Conventional wisdom suggests that the benefits of functional programming no longer apply in the presence of even a small amount of imperative code, as if the addition of imperative code effectively subtracts. And yet, as we show in this paper, combining functional programming with the special imperative language Esterel provides a multiplicative improvement to the benefits of functional programming. The key to the benefit of both Esterel and functional programming stems from a restriction that both share. As in functional programming, where only the inputs to a function determine its outputs, the state of an Esterel computation is fully determined by the program's input and the state that the computation had in the previous time step, where the notion of a time step is explicit in the language. Esterel's guarantee holds even though Esterel programs feature concurrent threads, mutable state, and the ability to create, suspend, and even terminate threads as the computation proceeds. This similarity is the root of the benefits that programmers accrue as they informally reason about their program's behavior. To illustrate these benefits, the bulk of this paper consists of an in-depth exploration of HipHop code (a mashup of JavaScript and Esterel) that implements a Sudoku solver, showing how it is possible to write code that is as easy to understand as if it were written in a pure functional programming style, even though it uses multiple threads, mutable state, thread preemption, and even thread abortion. Even better, concurrent composition and task canceling provide significant program structuring benefits that allow a clean decomposition and task separation in the solver.

We present HipHop.js, a synchronous reactive language that adds synchronous concurrency and preemption to Inspired from Esterel, HipHop.js simplifies the programming of non-trivial temporal behaviors as found in complex web interfaces or IoT controllers and the cooperation between synchronous and asynchronous activities. HipHop.js is compiled into plain sequential and executes on unmodified runtime environments. We use three examples to present and discuss HipHop.js: a simple web login form to introduce the language and show how it differs from and two real life examples, a medical prescription pillbox and an interactive music system that show why concurrency and preemption help programming such temporal applications.

Rich applications merge classical computing, client-server concurrency, web-based interfaces, and the complex time- and event-based reactive programming found in embedded systems. To handle them, we extend the Hop web programming platform by HipHop, a domain-specific language dedicated to event-based process orchestration. Borrowing the synchronous reactive model of Esterel, HipHop is based on synchronous concurrency and preemption primitives that are known to be key components for the modular design of complex reactive behaviors. HipHop departs from Esterel by its ability to handle the dynamicity of Web applications, thanks to the reflexivity of Hop. Using a music player example, we show how to modularly build a non-trivial Hop application using HipHop orchestration code.

HOP is a SCHEME-based language and system to build rich multi-tier web applications. We present HIPHOP, a new language layer within HOP dedicated to request and event orchestration. HIPHOP follows the synchronous reactive model of the Esterel and ReactiveC languages, originally developed for embedded systems programming. It is based on synchronous concurrency and preemption primitives, which are known to be key components for the modular design of complex temporal behaviors. Although the language is concurrent, the generated code is purely sequential and thread-free; HIPHOP is translated to HOP for the server side and to straight JavaScript for the client side. With a music playing example, we show how to modularly buid non-trivial orchestration code with HIPHOP