Haskell Prelude.read：无法解析字符串

来自哈斯克尔的例子http://learnyouahaskell.com/types-and-typeclasses http://learnyouahaskell.com/types-and-typeclasses

ghci> read "5" :: Int  
5  
ghci> read "5" :: Float  
5.0  
ghci> (read "5" :: Float) * 4  
20.0  
ghci> read "[1,2,3,4]" :: [Int]  
[1,2,3,4]  
ghci> read "(3, 'a')" :: (Int, Char)  
(3, 'a')  

但当我尝试时

read "asdf" :: String 

or

read "asdf" :: [Char]

我得到例外

Prelude.read 无解析

我在这里做错了什么？

这是因为您拥有的字符串表示形式不是 a 的字符串表示形式String，它需要在字符串本身中嵌入引号：

> read "\"asdf\"" :: String
"asdf"

这是这样的read . show === id for String:

> show "asdf"
"\"asdf\""
> read $ show "asdf" :: String
"asdf"

作为旁注，使用readMaybe函数来自Text.Read:

> :t readMaybe
readMaybe :: Read a => String -> Maybe a
> readMaybe "asdf" :: Maybe String
Nothing
> readMaybe "\"asdf\"" :: Maybe String
Just "asdf"

这避免了（在我看来）损坏read解析失败时引发异常的函数。