Show for String 的实例是怎么写的？

Question

我有一个关于定义类型 classes 实例的基本问题。我以 Show typeclass 为例，我只考虑 class 中的函数 show。像 Bool 这样的具体类型的 Show 实例很简单

instance Show Bool where
  show x = {function of x here}

但对于 String 则不是：

instance Show String where
  show x = {function of x here}

产生一个可以理解的错误

Illegal instance declaration for ‘Formatter String’
  (All instance types must be of the form (T t1 ... tn)
   where T is not a synonym.
   Use TypeSynonymInstances if you want to disable this.)
In the instance declaration for ‘Formatter String’

当然不允许以下内容：

instance Show [Char] where
  show x = {function of x here}

我可以定义一个新类型

newtype String2 = String2 String 
instance Formatter String2 where
  format (String2 x) = {function of x here}

但是不允许我显示 "test"，因为我可以在 Haskell 中显示。

我缺少class类型的什么基本特征？

Answer 1

Show类型类实际上有3个成员函数，show、showsPrec、showList。在 Show Char 的实例中，showList 函数被重载以输出引号并将所有字母拼在一起，没有分隔符：

来自 GHC.Show:

instance  Show Char  where
    showsPrec _ '\'' = showString "'\''"
    showsPrec _ c    = showChar '\'' . showLitChar c . showChar '\''

    showList cs = showChar '"' . showLitString cs . showChar '"'

其中 showLitString 定义为：

showLitString :: String -> ShowS
-- | Same as 'showLitChar', but for strings
-- It converts the string to a string using Haskell escape conventions
-- for non-printable characters. Does not add double-quotes around the
-- whole thing; the caller should do that.
-- The main difference from showLitChar (apart from the fact that the
-- argument is a string not a list) is that we must escape double-quotes
showLitString []         s = s
showLitString ('"' : cs) s = showString "\\"" (showLitString cs s)
showLitString (c   : cs) s = showLitChar c (showLitString cs s)

所以没有Show String实例，只是Show Char具体定义了如何在[Char]值上调用show。

Show for String 的实例是怎么写的？

How is the instance of Show for String written?

haskell

typeclass