Methods on Boolean


Why doesn’t the Boolean class have methods like not, if?

If I have a long chain such as a.b.c.e.f.g which consumes multiple lines, and it evaluates to a Boolean, but I need the inverse, I have to go back to the beginning and insert !. or worse to wrap the whole multi-line expression in parens, prepend and if and append the consequent expression.

It would seem logical if I could just use a not and if method like:

a.b.c.d.e.f.g.if{ doThis}
a.b.c.d.e.f.g.not.if{ doThis}


perhaps that’s an easy example to implement a case class, to effectively add methods to Boolean.


Sounds like a job for extension methods.

Good ideas. I would use these.

Brian Maso


I don’t know your use case, so this might not be appropriate.

Have you thought of using fold?


As I understand, extension methods are NOT back ported to 2.x. right?


I didn’t know fold worked with Boolean.


Extension methods are present in 2.x but are encoded as implicits. Example:

// Boolean extension methods
implicit class RichBoolean(val value: Boolean) extends AnyVal {
  def not: Boolean = !value
  def choice[T](ifTrue: => T, ifFalse: => T) = if (value) ifTrue else ifFalse
  def onTrue(action: => Unit): Unit = if (value) action
// somewhere else
object Xxx {
  import zzz.RichBoolean // bring extension methods into scope
  println(false.choice(5, 8))
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No, but when folks say “extension method” in the context of Scala 2, they really mean an implicit class. (Which can accomplish the same things.)


Sorry, I wasn’t clear. I should have said, that’s why I wasn’t sure if would fit your use case.

You have lots of Booleans. With that many, it might be better in a collection of some kind. That might not fit your case at all. A Seq[Boolean]( a, b, … ) or set might be better or perhaps worse. It depends on what you are trying to do. I was putting it out there for consideration.

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Question: When I copy/pate the implicit classic RichBoolean into the IntelliJ worksheet, it works as expected. However, if I copy it into a scala file, it get the error. What do I need to do to make is useful and why?

Error:(29, 16) `implicit' modifier cannot be used for top-level objects
implicit class RichBoolean(val value: Boolean) extends AnyVal {

Implicit classes need to be wrapped in classes or objects (plain objects or package objects). That’s because implicit class RichBoolean(value: Boolean) is expanded to:

implicit def RichBoolean(value: Boolean) = new RichBoolean(value)
class RichBoolean(value: Boolean)

and methods cannot be top level.


Boolean class does have a “not” method. It’s named unary_!. As syntactic sugar, methods named unary_!, unary_-, unary_+, or unary_~, can be invoked using the corresponding prefix operator. Conversely, in scala the 4 prefix operators are just syntactic sugar for methods of those names. There’s nothing stopping you from using the full method name as postfix. That said, of course true.unary_! doesn’t read that nice.


having not and if methods on Boolean reminds me of smalltalk.


I think that’s a good question and I would love to know the answer

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The answer to why Boolean does not have not and if methods?

It is always difficult to tell why something does not exist, unless you know of a specific effort and why it failed.

I would guess it was never introduced, because people are largely happy with the existing options !myBool and if(myBool) { … }. I would expect the existing options to be highly optimized by the compiler, so that alternatives would perform less well.


I don’t know the entire history of every possible change that did or didn’t make it into Scala, but I guess the most probable answer is: because nobody has cared enough to get those methods included in the std library.