Scalaz BooleanOps

In this post we’ll look at BooleanOps and the goodies it provides to work with Booleans. As always, we’ll go straight to examples.

Unless

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@ import scalaz._
import scalaz._
@ import Scalaz._
import Scalaz._
@ val t = true
t: Boolean = true
@ val f = false
f: Boolean = false
// executes the given side-effect if this boolean value is false
@ t unless "this won't print".println

@ f unless "this will print".println
"this will print"

As the comment for unless states:

Executes the given side-effect if this boolean value is false.

A mnemonic to remember the working is: “Unless the value is true, execute the side-effecting function”.

When

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// executes the given side-effect if this boolean value is true
@ t when "this will print".println
"this will print"

@ f when "this won't print".println

The “opposite” of unless is when which executes the function when the value is true. As the comment for when states:

Executes the given side-effect if this boolean value is true.

A mnemonic to remember the working is: “When the value is true, execute the side-effecting function”.

Folding a Boolean

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@ t fold[String]("this will be returned when true", "this will be returned when false")
res8: String = "this will be returned when true"
@ f fold[String]("this will be returned when true", "this will be returned when false")
res9: String = "this will be returned when false"

fold lets you decide what value to return depending on whether it is true or false.

Converting to an Option

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@ t option "this will create a Some() with this string in it"
res10: Option[String] = Some("this will create a Some() with this string in it")
@ f option "this will result in a None"
res11: Option[String] = None

option lets us convert a Boolean into an Option in a type-safe manner. A true results in a Some containing the value passed to option whereas a false results in an Option of whatever the type of the argument is.

Ternary Operator

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@ t ? "true" | "false"
res13: String = "true"
@ f ? "true" | "false"
res14: String = "false"

Scalaz also provides a ternary operator to work with Booleans. The ternary operator is actually a combination of ? and |. ? is the conditional operator that results in the creation of an object of Conditional and | is a method of that object.

Miscellaneous

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@ t ?? List(1, 2, 3)
res15: List[Int] = List(1, 2, 3)
@ f ?? List(1, 2, 3)
res16: List[Int] = List()

?? returns the given argument if the value is true, otherwise, the zero element for the type of the given argument. In our case, the “zero” element for List is an empty List. 

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@ t !? List(1, 2, 3)
res17: List[Int] = List()
@ f !? List(1, 2, 3)
res18: List[Int] = List(1, 2, 3)

!? is the opposite of ?? and returns the argument if the value is false or the zero element otherwise.

Conclusion

This brings us to the end of our post on BooleanOps. There’s a lot more functions provided but I’ve chosen to cover those which I feel will be the most useful.