Other fun variations so you can see the power of Kotlin:

A quick version by creating the string to write all at once:

File("somefile.txt").writeText(history.entries.joinToString("\n") { "${it.key}, ${it.value}" })
// or just use the toString() method without transform:
File("somefile.txt").writeText(x.entries.joinToString("\n"))

Or assuming you might do other functional things like filter lines or take only the first 100, etc. You could go this route:

File("somefile.txt").printWriter().use { out ->
    history.map { "${it.key}, ${it.value}" }
           .filter { ... }
           .take(100)
           .forEach { out.println(it) }
}

Or given an Iterable, allow writing it to a file using a transform to a string, by creating extension functions (similar to writeText() version above, but streams the content instead of materializing a big string first):

fun <T: Any> Iterable<T>.toFile(output: File, transform: (T)->String = {it.toString()}) {
    output.bufferedWriter().use { out ->
        this.map(transform).forEach { out.write(it); out.newLine() }
    }
}

fun <T: Any> Iterable<T>.toFile(outputFilename: String, transform: (T)->String = {it.toString()}) {
    this.toFile(File(outputFilename), transform)
}

used as any of these:

history.entries.toFile(File("somefile.txt")) {  "${it.key}, ${it.value}" }

history.entries.toFile("somefile.txt") {  "${it.key}, ${it.value}" }

or use default toString() on each item:

history.entries.toFile(File("somefile.txt")) 

history.entries.toFile("somefile.txt") 

Or given a File, allow filling it from an Iterable, by creating this extension function:

fun <T: Any> File.fillWith(things: Iterable<T>, transform: (T)->String = {it.toString()}) {
    this.bufferedWriter().use { out ->
        things.map(transform).forEach { out.write(it); out.newLine() }
    }
}

with usage of:

File("somefile.txt").fillWith(history.entries) { "${it.key}, ${it.value}" }

or use default toString() on each item:

File("somefile.txt").fillWith(history.entries) 

which if you had the other toFile extension already, you could rewrite having one extension call the other:

fun <T: Any> File.fillWith(things: Iterable<T>, transform: (T)->String = {it.toString()}) {
    things.toFile(this, transform)
}