As I was writing my comment, I think maybe I should start with DSL, but this isn't conclusive. Spark, Scalding, LINQ, RxJava, even Slick are examples of the types of DSLs I'd like to understand how to construct. They're essentially compilers.

Looking at the code for these technologies it's incredible how complex it is to implement the nice DSL that they expose. How much of that is accidental complexity?