How to code this configuration logic in Scala?

Question

This is a follow-up to my previous question

Suppose I use the following logic (in quasi-Java) to get a configuration parameter MyParam :

String myParam = null

if ((myParam = getFromArgs("MyParam")) != null)
   return myParam

if ((myParam = getFromSystemProperties("MyParam")) != null)
   return myParam

if ((myParam = getFromUserConfigFile("MyParam")) != null)
   return myParam

... // and so on

if (myParam == null)
   error("No MyParam")

How to code this logic in Scala? Is Applicative applicable here?

Answer 1

As I noted in a comment above, you can use a "First" monoid instance for Option (really you only need a semigroup) to do this—for example, with Scalaz:

import scalaz._, Scalaz._

def fo[A](a: A) = Option(a).fst

val param = (
  fo(myParam) |+|
  fo(getFromArgs("MyParam")) |+|
  fo(getFromSystemProperties("MyParam")) |+|
  fo(getFromUserConfigFile("MyParam"))
) getOrElse sys.error("No Param")

It's not necessarily preferable to the orElse versions using the standard library, but it highlights the relevant abstraction (note also that this version is lazy—the getX calls won't happen if they aren't needed).

Answer 2

List(getFromArgs _, getFromSystemProperties _, getFromUserConfigFile _).map{func=>
  Option(func("MyParam"))
}.reduce(_ orElse _).getOrElse(sys.error("No MyParam"))

Checkout scala Option cheatsheet. Option is powerful.

Answer 3

If your problem is only a matter of syntax, xiefei answer is exact.

If instead you are really looking for a flexible configuration I would suggest you warmly look to the Typesafe config project at : https://github.com/typesafehub/config/

Answer 4

(
  Option(getFromArgs("MyParam")) orElse
  Option(getFromSystemProperties("MyParam")) orElse
  Option(getFromUserConfigFile("MyParam")) getOrElse
  error("No MyParam")
)