Fetter

Fetter is a compiler plugin for Scala 2 that brings optional chaining to Scala. It allows you to write code like the following:

trait Person {
  val name: String
  val parent: Option[Person]
  val relatives: List[Person]
}

def foo(person: Option[Person]) = {
  val name: Option[String] = person?>name
  // desugars to: person.map(_.name)
  val parent: Option[Person] = person??>parent
  // desugars to: person.flatMap(_.name)
  val grandparentName: Option[String] = person??>parent??>parent?>name
  // desugars to: person.flatMap(_.parent.flatMap(_.parent.map(_.name)))
}

In fact, it works on any object for which map and flatMap are defined:

def getNames(people: List[Person]): List[String] = people?>name
def getRelatives(people: List[Person]): List[Person] = people??>relatives

val loadPerson: IO[Person] = ???
val loadName: IO[String] = loadPerson?>name

In general, the rule is that ?> desugars to map, and ??> desugars to flatMap.

What is Optional Chaining?

Optional chaining in a powerful syntactic feature found in some languages, such as Swift, Kotlin, JavaScript, and TypeScript. In these languages, when selecting from a nullable object, you may use ?.. If the object is null, the expression short-circuits and returns null. For example, you may write foo?.bar if foo is nullable. If foo is null, foo?.bar will be null, but if not, foo?.bar returns the same as foo.bar would.

For more information, see:

• Kotlin
• Swift
• JavaScript
• TypeScript

Using the Plugin

To use the plugin, first you must add the following external resolver to your `build.sbt``:

externalResolvers += "fetter resolver" at "https://maven.pkg.github.com/liam923/fetter"

Then, add the compiler plugin to the build.sbt file:

addCompilerPlugin("fetter" %% "fetter" % "1.0.0")