
    // By default the file write will replace
    // an existing file with the new data
    file.write (Array (1,2,3) map ( _.toByte))

    // another option for write is openOptions which allows the caller
    // to specify in detail how the write should take place
    // the openOptions parameter takes a collections of OpenOptions objects
    // which are filesystem specific in general but the standard options
    // are defined in the OpenOption object
    // in addition to the definition common collections are also defined
    // WriteAppend for example is a List(Create, Append, Write)
    file.write (List (1,2,3) map (_.toByte))

    // write a string to the file
    file.write("Hello my dear file")

    // with all options (these are the default options explicitely declared)
    file.write("Hello my dear file")(codec = Codec.UTF8)

    // Convert several strings to the file
    // same options apply as for write
    file.writeStrings( "It costs" :: "one" :: "dollar" :: Nil)

    // Now all options
    file.writeStrings("It costs" :: "one" :: "dollar" :: Nil,
                    separator="||\n||")(codec = Codec.UTF8)
  }

This is one of the features missing from standard Scala that I have found so useful that I add it to my personal library. (You probably should have a personal library, too.) The code goes like so:

def printToFile(f: java.io.File)(op: java.io.PrintWriter => Unit) {
  val p = new java.io.PrintWriter(f)
  try { op(p) } finally { p.close() }
}

and it's used like this:

import java.io._
val data = Array("Five","strings","in","a","file!")
printToFile(new File("example.txt")) { p =>
  data.foreach(p.println)
}

Similar to the answer by Rex Kerr, but more generic. First I use a helper function:

/**
 * Used for reading/writing to database, files, etc.
 * Code From the book "Beginning Scala"
 * http://www.amazon.com/Beginning-Scala-David-Pollak/dp/1430219890
 */
def using[A <: {def close(): Unit}, B](param: A)(f: A => B): B =
try { f(param) } finally { param.close() }

Then I use this as:

def writeToFile(fileName:String, data:String) = 
  using (new FileWriter(fileName)) {
    fileWriter => fileWriter.write(data)
  }

and

def appendToFile(fileName:String, textData:String) =
  using (new FileWriter(fileName, true)){ 
    fileWriter => using (new PrintWriter(fileWriter)) {
      printWriter => printWriter.println(textData)
    }
  }

etc.

A simple answer:

import java.io.File
import java.io.PrintWriter

def writeToFile(p: String, s: String): Unit = {
    val pw = new PrintWriter(new File(p))
    try pw.write(s) finally pw.close()
  }