Functional Reactive Programming Research Articles

Topology-Level Reactivity in Distributed Reactive Programs

Reactive programming is a popular paradigm to program event-driven applications, and it is often proposed as a paradigm to write distributed applications. One such type of application is *prosumer* applications, which are distributed applications that both produce and consume many events. We analyse the problems that occur when using a reactive programming language or framework to implement prosumer applications. We find that the assumption of an open network, which means prosumers of various types spontaneously join and leave the network, can cause a lot of code complexity or run-time inefficiency. At the basis of these issues lies *acquaintance management*: the ability to discover prosumers as they join and leave the network, and correctly maintaining this state throughout the reactive program. Most existing reactive programming languages and frameworks have limited support for managing acquaintances, resulting in accidental complexity of the code or inefficient computations. In this paper we present acquaintance management for reactive programs. First, we design an *acquaintance discovery* mechanism to create a *flock* that automatically discovers prosumers on the network. An important aspect of flocks is their integration with reactive programs, such that a reactive program can correctly and efficiently maintain its state. To this end we design an *acquaintance maintenance* mechanism: a new type of operator for functional reactive programming languages that we call `deploy-*`. The `deploy-*` operator enables correct and efficient reactions to time-varying collections of discovered prosumers. The proposed mechanisms are implemented in a reactive programming language called Stella, which serves as a linguistic vehicle to demonstrate the ideas of our approach. Our implementation of acquaintance management results in computationally efficient and idiomatic reactive code. We evaluate our approach quantitatively via benchmarks that show that our implementation is efficient: computations will efficiently update whenever a new prosumer is discovered, or a connected prosumer is dropped. To evaluate the distributed capabilities of our prototype implementation, we implement a use-case that simulates the bike-sharing infrastructure of Brussels, and we run it on a Raspberry Pi cluster computer. We consider our work to be an important step to use functional reactive programming to build distributed systems for open networks, in other words, distributed reactive programs that involve many prosumer devices and sensors that spontaneously join and leave the network.

Modal FRP for all: Functional reactive programming without space leaks in Haskell

Abstract Functional reactive programming (FRP) provides a high-level interface for implementing reactive systems in a declarative manner. However, this high-level interface has to be carefully reigned in to ensure that programs can in fact be executed in practice. Specifically, one must ensure that FRP programs are causal and can be implemented without introducing space leaks. In recent years, modal types have been demonstrated to be an effective tool to ensure these operational properties. In this paper, we present $\mathsf{Rattus}$ , a modal FRP language that extends and simplifies previous modal FRP calculi while still maintaining the operational guarantees for productivity, causality, and space leaks. The simplified type system makes $\mathsf{Rattus}$ a practical programming language that can be integrated with existing functional programming languages. To demonstrate this, we have implemented a shallow embedding of $\mathsf{Rattus}$ in Haskell that allows the programmer to write $\mathsf{Rattus}$ code in familiar Haskell syntax and seamlessly integrate it with regular Haskell code. Thus, $\mathsf{Rattus}$ combines the benefits enjoyed by FRP libraries such as Yampa, namely access to a rich library ecosystem (e.g., for graphics programming), with the strong operational guarantees offered by a bespoke type system. To establish the productivity, causality, and memory properties of the language, we prove type soundness using a logical relations argument fully mechanised in the Coq proof assistant.

Diamonds are not forever: liveness in reactive programming with guarded recursion

When designing languages for functional reactive programming (FRP) the main challenge is to provide the user with a simple, flexible interface for writing programs on a high level of abstraction while ensuring that all programs can be implemented efficiently in a low-level language. To meet this challenge, a new family of modal FRP languages has been proposed, in which variants of Nakano's guarded fixed point operator are used for writing recursive programs guaranteeing properties such as causality and productivity. As an apparent extension to this it has also been suggested to use Linear Temporal Logic (LTL) as a language for reactive programming through the Curry-Howard isomorphism, allowing properties such as termination, liveness and fairness to be encoded in types. However, these two ideas are in conflict with each other, since the fixed point operator introduces non-termination into the inductive types that are supposed to provide termination guarantees. In this paper we show that by regarding the modal time step operator of LTL a submodality of the one used for guarded recursion (rather than equating them), one can obtain a modal type system capable of expressing liveness properties while retaining the power of the guarded fixed point operator. We introduce the language Lively RaTT, a modal FRP language with a guarded fixed point operator and an `until' type constructor as in LTL, and show how to program with events and fair streams. Using a step-indexed Kripke logical relation we prove operational properties of Lively RaTT including productivity and causality as well as the termination and liveness properties expected of types from LTL. Finally, we prove that the type system of Lively RaTT guarantees the absence of implicit space leaks.

