Examples of using some Scala Option methods

Question

I have read the blog post recommended me here. Now I wonder what some those methods are useful for. Can you show examples of using forall (as opposed to foreach) and toList of Option?

Answer 1

• map: Allows you to transform a value "inside" an Option, as you probably already know for Lists. This operation makes Option a functor (you can say "endofunctor" if you want to scare your colleagues)
• flatMap: Option is actually a monad, and flatMap makes it one (together with something like a constuctor for a single value). This method can be used if you have a function which turns a value into an Option, but the value you have is already "wrapped" in an Option, so flatMap saves you the unwrapping before applying the function. E.g. if you have an Option[Map[K,V]], you can write mapOption.flatMap(_.get(key)). If you would use a simple map here, you would get an Option[Option[V]], but with flatMap you get an Option[V]. This method is cooler than you might think, as it allows to chain functions together in a very flexible way (which is one reason why Haskell loves monads).
• flatten: If you have a value of type Option[Option[T]], flatten turns it into an Option[T]. It is the same as flatMap(identity(_)).
• orElse: If you have several alternatives wrapped in Options, and you want the first one that holds actually a value, you can chain these alternatives with orElse: steakOption.orElse(hamburgerOption).orElse(saladOption)
• getOrElse: Get the value out of the Option, but specify a default value if it is empty, e.g. nameOption.getOrElse("unknown").
• foreach: Do something with the value inside, if it exists.
• isDefined, isEmpty: Determine if this Option holds a value.
• forall, exists: Tests if a given predicate holds for the value. forall is the same as option.map(test(_)).getOrElse(true), exists is the same, just with false as default.
• toList: Surprise, it converts the Option to a List.

Answer 2

Many of the methods on Option may be there more for the sake of uniformity (with collections) rather than for their usefulness, as they are all very small functions and so do not spare much effort, yet they serve a purpose, and their meanings are clear once you are familiar with the collection framework (as is often said, Option is like a list which cannot have more than one element).

forall checks a property of the value inside an option. If there is no value, the check pass. For example, if in a car rental, you are allowed one additionalDriver: Option[Person], you can do

additionalDriver.forall(_.hasDrivingLicense)

exactly the same thing that you would do if several additional drivers were allowed and you had a list.

toList may be a useful conversion. Suppose you have options: List[Option[T]], and you want to get a List[T], with the values of all of the options that are Some. you can do

for(option <- options; value in option.toList) yield value

(or better options.flatMap(_.toList))

Answer 3

I have one practical example of toList method. You can find it in scaldi (my Scala dependency injection framework) in Module.scala at line 72:

https://github.com/OlegIlyenko/scaldi/blob/f3697ecaa5d6e96c5486db024efca2d3cdb04a65/src/main/scala/scaldi/Module.scala#L72

In this context getBindings method can return either Nil or List with only one element. I can retrieve it as Option with discoverBinding. I find it convenient to be able to convert Option to List (that either empty or has one element) with toList method.