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scaladefaulthigher-order-functionsscala-option

Method call with option value or default parameter in Scala

I'm rather new to Scala and stumbled upon a small little issue that keeps bothering me. Let's say there is some method with default parameter

def foo(v: Any = "default"): String = s"called with parameter '$v'"

and an Option val opt: Option[String]. How to call this method with either the option value (if defined) or the default parameter? I mean despite the obvious solution

val result = if (opt.isDefined)
               from.here.to.foo(opt.get)
             else
               from.here.to.foo()

and having to type the method with (possibly long) object chain twice? Not to mention having more than one optional/default parameter...

All I could come up with is the unhelpful helper

def definedOrDefault[A, B](opt: Option[A], f0: => B, f1: A => B): B =
  if (opt.isDefined) f1(opt.get) else f0

but when not being able to mention default parameters in higher order functions... that's it. Reminds me of the bad old days with Java where method overloading creates the same problem.

Solution

  • It seems like you are using two concepts of value being optional in one place, i.e. optional parameter and Option. They don't like play well together, maybe it is better to use just one.

    If you always just pass value of Option to a method or pass nothing to get default value, maybe consider changing the function to accept the Option.

    def foo(v: Option[String]) = {
  val param = v getOrElse "default"
  s"called with parameter '$param'"
}

    If you still want to have default parameter you can change signature to 

    def foo(v: Option[String] = None)

    This approach however will require you to wrap your parameter into Some when you do a regular call e.g.

    foo(Some("regular param"))

    but it works well when you use Options. You can easly add more optional parameters as well.

    Here is an example

    def foo(v1: Option[String] = None, v2: Option[Int] = None) = {
  val param1 = v1 getOrElse "default"
  val param2 = v2 getOrElse 42
  s"'$param1', '$param2'"
}

foo() // "default", 42

foo(v2 = Some(12)) // "default", 12

foo(Some("asd"), Some(11)) // "asd", 11

val optionFromSomewhere = Some("a")
val anotherOptionFromSomewhere = Option.empty[Int]
foo(optionFromSomewhere, anotherOptionFromSomewhere) // "a", 42

    You could also introduce implicit conversion from Any to Option, that would let you to omit Some, but I don't think it is such a good idea

     implicit def any2option[A](e: A): Option[A] = Some(e)