A Functional Reactive Programming Language for Small-Scale Embedded Systems with Recursive Data Types

A Functional Reactive Programming Language for Small-Scale Embedded Systems with Recursive Data Types

A Ruby-Based Hardware/Software Co-Design Environment with Functional Reactive Programming: Mulvery

A Ruby-Based Hardware/Software Co-Design Environment with Functional Reactive Programming: Mulvery

Smoothly Navigating between Functional Reactive Programming and Actors

We formally define an elegant multi-paradigm unification of Functional Reactive Programming, Actor Systems, and Object-Oriented Programming. This enables an intuitive form of declarative programming, harvesting the power of concurrency while maintaining safety. We use object and reference capabilities to highlight and tame imperative features: reference capabilities track aliasing and mutability, and object capabilities track I/O. Formally, our type system limits the scope, impact and interactions of impure code. - Scope: Expressions whose input is pure will behave deterministically. - Impact: Data-races and synchronisation issues are avoided. The only way for an actor to behave nondeterministically, is by mutating its state based on message delivery order. - Interactions: Signals provide a functional boundary between imperative and functional code, preventing impure code from invalidating functional assumptions.

Reactive programming in Ada 2012 with RxAda

Abstract The ReactiveX API, also known as the Reactive Extensions in the .NET world, is a popular functional reactive programming framework for asynchronous, event-based, multithreaded programming. Although Ada built-in tasking reduces the dire needs for additional multithreading support of some other languages, the reactive approach has properties that are well-suited to the safety and maintainability culture predominant in the Ada world, such as complexity reduction, well-defined concurrency semantics, and enhanced legibility by means of concise and explicit information flows appealing to imperative reasoning. This work presents the design of a ReactiveX Ada implementation that aims to balance desirable library properties such as compile-time type-safety, amount of user-required generic instantiations, and a smooth learning curve for both library clients and maintainers. Concurrency design aspects of the library are detailed, showing how the Flat_Map and Thread abstractions have been implemented following Ada programming expectations, in particular with regard to task termination. In the intervening time from its first presentation, the library has gained implemented operators to the point of having all fundamental building blocks available. With RxAda, the Ada programmer can henceforth benefit from the abundant documentation existing for the language-agnostic ReactiveX approach without stepping out of the Ada tool chain.

Gavial: Programming the web with multi-tier FRP

Developing web applications requires dealing with their distributed nature and the natural asynchronicity of user input and network communication. For facilitating this, different researchers have explored the combination of a multi-tier programming language and functional reactive programming. However, existing proposals take this approach only part of the way (some parts of the application remain imperative) or remain naive, with no regard for avoiding glitches across network communication, network traffic overhead, compatibility with common APIs like XMLHttpRequest etc. In this paper, we present Gavial: the first mature design and implementation of multi-tier FRP that allows constructing an entire web application as a functionally reactive program. By applying a number of new ideas, we demonstrate that multi-tier FRP can in fact deal realistically with important practical aspects of building web applications. At the same time, we retain the declarative nature of FRP, where behaviors and events have an intuitive, compositional semantics and a clear dependency structure.

Runtime verification and validation of functional reactive systems

Abstract Many types of interactive applications, including reactive systems implemented in hardware, interactive physics simulations and games, raise particular challenges when it comes to testing and debugging. Reasons include de facto lack of reproducibility and difficulties of automatically generating suitable test data. This paper demonstrates that certain variants of functional reactive programming (FRP) implemented in pure functional languages can mitigate such difficulties by offering referential transparency at the level of whole programs. This opens up for a multi-pronged approach for assisting with testing and debugging that works across platforms, including assertions based on temporal logic, recording and replaying of runs (also from deployed code), and automated random testing using QuickCheck. When combined with extensible forms of FRP that allow for constrained side effects, it allows us to not only validate software simulations but to analyse the effect of faults in reactive systems, confirm the efficacy of fault tolerance mechanisms and perform software- and hardware-in-the-loop testing. The approach has been validated on non-trivial systems implemented in several existing FRP implementations, by means of careful debugging using a tool that allows the test or simulation under scrutiny to be controlled, moving along the execution time line, and pin-pointing of violations of assertions on personal computers as well as external devices.

