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Today's Topics:
1. Re: Issue installing reactive-banana-5.0.0.1 (Miguel Negrao)
----------------------------------------------------------------------
Message: 1
Date: Sat, 5 May 2012 15:26:08 +0100
From: Miguel Negrao <[email protected]>
Subject: Re: [Haskell-beginners] Issue installing
reactive-banana-5.0.0.1
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252
A 05/05/2012, ?s 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
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