The authors consider the user of modern functional languages, which support user-defined data types, polymorphic types and functions, pattern matching, and several kinds of modularity, as well as two important features not found in most conventional languages: lazy evaluation and higher order functions. Coroutines, modularity, and parallelism are discussed. The authors describe how a functional language provides these modern features and how to exploit its advantages. The examples are written in Miranda, the most widely used modern functional language, but the techniques can be used in almost any modern functional language. They use quadtrees because they illustrate these features and techniques nicely.