Fault-tolerant functional reactive programming (extended version)

Abstract Highly critical application domains, like medicine and aerospace, require the use of strict design, implementation, and validation techniques. Functional languages have been used in these domains to develop synchronous dataflow programming languages for reactive systems. Causal stream functions and functional reactive programming (FRP) capture the essence of those languages in a way that is both elegant and robust. To guarantee that critical systems can operate under high stress over long periods of time, these applications require clear specifications of possible faults and hazards, and how they are being handled. Modeling failure is straightforward in functional languages, and many functional reactive abstractions incorporate support for failure or termination. However, handling unknown types of faults, and incorporating fault tolerance into FRP, requires a different construction and remains an open problem. This work demonstrates how to extend an existing functional reactive framework with fault tolerance features. At value level, we tag faulty signals with reliability and probability information and use random testing to inject faults and validate system properties encoded in temporal logic. At type level, we tag components with the kinds of faults they may exhibit and use type-level programming to obtain compile-time guarantees of key aspects of fault tolerance. Our approach is powerful enough to be used in systems with realistic complexity, and flexible enough to be used to guide system analysis and design, validate system properties in the presence of faults, perform runtime monitoring, and study the effects of different fault tolerance mechanisms.

A fault-tolerant programming model for distributed interactive applications

Ubiquitous connectivity of web, mobile, and IoT computing platforms has fostered a variety of distributed applications with decentralized state. These applications execute across multiple devices with varying reliability and connectivity. Unfortunately, there is no declarative fault-tolerant programming model for distributed interactive applications with an inherently decentralized system model. We present a novel approach to automating fault tolerance using high-level programming abstractions tailored to the needs of distributed interactive applications. Specifically, we propose a calculus that enables formal reasoning about applications' dataflow within and across individual devices. Our calculus reinterprets the functional reactive programming model to seamlessly integrate its automated state change propagation with automated crash recovery of device-local dataflow and disconnection-tolerant distribution with guaranteed automated eventual consistency semantics based on conflict-free replicated datatypes. As a result, programmers are relieved of handling intricate details of distributing change propagation and coping with distribution failures in the presence of interactivity. We also provides proofs of our claims, an implementation of our calculus, and an empirical evaluation using a common interactive application.

Simply RaTT: a fitch-style modal calculus for reactive programming without space leaks

Functional reactive programming (FRP) is a paradigm for programming with signals and events, allowing the user to describe reactive programs on a high level of abstraction. For this to make sense, an FRP language must ensure that all programs are causal, and can be implemented without introducing space leaks and time leaks. To this end, some FRP languages do not give direct access to signals, but just to signal functions. Recently, modal types have been suggested as an alternative approach to ensuring causality in FRP languages in the synchronous case, giving direct access to the signal and event abstractions. This paper presents Simply RaTT , a new modal calculus for reactive programming. Unlike prior calculi, Simply RaTT uses a Fitch-style approach to modal types, which simplifies the type system and makes programs more concise. Echoing a previous result by Krishnaswami for a different language, we devise an operational semantics that safely executes Simply RaTT programs without space leaks. We also identify a source of time leaks present in other modal FRP languages: The unfolding of fixed points in delayed computations. The Fitch-style presentation allows an easy way to rules out these leaks, which appears not to be possible in the more traditional dual context approach.

Rhine: FRP with type-level clocks

Processing data at different rates is generally a hard problem in reactive programming. Buffering problems, lags, and concurrency issues often occur. Many of these problems are clock errors , where data at different rates is combined incorrectly. Techniques to avoid clock errors, such as type-level clocks and deterministic scheduling, exist in the field of synchronous programming, but are not implemented in general-purpose languages like Haskell. Rhine is a clock-safe library for synchronous and asynchronous Functional Reactive Programming (FRP). It separates the aspects of clocking, scheduling and resampling from each other, and ensures clock-safety at the type level. Concurrent communication is encapsulated safely. Diverse reactive subsystems can be combined in a coherent, declarative data-flow framework, while correct interoperability of data at different rates is guaranteed by type-level clocks. This provides a general-purpose framework that simplifies multi-rate FRP systems and can be used for game development, media applications, GUIs and embedded systems, through a flexible API with many reusable components.

P-FRP task scheduling with preemption threshold

Abort-and-Restart model is used in Priority-based Functional Reactive Programming in which higher priority tasks can preempt lower priority tasks and the lower priority tasks are aborted and restarted after the higher priority tasks have finished execution. This paper discusses a potential of improving schedulability in P-FRP systems by using preemption threshold that it allows a task to only disable preemption of tasks up to a specified threshold priority. Also, a sufficient schedulability test condition is studied in this paper for P-FRP tasks using preemption threshold, which is a critical problem to be solved in order to explore the potential benefit.

