Scala: how to use immutable values returned from function

Question

I am new to Scala and try to use it in a functional way. Here are my questions:

Why can't I create a new binding for 'cnt' variable with function return value using '<-' operator?

How can increment immutable variable in a functional way (similar to Haskell <-) ? For the sake of experiment I don't want to use mutable vars.

import scala.io.Source
object MyProgram {

  def main(args: Array[String]): Unit = {
    if (args.length > 0) {
      val lines = Source.fromFile(args(0)).getLines()
      val cnt = 0
      for (line <- lines) {
        cnt <- readLines(line, cnt)
      }
      Console.err.println("cnt = "+cnt)
    }
  }

  def readLines(line: String, cnt:Int):Int = {
    println(line.length + " " + line)
    val newCnt = cnt + 1
    return (newCnt)
  }
}

As for side effects, I could never expect that (line <- lines) is so devastating! It completely unwinds lines iterator. So running the following snippet will make size = 0 :

val lines = Source.fromFile(args(0)).getLines()
var cnt = 0
for (line <- lines) {
  cnt = readLines(line, cnt)
}
val size = lines.size

Is it a normal Scala practice to have well-hidden side-effects like this?

tip: in scala last statement of the function interpreted as result to return, thus `readLines` can be written as `def readLines(line: String, cnt:Int):Int = { println(line.length + " " + line); cnt + 1}` — om-nom-nom, Mar 19 '13 at 14:10
Scala encourages use of immutability, you have the option of using mutable data structures in situations like the one you have. You can idiomaticaly fold/reduce/recurse to eliminate var's but they are the underlying mechanisms used! — korefn, Mar 19 '13 at 14:17

score 2 · Accepted Answer · answered Mar 19 '13 at 14:04

2

You could fold on lines like so:

val lines = Source.fromFile(args(0)).getLines()
val cnt = lines.foldLeft(0) { case (count, line) => readLines(line, count) }
Console.err.println("cnt = "+cnt)

Your readLines method does side-effect with the call to println, but using foldLeft guarantees left-to-right processing of the list, so the output should be the same.

answered Mar 19 '13 at 14:04

sullivan at broad

68
5

See my addition to original question regarding side-effects in Scala – Anton Ashanin Mar 19 '13 at 14:32

score 1 · Answer 2 · edited May 23 '17 at 11:51

Why can't I reassign immutable 'cnt' variable with function return value using '<-' operator?

Why would you? If you has java experience, <- has the simular meaning as : in for(Item x: someCollection). It is just a syntactic sugar for taking current item from collection and naming it, it is not a bind operator in general.

Moreover, isn't reassign immutable oxymoron?

How can increment immutable variable in a functional way (similar to Haskell <-)?

Scala people usually use .zipWithIndex but this will work only if you're going to use counter inside for comprehension:

for((x, i) <- lines.zipWithIndex) { println("the counter value is" + i) }

So I think you need to stick with lines.count or use fold/reduce or = to assign new value to variable.

You right, I should not use "reassign" so I have changed my question to: "Why can't I create a new binding for 'cnt' variable with function return value using '<-' operator?" And from answers here I now understand that Scala does not provide programmer explicit binding mechanism for immutables like Haskell monads do. — Anton Ashanin, Mar 19 '13 at 14:18

score 0 · Answer 3 · answered Mar 19 '13 at 14:03

0

<- is not an operator, just a syntax used in for expressions. You have to use =. If you want to use <- it must be within the for-iteration-expression. And you cannot increment a val. If you want to modify that variable, make it a var.

answered Mar 19 '13 at 14:03

bluenote10

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Scala: how to use immutable values returned from function

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