Can I have instructions in each flatmap case? If not, how to increase counter?

Question

I have a flatmap that's going through a Sequence, and I'm using case in order to identify different elements in the Sequence, and replace the elements with something else. However, in some of those cases, I also want to increase a counter variable. Can I put an instruction inside a case? It seems that I can't, since what goes inside the case is what's going to replace the element of the Sequence. Is this possible at all? Here's the code that I tried, and it didn't work:

var location = 0
code.flatMap {
        case Define(label) => Word("")                            // Return an empty Word, don't increase location
        case CodeWord(word) => word; location = location + 4    // Return the Word and increase location by 4
        case Use(label) => Word(Assembler.encodeUnsigned(labelToValue(label))); location = location + 4
                                                                  // Convert label into Seq, then into Word. Increase location.
        case BeqBne(bits, label) =>{ // Are brackets necessary?
            Word(bits ++ encodeUnsigned(labelToValue(label) - (location + 4))); location = location + 4 }
        case _ => require(false, s"Encountered unsupported code $code.");
      }

It's important that I update the location variable because as you can see, in case BeqBne(bits, label) I need to use the current value of location. Should I create a function outside of flatmap which does all of that stuff and then just call the function and pass the values in each case inside of flatmap? Any advice is appreciated.

I might be helpful to provide some context for this code. From what I can glean from the above code snippet, I suspect that `flatMap`, or at least the way you're applying it, is not the most natural approach. Maybe I'm being picky, but the combination of `flatMap` with side effects sets off my spidey sense. — Jason Scott Lenderman, Oct 08 '15 at 02:22

score 2 · Accepted Answer · answered Oct 07 '15 at 22:44

When specifying multiple statements in a block of code the last statement is used as the return value. So you will need to increase the location variable first and then create the Word result.

E.g., change:

case Use(label) => {
  Word(Assembler.encodeUnsigned(labelToValue(label)));
  location = location + 4
}

To:

case Use(label) => {
  location = location + 4;
  Word(Assembler.encodeUnsigned(labelToValue(label)));
}

Or if you need to increment location after creating the result then store the result in a variable:

case BeqBne(bits, label) => {
  val result = Word(bits ++ encodeUnsigned(labelToValue(label) - (location + 4)));
  location = location + 4;
  result
}

Can I have instructions in each flatmap case? If not, how to increase counter?

1 Answers1