Fault tolerant functional reactive programming (functional pearl)

Highly critical application domains, like medicine and aerospace, require the use of strict design, implementation and validation techniques. Functional languages have been used in these domains to develop synchronous dataflow programming languages for reactive systems. Causal stream functions and Functional Reactive Programming capture the essence of those languages in a way that is both elegant and robust. To guarantee that critical systems can operate under high stress over long periods of time, these applications require clear specifications of possible faults and hazards, and how they are being handled. Modeling failure is straightforward in functional languages, and many Functional Reactive abstractions incorporate support for failure or termination. However, handling unknown types of faults , and incorporating fault tolerance into Functional Reactive Programming, requires a different construction and remains an open problem. This work presents extensions to an existing functional reactive abstraction to facilitate tagging reactive transformations with hazard tags or confidence levels. We present a prototype framework to quantify the reliability of a reactive construction, by means of numeric factors or probability distributions, and demonstrate how to aid the design of fault-tolerant systems, by constraining the allowed reliability to required boundaries. By applying type-level programming, we show that it is possible to improve static analysis and have compile-time guarantees of key aspects of fault tolerance. Our approach is powerful enough to be used in systems with realistic complexity, and flexible enough to be used to guide their analysis and design, to test system properties, to verify fault tolerance properties, to perform runtime monitoring, to implement fault tolerance during execution and to address faults during runtime. We present implementations in Haskell and in Idris.

Harmonizing Signals and Events with a Lightweight Extension to Java

The current demands for seamless connections with the surrounding environment make software more reactive. For example, such demands are evident in systems consisting of the Internet of Things. Such systems include a set of reactive values that are periodically updated in response to external or internal events to form a dataflow in the sense that such updates are propagated to other reactive values. Two effective approaches for realizing such reactive values have been proposed: the event mechanisms in event-based programming and the signals in functional-reactive programming. These two approaches are now becoming mixed in several languages such as Flapjax and REScala, which makes these languages notably expressive for modularizing the implementation of reactive software. For example, REScala provides a rich API that consists of functions converting events to signals and vice versa. In this paper, we explore another, simpler approach in the design space of reactive programming languages: the event mechanism is harmonized with signals, resulting in a simplified programming interface that is mostly based on signals. Based on this approach, we realize SignalJ, a simple extension of Java with events and signals. Our notable findings are (1) an event can be represented as an update of a signal and (2) such an effectful signal can be represented using annotations instead of introducing types and constructors for signals to further simplify the language. Another contribution of this paper is the formal model of SignalJ. As both mechanisms of events and signals may interfere with each other, this mixing sometimes results in surprising behavior. For example, the functional behavior of signals is affected by the imperative features of events. Thus, understanding the formal model of this mixing is actually important. The core calculus, Featherweight SignalJ (FSJ), was developed as an extension of Featherweight Java, and proofs are provided to ensure the soundness of FSJ.

Back to the future: time travel in FRP

Functional Reactive Programming (FRP) allows interactive applications to be modelled in a declarative manner using time-varying values. For practical reasons, however, operational constraints are often imposed, such as having a fixed time domain, time always flowing forward, and limiting the exploration of the past. In this paper we show how these constraints can be overcome, giving local control over the time domain, the direction of time and the sampling step. We study the behaviour of FRP expressions when time flows backwards, and demonstrate how to synchronize subsystems running asynchronously and at different sampling rates. We have verified the practicality of our approach with two non-trivial games in which time control is central to the gameplay.

Testing and debugging functional reactive programming

Many types of interactive applications, including video games, raise particular challenges when it comes to testing and debugging. Reasons include de-facto lack of reproducibility and difficulties of automatically generating suitable test data. This paper demonstrates that certain variants of Functional Reactive Programming (FRP) implemented in pure functional languages can mitigate such difficulties by offering referential transparency at the level of whole programs. This opens up for a multi-pronged approach for assisting with testing and debugging that works across platforms, including assertions based on temporal logic, recording and replaying of runs (also from deployed code), and automated random testing using QuickCheck. The approach has been validated on real, non-trivial games implemented in the FRP system Yampa through a tool providing a convenient Graphical User Interface that allows the execution of the code under scrutiny to be controlled, moving along the execution time line, and pin-pointing of violations of assertions on PCs as well as mobile platforms.

Functional reactive programming, refactored

Functional Reactive Programming (FRP) has come to mean many things. Yet, scratch the surface of the multitude of realisations, and there is great commonality between them. This paper investigates this commonality, turning it into a mathematically coherent and practical FRP realisation that allows us to express the functionality of many existing FRP systems and beyond by providing a minimal FRP core parameterised on a monad. We give proofs for our theoretical claims and we have verified the practical side by benchmarking a set of existing, non-trivial Yampa applications running on top of our new system with very good results.

Causal commutative arrows revisited

Causal commutative arrows (CCA) extend arrows with additional constructs and laws that make them suitable for modelling domains such as functional reactive programming, differential equations and synchronous dataflow. Earlier work has revealed that a syntactic transformation of CCA computations into normal form can result in significant performance improvements, sometimes increasing the speed of programs by orders of magnitude. In this work we reformulate the normalization as a type class instance and derive optimized observation functions via a specialization to stream transformers to demonstrate that the same dramatic improvements can be achieved without leaving the language.