hi dears kotliners my issue of the day: how can i ...

# announcements

[nfrankel]

hi dears kotliners
my issue of the day:
how can i have a higher-order function that takes a vararg?
something like:

fun foo( (vararg args: Any?) -> Unit) = ...

[Shawn]

vararg isn’t (yet?) supported on lambda parameters, you’ll have to use an anonymous fun

[Shawn]

as for mandating that as a parameter type… I think Array<Any?> might technically be what it is under the hood but I doubt the compiler considers the signatures compatible

[Nicholas Bilyk]

Shawn is correct, it would be KFunction1<Array<out Any?>, Unit> under the hood.

[Nicholas Bilyk]

fun vArgFunc(vararg t: String) {
	println("All ${t.joinToString()}")
}

fun t() {
	val z: KFunction1<Array<out String>, Unit> = ::vArgFunc
	z(arrayOf("one", "two", "three"))
}

[tseisel]

As a workaround, you can use the following signature :

fun foo(f: (Array<out Any?>) -> Unit)

, which is equivalent, and then simulate the vararg parameters with arrayOf(...).

[nfrankel]

thanks guys for your super quick answer
the array trick doesn’t in my case
as i want to genericize the following code:

var sort2 = fun(frequencies: Map<String, Int>) = frequencies
    .map { it.key to it.value }
    .sortedByDescending { it.second }

fun profile(f: (Map<String, Int>) -> List<Pair<String, Int>>): (Map<String, Int>) -> List<Pair<String, Int>> {
    val profilewrapper = fun(arg: Map<String, Int>): List<Pair<String, Int>> {
        val start = System.currentTimeMillis()
        val result = f.invoke(arg)
        val elapsed = start - System.currentTimeMillis()
        print("${f.toString()} took $elapsed secs")
        return result
    }
    return profilewrapper
}

fun main() {
    sort2 = profile(sort2)
}

so that profile() accepts a function that takes a vararg and return Any

[Nicholas Bilyk]

ah I see. You'll need profile1, profile2, profile3 etc for the number of arguments you'll want to parameterize

[Nicholas Bilyk]

fun <P1, R> profile1(f: (P1)->R)): (P1) -> R { ... }
fun <P1, P2, R> profile2(f: (P1, P2)->R)): (P1, P2) -> R { ... }

[nfrankel]

i thought about that @Nicholas Bilyk
but i’d be happy to have the vararg stuff if i can
though from your answer, it seems it’s not possible

[Nicholas Bilyk]

nope. vararg is just sugar around accepting an array parameter. You're thinking of something like ecmascript's arguments array, which Kotlin doesn't support

[nfrankel]

many thanks for your help!

[Nicholas Bilyk]

np 🙂

[Nicholas Bilyk]

what if you changed it instead of profile(p1, p2) to
profile { sort2(frequencies) }

[Nicholas Bilyk]

var sort2 = fun(frequencies: Map<String, Int>) = frequencies
    .map { it.key to it.value }
    .sortedByDescending { it.second }

inline fun <R> profile(inner: () -> R): R {
    
        val start = System.currentTimeMillis()
        val result = inner()
		val elapsed = start  -System.currentTimeMillis()
       
        print("took $elapsed millisecs")
        return result
  
}

fun main(args: Array<String>) {
    val sorted = profile { sort2(mapOf()) }
}

[nfrankel]

🤔
it’s not a real project
so my constraint is to be able to replace the sort2 variable with one wrapped by profile()
otherwise, yes, good point