Functional Reactive Programming

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Revision as of 16:24, 16 November 2012 by Ertes (talk | contribs) (→‎Introduction: Netwire 4 is now on HackageDB.)
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Functional Reactive Programming (FRP) integrates time flow and compositional events into functional programming. This provides an elegant way to express computation in domains such as interactive animations, robotics, computer vision, user interfaces, and simulation.


FRP is about domain-specific languages that capture the notion of time-varying values. Let's take Netwire 4 as an example. You can install it either from HackageDB,

   cabal install netwire

or by grabbing the latest source code via darcs:

   darcs get
   cd netwire
   cabal install

Imagine you have a simple GUI label that displays the number of seconds passed since program start. In an event-based model this is actually quite a complicated task. You would have to create a label and update it all the time using some form of timer/idle event. In Netwire you write:

   myLabel = time

Now let's say you want to have the same GUI, but the time should start at 10 and pass twice as fast, so you actually want to display twice the number of seconds passed plus 10:

   myLabel = 10 + 2*time

Imagine you want to display the string "yes" in a label:

   myLabel = "yes"

Now let's say you want to display "yes", when the space key is held down and "no" otherwise:

   myLabel = "yes" . keyDown Space <|> "no"

You want to display time while pressed and "Press space" while not:

   myLabel = fmap show time . keyDown Space <|> "Press space"

You want to display "yes" every other second and "no" otherwise:

   myLabel = "yes" . holdFor 1 (periodically 2) <|> "no"

Imagine doing that with event-based code.

Summary: FRP is about handling time-varying values like they were regular values.


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