What are plug-ins and what are they used for?

What are some examples of how Scala's compiler plugins are used in practice?

• Answer:

  We currently have 2 compiler plugins that we use at Quora, both written by (one with help from Scala compiler maintainer Paul Phillips as a consultant) and with some minor modifications by me.  These allow us to get some very useful features of Python that we wanted by extending Scala's syntax with these plugins. AutoArgs.  This plugin lets us get something similar to *args and **kwargs for certain Scala classes.  We use this for very quick and efficient serialization of classes, and in a way also backwards-compatible with our Python code.  We enable this plugin by mixing in this trait: trait AutoArgs { def autoArgs: Array[Any] = Array() // filled by compiler plugin def autoArgNames: Array[String] = Array() // filled by compiler plugin def autoKwargs: Map[String, Any] = Map((autoArgNames zip autoArgs): _*) } and we then make use of the autoArgs and autoKwargs defs in the class or its consumers. Decorate.  This plugin lets us implement the functionality of Python decorators [1] with Scala's static annotations.  We declare our own FunctionDecorator abstract class that extends from scala.annotation.StaticAnnotation.  Any decorator we want to write can then just inherit from FunctionDecorator and implement a single method decorate.  The compiler plugin fills in the rest, including support for decorator stacking. This allows function composition in the same simple and readable fashion as Python's syntax: @Count @Memoize(persistToDatabase = false) def fib(num: Int): Int = { // ... } In general though, we prefer to build functionality using Scala natively where possible. (We have resisted many ideas that we would want to implement with a compiler plugin...) But there are times where the benefits of code simplicity & maintainability outweigh the cost of writing these compiler plugins, and we will make that trade-off. [1] http://en.wikipedia.org/wiki/Python_syntax_and_semantics#Decorators