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haskell - 哈斯克尔。变量不在范围内：howManyTimes :: Integer -> [Integer] -> t

问题描述

我写了一个小代码，它计算列表中的字符或数字以及多少次。例如 howManyTimes 3 [2, 3, 4, 5, 3, 3, 1, 80, 3] 应该是 4

这是我的代码

howManyTimes y [] = 0
howManyTimes y xs = howManyIntern y (x:xs) acc 
                        | x == y = howManyIntern y xs (acc + 1)
                        | otherwise = howManyIntern y xs acc

我收到此错误消息。变量不在范围内：howManyTimes :: Integer -> [Integer] -> t

谁能给我一个提示？

标签： haskell

解决方案

我已经重写了我的代码。我不再收到错误但输出为 0

howManyTimes y xs = howManyIntern y xs 0 
    where howManyIntern y [] acc = 0 
          howManyIntern y (x:xs) acc | x == y = howManyIntern y xs (acc + 1) 
                                     | otherwise = howManyIntern y xs acc

由于表达式：

howManyIntern y [] acc = 0

每当它完成计数时返回 0。很容易通过 returnacc来修复：

howManyIntern y [] acc = acc

另一种使用现有功能的方法：

howManyTimes y xs = length $ filter (==y) xs

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