Justin du Coeur (jducoeur) wrote,
Justin du Coeur

The Stackable Trait Pattern

Time for a small Scala Burble of the Day.

NSTIW, in a discussion on the akka-user mailing list (I finally committed to getting involved in the Akka community, and unsurprisingly have immediately started making trouble). Somebody suggested a tweak to my code which appeared to involve a trait (Scala's interface-plus-implementation mixins that make the language so powerful) using super to call a method on the *child* class. I pointed out that that was wrong -- but it turns out not to have been as wrong as I thought.

Enter the Stackable Trait Pattern, which is surely one of Scala's cleverer ideas -- a typesafe, statically-compiled way for mixins to work together similarly to how they do in dynamic languages. Basically, you build your concrete object out of a base class and any number of traits. Those traits can work *together* in a highly pluggable way. In this model, "super" doesn't just mean "the superclass" as it does in most static languages; instead, when used in this way, it means "the previous version of this method in the mixin list". Hence the "stackable" -- each trait acts as a *modifier* on the method, in an order decided when you actually build the final object.

It's a subtlety, but a really useful one -- it means that you can basically create a library of legos, which can be assembled in different configurations simply by constructing your concrete object with your traits in the desired order of "inheritance". Neat...
Tags: programming, scala

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