在Haskell中使用包含在純函式中的不純函式的問題

我正在嘗試使用包裝在純函式中的 readFile 函式。

使用的函式（在我的學校里，如果我想使用它，我必須重新撰寫所有內置函式）

myNth :: [a] -> Int -> a
myNth [] n = error "error"
myNth (x:xs) 0 = x
myNth (_:xs) n = myNth xs (n - 1)

myTail :: [a] -> [a]
myTail (_:a) = a
myTail [] = error "empty list"

pointStrToInt :: String -> (Int, Int, Int)
pointStrToInt point = (read(myTail(myNth(split ',' point) 0)), read(myNth(split ',' point) 1), read(myNth(split ')' (myNth(split ',' point) 2)) 0))

fileToPointArray :: [String] -> [(Int, Int, Int)]
fileToPointArray = map pointStrToInt

split :: Eq a => a -> [a] -> [[a]]
split d [] = []
split d s = x : split d (drop 1 y) where (x,y) = span (/= d) s

問題功能：

closest :: FilePath -> (Int, Int, Int) -> (Int, Int, Int) 
closest path a = do
                    let content = readFile path
                    let closestPoint = pointStrToInt (myNth(lines (content)) 0)
                    --print (closestPoint)
                    let arr = fileToPointArray (lines content)
                    --print (arr)
                    let p = doClosest arr a closestPoint
                    --print (p)
                    p

    where
        doClosest :: [(Int, Int, Int)] -> (Int, Int, Int) -> (Int, Int, Int) -> (Int, Int, Int)
        doClosest [] _ a = a
        doClosest (x:xs) a (b, c, d) = if ((distance (itofPoint x) (itofPoint a)) < (distance (itofPoint x) (itofPoint (b, c, d))))
                                       then (doClosest xs a x)
                                       else (doClosest xs a (b, c, d))

主要目標是獲取與作為引數傳遞的給定點的最近點。原型必須是這樣的：

closest :: FilePath -> (Int, Int, Int) -> (Int, Int, Int)

但我不能使用不純函式（readFile），因為該函式不是 IO()。

我該如何解決？

編輯：-該檔案包含一組 (x,y,z) 格式的點

-距離函式回傳2個點的距離

-檔案可以在任何執行中更改

uj5u.com熱心網友回復：

你已經陷入do困境；你只是沒有利用它。content應該從 中提取， readFile path而不是簡單地參考IO它創建的動作。您還需要將回傳值更改為IO (Int, Int, Int)：

closest :: FilePath -> (Int, Int, Int) -> IO (Int, Int, Int) 
closest path a = do
                    content <- readFile path
                    let closestPoint = pointStrToInt (myNth(lines (content)) 0)
                    let arr = fileToPointArray (lines content)
                    let p = doClosest arr a closestPoint
                    return p

    where
        doClosest :: [(Int, Int, Int)] -> (Int, Int, Int) -> (Int, Int, Int) -> (Int, Int, Int)
        doClosest [] _ a = a
        doClosest (x:xs) a (b, c, d) = if ((distance (itofPoint x) (itofPoint a)) < (distance (itofPoint x) (itofPoint (b, c, d))))
                                       then (doClosest xs a x)
                                       else (doClosest xs a (b, c, d))

更簡單一點的是保持closest純凈并將其與readFile包裝器結合。

-- Note the order of arguments is reversed, so that we
-- can partially apply it later...
closest :: (Int, Int, Int) -> String -> (Int, Int, Int) 
closest content a = let closestPoint = pointStrToInt (myNth(lines (content)) 0)
                        arr = fileToPointArray (lines content)
                    in doClosest arr a closestPoint

    where
        doClosest :: [(Int, Int, Int)] -> (Int, Int, Int) -> (Int, Int, Int) -> (Int, Int, Int)
        doClosest [] _ a = a
        doClosest (x:xs) a (b, c, d) = if ((distance (itofPoint x) (itofPoint a)) < (distance (itofPoint x) (itofPoint (b, c, d))))
                                       then (doClosest xs a x)
                                       else (doClosest xs a (b, c, d))

doIt :: FilePath -> (Int, Int, Int) -> IO (Int, Int, Int)
doIt path a = fmap (closest a) (readFile path)

（如果您不想或無法更改 中的引數順序closest，則可以fmap (flip closest a) (readFile path)改用。）

標籤：哈斯克尔 函数式编程 读取文件

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