Given the widespread use of memory-safe languages, students must understand garbage collection well. Following a constructivist philosophy, an effective approach would be to have them implement garbage collectors. Unfortunately, a full implementation depends on substantial knowledge of compilers and runtime systems, which many courses do not cover or cannot assume.

This paper presents Flapjax, a language designed for contemporary Web applications. These applications communicate with servers and have rich, interactive interfaces. Flapjax provides two key features that simplify writing these applications. First, it provides event streams, a uniform abstraction for communication within a program as well as with external Web services. Second, the language itself is reactive: it automatically tracks data dependencies and propagates updates along those dataflows. This allows developers to write reactive interfaces in a declarative and compositional style.

Functional Reactive Programming (FRP) extends traditional functional programming with dataflow evaluation, making it possible to write interactive programs in a declarative style. An FRP language creates a dynamic graph of data dependencies and reacts to changes by propagating updates through the graph. In a transparent FRP language, the primitive operators are implicitly lifted, so they construct graph nodes when they are applied to time-varying values. This model has some attractive properties, but it tends to produce a large graph that is costly to maintain. In this paper, we develop a transformation we call lowering, which improves performance by reducing the size of the graph. We present a static analysis that guides the sound application of this optimization, and we present benchmark results that demonstrate dramatic improvements in both speed and memory usage for real programs.