Replace if-without-else in Scala

Question

How do you usually use to replace if-without-else in Scala in functional way?

For example, like this typical pattern in imperative style:

var list = List("a", "b", "c")

if (flag) { // flag is boolean variable
    // do something inside if flag is true
    list = "x" :: list
}
// if flag is false, nothing happened

I'm thinking like this to make it functional:

val tempList = List("a", "b", "c")
val list = if (flag) "x" :: tempList else tempList

Could there be a better way without using intermediary variable?

So anyone can share how do you eliminate if-without-else in scala?

In functional programming, there usually is no if without an else - you always need to get some value back. You only might not need to declare it explicitly, when you use a function that provides a default value. — Bergi
– Bergi, Commented Feb 1, 2015 at 14:34
Your second version looks fine to me. Clear what's going on, no mutable vars, no problem — The Archetypal Paul
– The Archetypal Paul, Commented Feb 1, 2015 at 15:19

Rex Kerr · Accepted Answer · 2015-02-01 23:16:59Z

4

It's generally better to avoid cluttering your namespace with temporary variables. So something like this would be better:

val list = {
  val temp = List("a", "b", "c")
  if (flag) "x" :: temp else temp
}

or

val list = List("a", "b", "c") match {
  case x if flag => "x" :: x
  case x => x
}

If you find yourself doing this a lot and performance is not a big concern, it might be handy to define an extension method. I have one in my personal library that looks like this:

implicit class AnythingCanPickFn[A](private val underlying: A) extends AnyVal {
  /** Transforms self according to `f` for those values where `p` is true. */
  @inline def pickFn(p: A => Boolean)(f: A => A) =
    if (p(underlying)) f(underlying) else underlying
}

You'd use this one like so:

List("a", "b", "c").pickFn(_ => flag){ "x" :: _ }

edited Feb 1, 2015 at 23:16

answered Feb 1, 2015 at 16:27

Rex Kerr

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3 Comments

Gabriele Petronella Over a year ago

I totally agree with the less local "variables" principle. For even less cluttering, you can replace the x in the first match with a _

Bergi Over a year ago

Shouldn't that be "x" :: x in that match case?

Rex Kerr Over a year ago

@Bergi - Yes it should. Fixed, thanks! (I should know better than to try to rename variables after testing.)

som-snytt · Accepted Answer · 2015-02-01 23:48:56Z

2

You could skin the cat by making the optional part optional:

scala> val flag = true
flag: Boolean = true

scala> Option("x").filter(_ => flag).toList ::: "a" :: "b" :: "c" :: Nil
res0: List[String] = List(x, a, b, c)

scala> val flag = false
flag: Boolean = false

scala> Option("x").filter(_ => flag).toList ::: "a" :: "b" :: "c" :: Nil
res1: List[String] = List(a, b, c)

or conversely

scala> sys.props.get("flag")
res3: Option[String] = None

scala> sys.props.get("flag").map(_ => "x").toList ::: List("a","b","c")
res4: List[String] = List(a, b, c)

scala> sys.props("flag") = "true"

scala> sys.props.get("flag").map(_ => "x").toList ::: List("a","b","c")
res6: List[String] = List(x, a, b, c)

answered Feb 1, 2015 at 23:48

som-snytt

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Comments

user1498953 · Accepted Answer · 2015-02-01 19:30:42Z

0

It can be done using scalaz's ternary operator

import scalaz._
import Scalaz._

val temp = List("a", "b", "c")
val list = (flag) ? ("x" :: temp) | temp


libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.0.6"

answered Feb 1, 2015 at 19:30

user1498953

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