Question

我有这段代码：

main :: IO ()
main = do
let list = []
handle <- openFile "polycake.in" ReadMode
contents <- hGetContents handle
let singlewords = words contents
    list = fileToList singlewords
    n = head list
    list' = drop 1 list
    polygons = polyList n list'
mapM_ print polygons
hClose handle

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

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'
    -- Calc Perim Here
    --Just (under, over) = calcPerim (fromIntegral y) points :: Maybe 
(Length, Length)
    poly = Polygon { v = v, y = y, points = points}
    nextPoly = polyList (n-1) list''
in (poly:nextPoly)

getPoints :: [Int] -> [Point]
getPoints [] = error "Can't create a point (None)"
getPoints [_] = error "Can't create a point (Only One)"
getPoints (k:v:t) = (fromIntegral k, fromIntegral v) : getPoints t

但getPoints正在获得一个空列表。在读入所有顶点之后，最后一对应该是对getPoints的最后一次调用，但似乎它正在使用空列表发送另一个调用。

    Polygon {v = 4, y = 2, points = [(0.0,0.0),(4.0,0.0),(4.0,4.0),(0.0,4.0)polycake.hs: Can't create point (None)
    CallStack (from HasCallStack):
    error, called at polycake.hs:54:16 in main:Main

以下是示例输入：

如果第一行是多边形的数量，第二行的第一个值是多边形中的顶点数量，第二个值是计算中使用的值。因此，第二行上的4,2表示4个顶点，将接下来的4行作为顶点对。 6,10创建具有6个顶点等的下一个多边形。

我只是不确定空列表来自哪里来尝试创建点。

Answer 1

空列表来自：

getPoints (k:v:t) = (fromIntegral k, fromIntegral v) : getPoints t
--                                                     ^^^^^^^^^^^

空列表与模式t匹配。（任何列表都匹配模式t。）

你如何修复它取决于你，但可能最简单的修复（也是一个非常合理的修复）是替换你的基本情况：

getPoints [] = []

当它应该包含2个元素Haskell时发送方法空列表

1 个答案: