Haskell 无法将预期类型“字符串"与实际类型“字符"匹配 [英] Haskell Couldn't match expected type 'String' with actual type 'Char'

问题描述

我想知道为什么会出现此错误.这是一个分配，我要将整数从整数转换为十六进制值.当我通过 16 修改整数值时，我调用了这个辅助转换函数.(与整数值连接，然后在递归调用中除以 16)

I am wondering why I'm getting this error. It's for an assignment where i'm to convert from an integer to a hex value. I call this helper conversion function when I mod the integer value by 16. (concatenated with the integer value which I then divide by 16 in a recursive call)

这是我的代码:

    changeToHex :: Integer -> String
    --main function

    toHex :: Integer -> String
    toHex x
        |x == 0         = '0'
        |x == 1         = '1'
        |x == 2         = '2'
        |x == 3         = '3'
        |x == 4         = '4'
        |x == 5         = '5'
        |x == 6         = '6'
        |x == 7         = '7'
        |x == 8         = '8'
        |x == 9         = '9'
        |x == 10        = 'A'
        |x == 11        = 'B'
        |x == 12        = 'C'
        |x == 13        = 'D'
        |x == 14        = 'E'
        |x == 15        = 'F'

推荐答案

使用单引号 ('F') 为您提供 Char 文字.对于 String 文字，实际上是 Char 值的列表，您应该使用双引号 ("F").

Using single quotes ('F') gives you a Char literal. For a String literal, which is in fact a list of Char values, you should use double quotes ("F").

由于 String 是 [Char] 的别名，如果您想从 Char 转换为 String，您可以仅将 Char 包装在一个元素列表中.这样做的函数可能如下所示:

Since String is an alias for [Char], if you want to convert from a Char to a String, you can merely wrap the Char in a one-element list. A function to do so might look like:

stringFromChar :: Char -> String
stringFromChar x = [x]

这通常是内联编写的，如 (:[])，相当于 x ->;(x : []) 或 x ->[x].

This is typically written inline, as (:[]), equivalent to x -> (x : []) or x -> [x].

顺便说一句，您可以大大简化您的代码，例如使用 Enum 类型类:

As an aside, you can simplify your code considerably, using for example the Enum typeclass:

toHexDigit :: Int -> Char
toHexDigit x
  | x < 0 = error "toHex: negative digit value"
  | x < 10 = toEnum $ fromEnum '0' + x
  | x < 15 = toEnum $ fromEnum 'A' + x - 10
  | otherwise = error "toHex: digit value too large"

更一般地说，任何时候你都有这样的功能:

More generally, any time you have a function like:

f x
  | x == A = ...
  | x == B = ...
  | x == C = ...
  ...

您可以使用 case 将其转换为更少重复、更高效的等价物:

You can convert that to a less repetitious, more efficient equivalent with case:

f x = case x of
  A -> ...
  B -> ...
  C -> ...
  ...

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