Question on Scala Closure (From "Programming in Scala")

Posted by Ekkmanz on Stack Overflow See other posts from Stack Overflow or by Ekkmanz
Published on 2009-07-05T04:47:06Z Indexed on 2010/04/19 4:33 UTC
Read the original article Hit count: 492

Filed under:
|

I don't understand why authors said that Code Listing 9.1 from "Programming in Scala" use closure. In chapter 9, they show how to refactor code into more less duplicated form, from this original code:

object FileMatcher {
  private def filesHere = (new java.io.File(".")).listFiles
  def filesEnding(query: String) =
    for (file <- filesHere; if file.getName.endsWith(query))
      yield file
  def filesContaining(query: String) =
    for (file <- filesHere; if file.getName.contains(query))
      yield file
  def filesRegex(query: String) =
    for (file <- filesHere; if file.getName.matches(query))
      yield file
}

To the second version:

object FileMatcher {
  private def filesHere = (new java.io.File(".")).listFiles
  def filesMatching(query: String,
    matcher: (String, String) => Boolean) = {
      for (file <- filesHere; if matcher(file.getName, query))
        yield file
    }    
  def filesEnding(query: String) =
    filesMatching(query, _.endsWith(_))
  def filesContaining(query: String) =
    filesMatching(query, _.contains(_))
  def filesRegex(query: String) =
    filesMatching(query, _.matches(_))
}

Which they said that there is no use of closure here. Now I understand until this point. However they introduced the use of closure to refactor even some more, shown in Listing 9.1:

object FileMatcher {
  private def filesHere = (new java.io.File(".")).listFiles
  private def filesMatching(matcher: String => Boolean) =
    for (file <- filesHere; if matcher(file.getName))
      yield file
  def filesEnding(query: String) =
    filesMatching(_.endsWith(query))
  def filesContaining(query: String) =
    filesMatching(_.contains(query))
  def filesRegex(query: String) =
    filesMatching(_.matches(query))
}

Now they said that query is a free variable but I don't really understand why they said so? Since ""query"" seems to be passed from top method down to string matching function explicitly.

© Stack Overflow or respective owner

Related posts about scala

Related posts about closures