Naive file sync implementation in Kotlin

file-sync.kt

fun sync(from: File, to: File, detectChange: (File, File) -> Boolean, filePredicate : (File) -> Boolean = {true}) {
    require(from.isDirectory)
    require(to.isDirectory)

    from.walkTopDown()
            .filter { it.isDirectory || filePredicate(it) }
            .asSequence()
            .forEach { file ->
                val relative = file.relativeTo(from)
                val destFile = File(to, relative)

                if (file.isDirectory) {
                    if (destFile.isFile) {
                        destFile.delete()
                    }

                    destFile.mkdirsOrFail()
                } else if (destFile.isDirectory) {
                    destFile.deleteRecursively()
                }

                if (file.isFile && (!destFile.exists() || detectChange(file, destFile))) {
                    file.copyTo(destFile, true)
                }
            }

    to.walkTopDown()
        .filter { it.isDirectory || filePredicate(it) }
        .asSequence()
        .forEach { file ->
            val relative = file.relativeTo(to)
            val fromFile = File(from, relative)

            if (!fromFile.exists()) {
                file.deleteRecursively()
            }
        }
}

private fun File.mkdirsOrFail() {
    if (!mkdirs() && !exists()) {
        throw IOException("Failed to create directory $this")
    }
}