Question

想再询问您一道Haskell的题目，找下面程序的8个error是什么，非常感谢！

定义merge sort的一个函数，一共有8个error。
mergeSort :: ([a] -> [a] -> [a]) -> [a] -> [a] -> [a]

mergeSort merge xs
| length xs <= 2 =xs
| else
== merge (mergeSort merge first)
(mergeSort merge second)
where first = take half ys
second = drop half xs
halve = (length xs) div 2

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Answer 1

其实你guard的用法没有问题, 只是有点语法错误

mergeSort merge xs
| length xs <= 2 =xs
| otherwise = merge (mergeSort merge first) (mergeSort merge second)
where first = take half ys
second = drop half xs
halve = (length xs) div 2

应该用otherwise 而不是else

Answer 2

这是正确代码

mergeSort :: ([a] -> [a] -> [a]) -> [a] -> [a]

mergeSort merge xs =
if length xs <= 2 then xs
else merge (mergeSort merge first)
(mergeSort merge second)
where
first = take halve xs
second = drop halve xs
halve = (length xs) `div` 2