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Thursday, December 15, 2011

Various examples in Haskell's FRP.Reactive

After playing with Flapjax library in Javascript, I moved to Reactive to learn more about FRP. Because research on Functional Reactive Programming is most active in Haskell, I thought it would be better to do that. Reactive seems to be a nice library, but unfortunately I couldn't find many working code examples. So I show some of them as my exercise. To write this, I owe a maoe's great article in Japanese.

(This page has been translated into Spanish language by Maria Ramos from

As I didn't have much time, I couldn't write a good explanation now. But still I hope it helps some people who learn Reactive like me. I used Haskell Platform 2010 (slightly old) and did cabal install reactive --enable-documentation to install Reactive.

The first example shows "Hello, World!" after three seconds. atTime generates a timer event, and <$> convert this event to IO action (\_ -> putStrLn "Hello, World!") which writes a string.

This is as same as above, but it makes events each second.

This makes running Fibonnaci numbers. You can use scanlE to process previous value and current value of the event in a function. In this case, (0, 1) is the initial value, and when an event occurs, the function \(n0, n1) _ -> (n1, n0 + n1) calculates next value, and the result (the first case is (1, 1)) is used as a next argument when a new event occurs.

It shows characters as you type. It looks difficult but you don't have to worry about run function. The important part is machine :: Event Char -> Event (IO ()) that convert a character input event to an IO action.

This example shows how to merge two events. onType is same as machine in the previous example, and onClock is same as helloMany.hs example. I used `mappend` to merge the two events

This shows a simple state machine. The function next defines the state machine, and mealy_ convert the definition to an event. zipE is another way to merge two events. Unlike mappend, you can see two values in the two events in a same time.

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