具有两个where子句的Haskell方法

Cilenco 发表于 Dev

奇伦科

我想编写一个具有两个where子句的Haskell函数，但从编译器中收到此错误：

输入中的语法错误（意外的符号“ changeNew”）

changeItem oPos nPos (x:xs) 
                        | oPos < nPos = changeOld
                        | oPos > nPos = changeNew
                        | otherwise = x : changeItem (oPos-1) (nPos-1) xs
                        where changeOld
                            | oPos == 0 = (xs !! nPos) : changeItem x (nPos-1) xs
                            | nPos == 0 = oPos : xs
                            | otherwise = x : changeItem (oPos-1) (nPos-1) xs
                        changeNew
                            | oPos == 0 = nPos : xs
                            | nPos == 0 = (xs !! oPos) : changeItem (oPos-1) x xs
                            | otherwise = x : changeItem (oPos-1) (nPos-1) xs

代码有什么问题？为什么不能声明两个where子句？

克里斯·泰勒

where子句中定义的名称需要在左侧对齐，像这样

changeItem oPos nPos (x:xs) 
                        | oPos < nPos = changeOld
                        | oPos > nPos = changeNew
                        | otherwise = x : changeItem (oPos-1) (nPos-1) xs
                        where 
                          changeOld
                            | oPos == 0 = (xs !! nPos) : changeItem x (nPos-1) xs
                            | nPos == 0 = oPos : xs
                            | otherwise = x : changeItem (oPos-1) (nPos-1) xs
                          changeNew
                            | oPos == 0 = nPos : xs
                            | nPos == 0 = (xs !! oPos) : changeItem (oPos-1) x xs
                            | otherwise = x : changeItem (oPos-1) (nPos-1) xs

尽管更“常用”的Haskell样式会将所有内容移至左侧，以防止您的代码进入右下角。

changeItem oPos nPos (x:xs) 
  | oPos < nPos = changeOld
  | oPos > nPos = changeNew
  | otherwise = x : changeItem (oPos-1) (nPos-1) xs
 where 
  changeOld
    | oPos == 0 = (xs !! nPos) : changeItem x (nPos-1) xs
    | nPos == 0 = oPos : xs
    | otherwise = x : changeItem (oPos-1) (nPos-1) xs
  changeNew
    | oPos == 0 = nPos : xs
    | nPos == 0 = (xs !! oPos) : changeItem (oPos-1) x xs
    | otherwise = x : changeItem (oPos-1) (nPos-1) xs

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