Reading a file path from property and then reading the file idiomatic Scala - scala

Question

I want to read in the path of a file from configureation and then read the file in an idomatic Scala way. This is the code I have so far:
val key: Option[String] = {
val publicKeyPath: Option[String] = conf.getString("bestnet.publicKeyFile")
publicKeyPath match {
case Some(path) => {
Future {
val source = fromFile(s"./$path")
val key: String = source.getLines.toIterable.drop(1).dropRight(1).mkString
source.close()
key
} onComplete {
case Success(key) => Success(key)
case Failure(t) => None
}
}
case None => None
}
}
However this is not working since Im getting the error Expression of type Unit does not conform to Option[String]
What am I getting wrong and is my approach idiomatic Scala or should it be done in some other way?

Answer 1

If you want to return the contents as String there is no need to use a Future. E.g. the following would do:
val key: Option[String] = {
val publicKeyPath: Option[String] = conf.getString("bestnet.publicKeyFile")
publicKeyPath match {
case Some(path) =>
val source = fromFile(s"./$path")
val key: String = source.getLines.toIterable.drop(1).dropRight(1).mkString
source.close()
Some(key)
case None =>
None
}
}
The pattern of transforming the value of a Some(_) can be done more idiomatic using the higher-level function map, i.e.:
val key: Option[String] = {
val publicKeyPath = conf.getString("bestnet.publicKeyFile")
publicKeyPath.map(path => {
val source = fromFile(s"./$path")
val key = source.getLines.toIterable.drop(1).dropRight(1).mkString
source.close()
key
})
}
A more idiomatic way to do resource management (i.e. closing the Source) is by using the "Loan Pattern". For example:
def using[A](r: Resource)(f: Resource => A): A = try {
    f(r)
} finally {
r.dispose()
}
val key: Option[String] = {
val publicKeyPath = conf.getString("bestnet.publicKeyFile")
publicKeyPath.map(path =>
using(fromFile(s"./$path"))(source =>
source.getLines.toIterable.drop(1).dropRight(1).mkString
)
)
}
Scala is a flexible language and it is not uncommon to define such an abstraction in user-land (whereas in Java, the using abstraction is a language feature).
If you need non-blocking parallel code you should return a Future[String] instead of an Option[String]. This complicates the automatic resource management, since code is executed at a different time. Anyway, this should give you some pointers for improving your code.