Scala syntax to list files

[Shairon Toledo]

Hi guys,

I'm trying figure out an elegant way do list all files recursively using scala syntax( no libs), I ended with the code below 

    def fetchFiles(path:String)(op:File => Unit) = {

      for (file <- new File(path).listFiles if !file.isHidden){

        op(file)

        if (file.isDirectory)

          fetchFiles(file.getAbsolutePath)(op)

      } 

    }

    fetchFiles("/", println)

    

I've tried implement it just using "for", in different ways, no success.  

--
Shairon Toledo
http://hashcode.me

[Jesse Eichar]

Watch out for the null pointer exception caused by listFiles returning null :)

[√iktor Ҡlang]

 def fetchFiles(path:String)(op:File => Unit): Unit =

      for (files <- Option(new File(path).listFiles); file <- files if !file.isHidden){

        op(file)

        if (file.isDirectory)

          fetchFiles(file.getAbsolutePath)(op)

      } 

fetchFile("/")(println)

On Fri, Mar 16, 2012 at 8:04 PM, Shairon Toledo <shairon...@gmail.com> wrote:

--
Viktor Klang

Akka Tech Lead

Typesafe - The software stack for applications that scale

Twitter: @viktorklang

[Josh Suereth]

val path = "some file"

// Gives us all directories and files in depth-first-searchy fashion.

object files extends Traversable[File] {

  def foreach[U](f: File => U) = {

     def walk(file: File): Unit = 

        if(f.isDirectory)  {

          f(file)

          for {

            files <- Option(file.listFiles)

            file <- files

          } walk(file)

        } else f(file)

     walk(new File(path))

  }

}

files filter (_.getName == ".txt.") // text files

2012/3/16 √iktor Ҡlang <viktor...@gmail.com>

[Brian Schlining]

Hi Shairon,

If you're using Java 7 you can do the following:

import java.io._

import java.nio.file._

import java.nio.file.attribute._

def fetchFiles(path: String)(op: File => Unit) {

    Files.walkFileTree((new File(path)).toPath, new SimpleFileVisitor[Path]{

        override def preVisitDirectory(dir: Path, attrs: BasicFileAttributes) = {

            op(dir.toFile)

            FileVisitResult.CONTINUE

        }

        override def visitFile(file: Path, attrs: BasicFileAttributes): FileVisitResult = {

            op(file.toFile)

            FileVisitResult.CONTINUE

        }

    })

}

fetchFiles("/") { println }

[Daniel Sobral]

You really ought to write some Traversable collections, y'know?

2012/3/16 Josh Suereth <joshua....@gmail.com>:

--
Daniel C. Sobral

I travel to the future all the time.

[Josh Suereth]

You should see my code...

[Shairon Toledo]

Nice code Josh, I understand by eye your strategy, however it didn't run here, I get 

value isDirectory is not a member of java.io.File => U

        if(f.isDirectory)  {

neither inside of object Main{} nor in scala console.

And another question, look in highlights 

 object files extends Traversable[File] {
   def foreach[U](f: File => U) = {
      def walk(file: File): Unit =
         if(f.isDirectory)  {
           f(file)
           for {
             files <- Option(file.listFiles)
             file <- files
           } walk(file)
         } else f(file)
      walk(new File(path))
   }
}

newbie mode: In that context, I didn't get which "files" (green or red) is the reference for the object files and what's return of Option evaluation. Could you clarify? I accept RTFM as answer :)

--
[ ]'s
Shairon Toledo
http://www.google.com/profiles/shairon.toledo

[√iktor Ҡlang]

On Sat, Mar 17, 2012 at 3:02 PM, Shairon Toledo <shairon...@gmail.com> wrote:

Nice code Josh, I understand by eye your strategy, however it didn't run here, I get 

value isDirectory is not a member of java.io.File => U

        if(f.isDirectory)  {

That should be: if (file.isDirectory) {

[Josh Suereth]

I'm running into naming conflics here.  Let's try again:

 object files extends Traversable[File] {
   def foreach[U](f: File => U) = {
      def walk(file: File): Unit =

         if(file.isDirectory)  {
           f(file)
           for {
             nextfiles <- Option(file.listFiles)
             nextfile <- nextfiles
           } walk(nextfile)
         } else f(file)
      walk(new File(path))
   }
}

Note:  That version of foreach has stack issues.  I'm mostly trying to show you how creating a *collection* of files can be really simple.  You can make your own "files" collection for different traversal strategies (depth-first, breadth-first) etc.   Dealing with a collection of files should give you access to all the fun powers of collections in Scala, like map/flatMap/filter/groupBy etc.

So, hopefully, I showed you enough of walk to get the gist and write your own custom versions.  In my experience, breadth-first search is *usually* a better option for these file traversables.

Hope that helps!

- Josh

[Shairon Toledo]

Yeap indeed I'm from ruby world, Traversable is similar to Enumerator mix-in.

Thx.