Functional reactive programming
||This article provides insufficient context for those unfamiliar with the subject. (April 2015)|
Functional reactive programming (FRP) is a programming paradigm for reactive programming (asynchronous dataflow programming) using the building blocks of functional programming (e.g. map, reduce, filter). FRP has been used for programming graphical user interfaces (GUIs), robotics, and music, aiming to simplify these problems by explicitly modeling time.
Formulations of FRP
Formulations such as Event-Driven FRP and Elm require that updates are discrete and event-driven. These formulations have pushed for practical FRP, focusing on a semantics that have a simple API that can be implemented efficiently in a setting such as robotics or in a web-browser.
In these formulations, it is common that the ideas of behaviors and events are combined into signals that always have a current value, but change discretely.
The earliest formulation of FRP used a continuous semantics, aiming to abstract over many operational details that are not important to the meaning of a program. The key properties of this formulation are:
- Modeling values that vary over continuous time, called "behaviors" and later "signals".
- Modeling "events" which have occurrences at finitely many points in time.
- The system can be changed in response to events, generally termed "switching."
- The separation of evaluation details such as sampling rate from the reactive model.
This semantic model of FRP in side-effect free languages is typically in terms of continuous functions, and typically over time.
There are two types of FRP systems, push-based and pull-based. Push-based systems take events and push them through a signal network to achieve a result. Pull-based systems wait until the result is demanded, and work backwards through the network to retrieve the value demanded.
Some FRP systems such as Yampa use sampling. On a regular interval, samples are pushed through a signal network. This approach has two drawbacks: it is very computation intensive to process samples on a regular interval, and the network has to wait up to the duration of the sampling interval to find out about changes to the input. Sampling is an example of push-based FRP.
The Reactive and Etage libraries on Hackage introduced an approach called push-pull FRP, which combines the best of push-based and pull-based FRP. In this approach, only when the next event on a purely defined stream (such as a list of fixed events with times) is demanded, that event is constructed. These purely defined streams act like lazy lists in Haskell. That is the pull-based half. The push-based half is used when events external to the system are brought in. The external events are pushed to consumers, so that they can find out about an event the instant it is issued.
- Czaplicki, Evan (Apr 2012), Elm: Concurrent FRP for Functional GUIs (PDF) (thesis), Harvard.
- Nilsson, Henrik; Courtney, Antony; Peterson, John (Feb 2011) , "Functional Reactive Programming, Continued", Haskell Workshop (PDF) (2).
- Taha, Walid; Wan, Zhanyong; Hudak, Paul (2002), "Event-Driven FRP", PADL (PDF), Yale.
- Czaplicki, Evan; Chong, Stephen (2013), "Asynchronous Functional Reactive Programming for GUIs", PLDI, Harvard.
- Wan, Zhanyong; Taha, Walid; Hudak, Paul (Feb 2011), "Real-Time FRP", ICFP (PDF) (1).
- Elliott, Conal; Hudak, Paul (1997), "Functional Reactive Animation", ICFP.
- Courtney, Antony; Elliott, Conal (Feb 2011) , "Genuinely Functional User Interfaces", Haskell Workshop, Yale.
- "What is Functional Reactive Programming?" An intro to FRP with live demos.
- Haskell-related FRP research
- "Deprecating the Observer Pattern with Scala⋅React," Scala.React, an FRP Scala implementation
- What is (functional) reactive programming? Stackoverflow Answers.