警卫上的解析错误

Question

我正在实现归并排序的归并子程序。编译时收到此错误消息：输入时解析错误 |

merge :: [Int] -> [Int] -> [Int]
merge xs ys = go xs ys []
    where go xs ys zs =
            |null ys              = zs ++ xs                     
            |null xs              = zs ++ ys                        
            |(head xs) <= head ys = go (tail xs) ys (head xs : zs)  
            |otherwise            = go xs (tail ys) (head ys : zs)

谁能告诉我为什么？还有更简洁的方法吗？谢谢。

Answer 1

您可以通过删除 go xs ys zs 之后的 = 来修复语法错误 - 当使用守卫定义某些内容时，= 仅出现在每个守卫之后，就像您已经拥有的那样。

除此之外，如果您更多地使用模式匹配，您的代码会更简洁。您可以使用模式[] 来识别空列表和(x:xs) 来识别非空列表并绑定头部，而不是使用null 检查列表是否存在并尾随名称x 和xs 等：

merge :: [Int] -> [Int] -> [Int]
merge xs ys = go xs ys []
    where go xs [] zs          = zs ++ xs
          go [] ys zs          = zs ++ ys
          go (x:xs) (y:ys) zs
              | x <= y         = go xs (y:ys) (x:zs)
              | otherwise      = go (x:xs) ys (y:zs)