Implementing ifTrue, ifFalse, ifSome, ifNone, etc. in Scala to avoid if(…) and simple pattern matching

Question

In Scala, I have progressively lost my Java/C habit of thinking in a control-flow oriented way, and got used to go ahead and get the object I\'m interested in first, and the

Answer 1

First: we probably cannot reuse else, as it is a keyword, and using the backticks to force it to be seen as an identifier is rather ugly, so I'll use otherwise instead.

Here's an implementation attempt. First, use the pimp-my-library pattern to add ifTrue and ifFalse to Boolean. They are parametrized on the return type R and accept a single by-name parameter, which should be evaluated if the specified condition is realized. But in doing so, we must allow for an otherwise call. So we return a new object called Otherwise0 (why 0 is explained later), which stores a possible intermediate result as a Option[R]. It is defined if the current condition (ifTrue or ifFalse) is realized, and is empty otherwise.

class BooleanWrapper(b: Boolean) {
  def ifTrue[R](f: => R) = new Otherwise0[R](if (b) Some(f) else None)
  def ifFalse[R](f: => R) = new Otherwise0[R](if (b) None else Some(f))
}
implicit def extendBoolean(b: Boolean): BooleanWrapper = new BooleanWrapper(b)

For now, this works and lets me write

someTest ifTrue {
  println("OK")
}

But, without the following otherwise clause, it cannot return a value of type R, of course. So here's the definition of Otherwise0:

class Otherwise0[R](intermediateResult: Option[R]) {
  def otherwise[S >: R](f: => S) = intermediateResult.getOrElse(f)
  def apply[S >: R](f: => S) = otherwise(f)
}

It evaluates its passed named argument if and only if the intermediate result it got from the preceding ifTrue or ifFalse is undefined, which is exactly what is wanted. The type parametrization [S >: R] has the effect that S is inferred to be the most specific common supertype of the actual type of the named parameters, such that for instance, r in this snippet has an inferred type Fruit:

class Fruit
class Apple extends Fruit
class Orange extends Fruit

val r = someTest ifTrue {
  new Apple
} otherwise {
  new Orange
}

The apply() alias even allows you to skip the otherwise method name altogether for short chunks of code:

someTest.ifTrue(10).otherwise(3)
// equivalently:
someTest.ifTrue(10)(3)

Finally, here's the corresponding pimp for Option:

class OptionExt[A](option: Option[A]) {
  def ifNone[R](f: => R) = new Otherwise1(option match {
    case None => Some(f)
    case Some(_) => None
  }, option.get)
  def ifSome[R](f: A => R) = new Otherwise0(option match {
    case Some(value) => Some(f(value))
    case None => None
  })
}

implicit def extendOption[A](opt: Option[A]): OptionExt[A] = new OptionExt[A](opt)

class Otherwise1[R, A1](intermediateResult: Option[R], arg1: => A1) {
  def otherwise[S >: R](f: A1 => S) = intermediateResult.getOrElse(f(arg1))
  def apply[S >: R](f: A1 => S) = otherwise(f)
}

Note that we now also need Otherwise1 so that we can conveniently passed the unwrapped value not only to the ifSome function argument, but also to the function argument of an otherwise following an ifNone.

Answer 2

Why don't just use it like this:

val idiomaticVariable = if (condition) { 
    firstExpression
  } else { 
    secondExpression 
  } 

?

IMO, its very idiomatic! :)

Answer 3

You may be looking at the problem too specifically. You would probably be better off with the pipe operator:

class Piping[A](a: A) { def |>[B](f: A => B) = f(a) }
implicit def pipe_everything[A](a: A) = new Piping(a)

Now you can

("fish".length > 5) |> (if (_) println("Hi") else println("Ho"))

which, admittedly, is not quite as elegant as what you're trying to achieve, but it has the great advantage of being amazingly versatile--any time you want to put an argument first (not just with booleans), you can use it.

Also, you already can use options the way you want:

Option("fish").filter(_.length > 5).
  map (_ => println("Hi")).
  getOrElse(println("Ho"))

Just because these things could take a return value doesn't mean you have to avoid them. It does take a little getting used to the syntax; this may be a valid reason to create your own implicits. But the core functionality is there. (If you do create your own, consider fold[B](f: A => B)(g: => B) instead; once you're used to it the lack of the intervening keyword is actually rather nice.)

---

Edit: Although the |> notation for pipe is somewhat standard, I actually prefer use as the method name, because then def reuse[B,C](f: A => B)(g: (A,B) => C) = g(a,f(a)) seems more natural.