Scala function map

Question

I am just starting to learn scala, and perhaps I am a little ambitious. I wanted to map a String to a function that is from a String to Boolean. Unfortunately, I don't really understand the feedback I get from the compiler. Here is the code:

class StdRead {
    def read(key : Array[Int], value : Array[Int],seperator : Array[Char],
        filter : Map[Int, String => Boolean], write : String => Unit) {
    for( ln <- stdin.getLines ) {
        val inarr = ln split seperator 
        for(i <- key) {
            val func = filter get i //func : Option[(String) => Boolean]
            val f = func getOrElse Unit //f : java.lang.Object
            val res = func(inarr(i)) //Error, doesn't work
        }
        //more stuff        
    } 
}

First of all, why do I get an Option, is that the only way to access the function. Secondly, why is the return of getOrElse an object? Shouldn't that be something I can use to invoke the function? It would be great if someone could give me a simple example as to what I need to do here.

Thanks in advance.

@DanielC.Sobral some people have different cultures, and i think it is quite ethno-centric of you to propose that your culture is the defacto one. Like the OP, I too am from a culture that sometimes uses the E form than the A form. — bharal
– bharal, Commented Jul 18, 2013 at 15:17
@bharal Sorry if you feel that way. I could find no dictionary which shows "seperator" as a correct spelling. I see no reason why not help others improve their spelling on what could be honest spelling mistakes. Instead of berating me for helping others, you should have helped me by giving actual examples of cultures with that spelling and references confirming that this is, indeed, a valid spelling on your culture, instead of, perhaps, a mistake on your part. — Daniel C. Sobral
– Daniel C. Sobral, Commented Jul 19, 2013 at 18:01

Kris Nuttycombe · Accepted Answer · 2011-03-30 18:27:52Z

4

As Rex noted, the type of Map.get is A => Option[B], whereas the apply method has the type A => B, but that's a lie because it may throw an exception. In general, I'd advise against using the apply form because of this lie.

In your specific example, you probably shouldn't be using getOrElse at all; instead, since what you're doing is just producing a side effect anyway, try something like the following. I've changed the name 'filter' because that's actually also the name of an important method on Map and other iterables, and overloading that name is a needless source of confusion.

class StdRead {
  def read(key : Array[Int], value : Array[Int],seperator : Array[Char],
      stringTests : Map[Int, String => Boolean], write : String => Unit) {
    for { 
      ln <- stdin.getLines 
      val inarr = ln split seperator 
      i <- key
      stringTest <- stringTests.get(i)
    } {
      stringTest(inarr(i))
    }
  }
}

Notice that you can declare values inside of the expressions of a for comprehension. Remember that these are not loops!

edited Mar 30, 2011 at 18:27

answered Mar 24, 2011 at 4:10

Kris Nuttycombe

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2 Comments

delmet Over a year ago

Thanks for the reply. I need to look into that incomprehensible for "comprehension". More rope to hang myself with. Thanks!!!

Daniel C. Sobral Over a year ago

Great example of for comprehension!

Rex Kerr · Accepted Answer · 2011-03-24 03:47:27Z

3

If you are certain that a map has a certain key you can just grab it directly: filter(i). The get method is used if you are not sure (or to be more careful about errors). It returns an option, from which you have correctly decided to use getOrElse. However, you want getOrElse( (s:String) => /* some boolean */), that is, you must also return a function from String to Boolean there. Then you can apply f, which will be either a successfully obtained function from the map, or whichever default function you supplied in getOrElse.

What happened when you told it to return Unit was that it thought, so to speak, "Well, you say if it's there it's a function, and if it's not it's a Unit...that means that all I can say about the type is that it's an Object."

Anyway, something like

val f = func getOrElse (s: String) => true
val res = inarr.filter(f)

will do what I think you're trying to do (which is to filter the array inarr using the function retrieved from the map).

answered Mar 24, 2011 at 3:47

Rex Kerr

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1 Comment

delmet Over a year ago

AH! I get it. Thanks man. I thought I was doing my self a favor by using Unit and deferring the decision to decide what comes there to later. All the while, I was shooting myself in the foot.

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