Kotlin: Pass and consume a 2 parameter function?

Question

I'm trying to learn functional programming in Kotlin and am having difficulty making this code work:

import java.util.*

fun caseName(br: String, c: Int): String {
  if (c == 0) {
    return br.toLowerCase()
    } else {
    return br.toUpperCase()
    }
}
fun mapIt(ns: ArrayList<String>, f: (String, Int) -> String): List<String> {
  val coll: List<String> = ns.map {it -> f(it, _)}
  return coll
}

fun main(args: Array<String>) {
  val names = arrayListOf("Joe", "Bill", "Murrary")
  val cased = mapIt(names, (::caseName)(_, 0))
  println(cased.first())
}

How do I get mapIt to recognize the case flag when mapping over the list?

Thank you!

EDIT: The case above is a simplified version of the following, which does not work either...

data class Borrower(val name: String, val maxBooks: Int) {
    companion object {

        fun getName(br: Borrower, c: Int): String {
          if (c == 0) {
            return br.name.toLowerCase()
            } else {
            return br.name.toUpperCase()
            }
        }

        fun findBorrower(n: String, brs: ArrayList<Borrower>, f: (Borrower) -> String): Borrower? {
            val coll: List<Borrower> = brs.filter { it -> f(it) == n }
            if (coll.isEmpty()) {
                return null
            } else return coll.first()
        }
    }
}

fun main(args: Array<String>) {
    val br1 = Borrower(name = "Borrower1", maxBooks = 1)
    val br2 = Borrower(name = "Borrower2", maxBooks = 2)
    val br3 = Borrower(name = "Borrower3", maxBooks = 3)
    val br4 = Borrower(name = "borrower4", maxBooks = 4)
    val br5 = Borrower(name = "BORROWER5", maxBooks = 5)
    val brs1 = arrayListOf(br1, br2, br3, br4, br5)

    val c = Borrower.findBorrower("borrower3", brs1, {Borrower.Companion::getName(it, 0)})
    println(c)
}

Oliver Charlesworth · Accepted Answer · 2017-06-08 20:22:49Z

5

There are a couple of issues here:

Your objectives are contradictory here - you want mapIt to take a 2-arg function, but it also looks like you're trying to perform a partial application of caseName, which would of course result in a 1-arg function.
Underscores don't do what you think in Kotlin (it looks like maybe you come from a Scala background?).

It looks like you want something either like this:

// f is a *single*-arg function
fun mapIt(ns: ArrayList<String>, f: (String) -> String): List<String> {
    return ns.map(f)
}

fun main(args: Array<String>) {
    val names = arrayListOf("Joe", "Bill", "Murrary")
    val cased = mapIt(names, { caseName(it, 0) })  // Partial application
    println(cased.first())
}

or like this:

// f is a two-arg function
fun mapIt(ns: ArrayList<String>, f: (String, Int) -> String): List<String> {
    return ns.map { f(it, 0) }
}

fun main(args: Array<String>) {
    val names = arrayListOf("Joe", "Bill", "Murrary")
    val cased = mapIt(names, ::caseName)
    println(cased.first())
}

edited Jun 8, 2017 at 20:22

answered Jun 8, 2017 at 19:38

Oliver Charlesworth

274k34 gold badges591 silver badges687 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

ericky Over a year ago

Thank you for your help. Yes I do have some Scala experience but was using the underscores here as a means to describe the holes in my knowledge! I was after the single-arg function and see the error in my thinking - missed the partial application. Would you mind looking at my edit - which inspired the simplified example that I began with? I believe there is a companion object issue? Thank you.

glee8e · Accepted Answer · 2017-06-08 23:56:28Z

1

Your original code (the Borrower one) is actually very close to success, so I'm going to ignore your werid simplified version. You just got one line wrong:

val c = Borrower.findBorrower("borrower3", brs1, {Borrower.Companion::getName(it, 0)})
                                                                    ^^
                                                        Why you are using double colon?

The last parameter is a lambda expression, and it just make no sense to use a function reference inside a lambda, even in scala. Just use plain dots and it will be fine.

val c = Borrower.findBorrower("borrower3", brs1, {Borrower.Companion.getName(it, 0)})

Since members of companion objects can be called with just the main types' name, it can be further simplified into

val c = Borrower.findBorrower("borrower3", brs1, {Borrower.getName(it, 0)})

answered Jun 8, 2017 at 23:56

glee8e

6,4795 gold badges34 silver badges52 bronze badges

1 Comment

ericky Over a year ago

I was using "::" because I thought I had to call a Bound Class Reference. Thank you both for your help.

Collectives™ on Stack Overflow

Kotlin: Pass and consume a 2 parameter function?

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related