Haskell：模式中的解析错误

发布于 2024-12-22 01:11:20 字数 300 浏览 1 评论 0原文

谁愿意告诉我这段代码有什么问题（语法上）？

-- merge two sorted lists
mergeX [] b res = b ++ res
mergeX a [] res = a ++ res
mergeX a:as b:bs res
    | a > b     = mergeX as b:bs a:res
    | otherwise = mergeX a:as bs b:res

口译员：

模式解析错误：mergeX

原文

Who likes to tell me what is wrong with this code (syntactically)?

-- merge two sorted lists
mergeX [] b res = b ++ res
mergeX a [] res = a ++ res
mergeX a:as b:bs res
    | a > b     = mergeX as b:bs a:res
    | otherwise = mergeX a:as bs b:res

Interpreter:

Parse error in pattern: mergeX

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ぶ宁プ宁ぶ 2024-12-29 01:11:20

你需要一些括号：

mergeX [] b res = b ++ res
mergeX a [] res = a ++ res
mergeX (a:as) (b:bs) res
    | a > b     = mergeX as (b:bs) (a:res)
    | otherwise = mergeX (a:as) bs (b:res)

原因是因为 : 的优先级低于函数应用，所以 mergeX a:as b:bs res 将被解析为：

(mergeX a):(as b):(bs res)

这是一个错误。

You need some parenthesis:

mergeX [] b res = b ++ res
mergeX a [] res = a ++ res
mergeX (a:as) (b:bs) res
    | a > b     = mergeX as (b:bs) (a:res)
    | otherwise = mergeX (a:as) bs (b:res)

The reason is because : has a lower precedence than function application, so mergeX a:as b:bs res will be parsed as: