What is the proper way to code a read-while loop in Scala?

Question

What is the \"proper\" of writing the standard read-while loop in Scala? By proper I mean written in a Scala-like way as opposed to a Java-like way.

Here is the code I

Answer 1

What Rex Kerr suggests in his comment is the following:

val md = MessageDigest.getInstance("MD5")
val input = new FileInputStream("foo.txt")
val buffer = new Array[ Byte ]( 1024 )
Stream.continually(input.read(buffer))
  .takeWhile(_ != -1)
  .foreach(md.update(buffer, 0, _))
md.digest

The key is the Stream.continually. It gets an expression which is evaluated continually, creating an infinite Stream of the evaluated expression. The takeWhile is the translation from the while-condition. The foreach is the body of the while-loop.

Answer 2

What about a curried function? You 11 lines of Scala code become:

val md = MessageDigest.getInstance(hashInfo.algorithm)
val input = new FileInputStream("file")
iterateStream(input){ (data, length) => 
    md.update(data, 0, length)
}
md.digest

The iterateStream function on line 3, which you could add to a library is:

def iterateStream(input: InputStream)(f: (Array[Byte], Int) => Unit){
    val buffer = new Array[Byte](512)
    var curr = input.read(buffer)
    while(curr != -1){
        f(buffer, curr)
        curr = input.read(buffer)
    }
}

The ugly duplicated code (where the input is read) ends up in the library, well tested and hidden away from the programmer. I feel that the first block of code is less complex than the Iterator.continually solution.

Answer 3

Based on Rex's post that he mentioned:

Stream.continually(input.read(buffer)).takeWhile(_ != -1).foreach(md.update(buffer, 0, _))

You should replace the var readLen + while {...} lines with it, it produces the same result.

As Rex mentioned, it works with scala 2.8.