[Haskell-beginners] Issue installing reactive-banana-5.0.0.1

[Miguel Negrao]

A 05/05/2012, às 08:40, Heinrich Apfelmus escreveu:

> Miguel Negrao wrote:
>> One other question, is it possible in reactive-banana to define
>> “recursive” event streams. For instance consider a stream which
>> receives numbers between 0.0 and 1.0.  If the last outputted value
>> was between 0.8 an 1.0 then output 1-x otherwise output x. After that
>> it only leta numbers through if they are between 0.0 and 0.2 or
>> between 0.8 and 1.0.
> 
> The standard way to do recursion in reactive-banana is to use multiple recursion between a  Behavior  and an  Event . See also
> 
>  http://stackoverflow.com/a/7852344/403805

ok, I will study that.
> 
> Note that the specification you gave does not require recursion, though. Here an implementation of your example.
> 
>    import Reactive.Banana
> 
>    example :: Event t Double -> Event t Double
>    example e = filterJust e2
>        where
>        e2 = f <$> bIsFirst <@> e
> 
>        bIsFirst = stepper True $ False <$ e
> 
>        between x a b = a < x && x < b
> 
>        f True x
>            | between x 0.8 1.0 = Just $ 1 - x
>            | otherwise         = Just $ x
>        f False x
>            | between x 0.8 1.0 = Just $ x
>            | between x 0.0 0.2 = Just $ x
>            | otherwise         = Nothing
> 
> Here an example output
> 
>    GHCi> interpretModel example [[0.9],[0.3],[0.4]]
>    [[9.999999999999998e-2],[],[]]

Hum, that’s not exactly what I wanted. So if it’s the first event just let it through, and then filter it. If it’s not the first event, then do the inversion (1-x) or not depending on the last outputted value, and then filter it.
An input of 
	[[0.9],[0.5],[0.1],[0.9],[0.9]]
should produce
	[[0.9],[],[0.9],[0.1],[0.9]]

The following code is not correct but it’s closer to what I described:

module Main where

import Reactive.Banana
   
main :: IO()
main = do
        list <- interpretModel example [[0.9],[0.3],[0.4],[0.15],[0.87]]
        putStrLn $ show list   
  
example :: Event t Double -> Event t Double
example e = filterede2
        where
        filterede2 = filterE (\x->between x 0.0 0.2 && between x 0.8 1.0)  e2
        e2 = f <$> bIsFirst <@> e <@> e2

        bIsFirst = stepper True $ False <$ e

        between x a b = a < x && x < b

        f True x y = x
        f False x y
                | between y 0.8 1.0 = 1 - x
                | otherwise = x

best,
Miguel