我是一个相当新的 Haskell 程序员,我正在尝试弄清楚如何将一些值转换为代数数据类型。
我有一个记录数据类型:
data OrbitElements = OrbitElements { epoch :: Double,
ecc :: Double,
distPeri :: Double,
incl :: Double,
longAscNode :: Double,
argPeri :: Double,
timePeri :: Double,
meanMotion :: Double,
meanAnomaly :: Double,
trueAnomaly :: Double,
semiMajorAxis :: Double,
distApo :: Double,
period :: Double
}
我正在从一个文本文件中提取一些信息,这些信息最终出现在一个 Doubles 列表中。有没有一种简单的方法可以用列表初始化这种数据类型?我可以单独调用每个 setter ,但是当我已经拥有列表中的所有值时,这似乎非常低效。
let d = [2456382.5,6.786842103348031e-3,0.7184187640759256,3.394660181513041,76.64395338801751,55.2296201483587,2456457.141012543,1.602144936476915,240.4142797010899,239.7408018186761,0.7233278761603762,0.7282369882448266,224.6987721295883]
let o = OrbitElements
let epoch o = d !! 0
let ecc o = d !! 1
-- and so on
我错过了什么?
最佳答案
最直接的方法是手动完成:
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
但是,有一种更性感的方式,即逐个元素地解析它们,这样更不容易出错,并且更能描述我们正在尝试做的事情:
首先我们定义两个解析器,其中一个从列表中请求一个新元素(如果列表为空则失败),第二个解析器匹配列表的末尾(如果列表不为空则失败):
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
现在我们可以定义
fromList
应用风格:fromList' :: [Double] -> Maybe OrbitElements
fromList' = evalStateT $ OrbitElements
<$> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<*> getElem
<* endOfList
关于list - 从列表初始化代数数据类型,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15734573/