How to write to text file - after reading from input? - file

Question

Alternative title: how does one "nicely" "update" the contents of text files with Kotlin?
I have seen how do i write to a file in kotlin.
Basically, the suggested solution there is based on creating a File object, to then write to it: File("somefile.txt").printWriter().use ...
But well: I already have code like this:
File(fname).bufferedReader().lineSequence().flatMap { transform(it) }
where transform() is my own method that either returns the original line, or some updates lines when some specific match matched. (its signature is transform(line : String) : Sequence<String>)
Sure: I could assign the result of flatMap() to some temporary variable, to then use that with printWriter().use.
But is there a nice, canonical way that works without a temporary variable?

Answer 1

Note that your reading code (File().bufferedReader().lineSequence()) can already be simplified to:
File(fname).useLines {
it.flatMap(::transform)
}
Now regarding writing without putting it in an intermediate variable...
File("writeTo").bufferedWriter().use { writer ->
File("readFrom").useLines { lines ->
lines.flatMap(::transform)
.forEach {
writer.appendln(it)
}
}
}
But I am not yet really convinced about it... I don't really like it ;-)
I rather like something like:
File("readFrom").useLines {
..
}.writeTo("writeTo")
but that is not going to work as long as I use useLines ;-)
But here is something similar:
File("readFrom").useLines {
it.flatMap(::transform)
.writeTo(File("writeTo"))
}
But this requires the following extension function then:
fun Sequence<String>.writeTo(file: File) {
file.bufferedWriter().use { writer -> forEach { writer.appendln(it) } }
}
On the other hand... I always wanted to have a sequence.toFile-writer ;-) adapted it already to use a delimiter as parameter...