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2013-03-31T21:30:42.380
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The most direct way is to just do it by hand: <pre><code>fromList :: [Double] -> Maybe OrbitElements fromList [ _epoch , _ecc , _distPeri , _incl , _longAscNode , _argPeri , _timePeri , _meanMotion , _meanAnomaly , _trueAnomaly , _semiMajorAxis , _distApo , _period ] = Just $ OrbitElements _epoch _ecc _distPeri _incl _longAscNode _argPeri _timePeri _meanMotion _meanAnomaly _trueAnomaly _semiMajorAxis _distApo _period fromList _ = Nothing </code></pre> However, there is a slightly sexier way, which is to parse them element by element, which is less error-prone and more descriptive of what we are trying to do: First we define two parsers, one of which requests a new element from the list (or fails if the list is empty), and the second of which matches the end of the list (or fails if the list is not empty): <pre><code>import Control.Applicative import Control.Monad import Control.Monad.Trans.State getElem :: StateT [Double] Maybe Double getElem = do s <- get case s of [] -> mzero x:xs -> do put xs return x endOfList :: StateT [Double] Maybe () endOfList = do s <- get case s of [] -> return () _ -> mzero </code></pre> Now we can define <code>fromList</code> in Applicative style: <pre><code>fromList' :: [Double] -> Maybe OrbitElements fromList' = evalStateT $ OrbitElements <$> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <*> getElem <* endOfList </code></pre>
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1. POInitializing algebraic data type from list
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1. USGabriel Gonzalez
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1. COThanks, this confirmed my suspicions, and any answer that starts out 'First, we define two parsers..." is great in my book. :)
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 USJames Curbo
2. COThe "by hand" option becomes a littler nicer when combined with [`-XRecordWildCards`](http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/syntax-extns.html#record-wildcards): `fromList [epoch,ecc,distPeri,incl,longAscNode,argPeri,timePeri,meanMotion,meanAnomaly,trueAnomaly,semiMajorAxis,distApo,period] = Just OrbitElements{..}`.
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