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Today's Topics:
1. Re: Issue installing reactive-banana-5.0.0.1 (Miguel Negrao)
----------------------------------------------------------------------
Message: 1
Date: Tue, 8 May 2012 21:03:07 +0100
From: Miguel Negrao <miguel.negrao-li...@friendlyvirus.org>
Subject: Re: [Haskell-beginners] Issue installing
reactive-banana-5.0.0.1
To: beginners@haskell.org
Message-ID: <9b71adab-6229-4601-94cf-a3ecebda3...@friendlyvirus.org>
Content-Type: text/plain; charset=iso-8859-1
A 07/05/2012, ?s 13:12, Heinrich Apfelmus escreveu:
> Miguel Negrao wrote:
>> A 06/05/2012, ?s 14:31, Heinrich Apfelmus escreveu:
>>> Ah, ok, then I don't understand your specification.
>>>
>>> Could you give a specification in terms of a simple list transformation
>>>
>>> example :: [Double] -> [Double]
>>>
>>> ? All list functions are allowed, we can then transform it into a
>> > style that uses only the combinators available in reactive-banana.
>> Ok, this should demonstrate an example of what I mean:
>> module Main where
>> main :: IO()
>> main = print $ test [0.9,0.1,0.2,0.8]
>> --should output [0.9,0.1,0.8,0.8]
>> test :: [Double]->[Double]
>> test (x:xs) = x : test1 xs x
>> test [] = []
>> test1:: [Double]->Double->[Double]
>> test1 (x:xs) lastValue = let
>> y = if lastValue>=0.8 then x else 1.0-x
>> in if (y<=0.2) || (y>=0.8) then y : test1 xs y else test1 xs lastValue
>> test1 [] _ = []
>
> You can reformulate this function in terms of the mapAccum combinators from
> Data.List. Once you have done this, you can easily adapt it to the mapAccum
> combinator from reactive-banana.
>
> test :: Event t Double -> Event t Double
> test e = filterJust $ fst $ mapAccum Nothing $ next <$> e
> where
> next x Nothing = (Just x, Just x)
> next x (Just lastValue) =
> let y = if lastValue>=0.8 then x else 1.0-x
> in if (y<=0.2) || (y>=0.8)
> then (Just y , Just y)
> else (Nothing, Just lastValue)
Thanks, mapAccum is what I was missing.
best,
Miguel
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