从Haskell中的文本文件创建多边形列表

杰克·麦克劳克林

我已经阅读了一个数字文件并从中创建了一个数字列表，现在，我想根据列表中的数字创建一个多边形列表，因为它们具有特定的布局/图案。

type Point = (Double, Double)

data Polygon = Polygon { v :: Int
                       , y :: Int
                       , points :: [Point]  
                       }

main = do  
    let list = []
    handle <- openFile "polycake.in" ReadMode
    contents <- hGetContents handle
    let singlewords = words contents
        list = fileToList singlewords
        n = list!!0
        list' = drop 1 list
        polygons = polyList n list'  
    hClose handle 

fileToList :: [String] -> [Int]
fileToList = map read

polyList :: Int -> [Int] -> [Polygon]
polyList n [] = return []
polyList n list = do
    let v = list!!0
        y = list!!1
        list' = drop 2 list
        pointList = take (2*v) list'
        points = getPoints pointList
        list'' = drop (2*v) list'
        poly = Polygon { v = v, y = y, points = points}
        nextPoly = polyList (n-1) list''
    return (poly:nextPoly)

getPoints :: [Int] -> [Point]
getPoints (k:v:t) = (fromIntegral k, fromIntegral v) : getPoints t

但是我得到的错误：

polycake.hs:27:17: error:
     • Couldn't match type ‘[a0]’ with ‘Polygon’
      Expected type: [Polygon]
        Actual type: [[a0]]
     • In the expression: return []
      In an equation for ‘polyList’: polyList n [] = return []
   |
27 | polyList n [] = return []
   |                 ^^^^^^^^^

polycake.hs:37:5: error:
    • Couldn't match type ‘[Polygon]’ with ‘Polygon’
      Expected type: [Polygon]
        Actual type: [[Polygon]]
    • In a stmt of a 'do' block: return (poly : nextPoly)
      In the expression:
        do let v = list !! 0
               y = list !! 1
               ....
           return (poly : nextPoly)
      In an equation for ‘polyList’:
          polyList n list
            = do let v = ...
                     ....
                 return (poly : nextPoly)
   |
37 |     return (poly:nextPoly)
   |     ^^^^^^^^^^^^^^^^^^^^^^

我不确定为什么会出现这些错误。

样品polycake.in：

因此，第一行是测试用例的数量，每个测试用例以2个数字开头，第一个是顶点数，第二个是用于后续计算的ay值。然后，以下v对是顶点或点。

切普纳

您正在滥用do符号和return功能。return不仅仅是用于指定函数返回值的关键字；它用于在Monadic类型的值内部“包装”一个值。虽然[]是monad，但您并未在函数中使用它。

polyList :: Int -> [Int] -> [Polygon]
polyList n [] = []
polyList _ [x] = error "Too few points remaining"
polyList n (v:y:list') =
    let pointList = take (2*v) list' -- Note: list' may not *have* 2*v points
        points = getPoints pointList
        list'' = drop (2*v) list'
        poly = Polygon { v = v, y = y, points = points}
        nextPoly = polyList (n-1) list''
    in (poly:nextPoly)

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