来自Haskell的例子 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')
但是当我尝试时
read "asdf" :: String
或
read "asdf" :: [Char]
我得到例外
前奏.阅读 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String
的字符串表示形式,它需要在字符串本身中嵌入引号:
> read ""asdf"" :: String
"asdf"
这样read . show === id
String
:
> show "asdf"
""asdf""
> read $ show "asdf" :: String
"asdf"
作为旁注,最好改用 Text.Read
中的 readMaybe
函数:
> :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)损坏的read
函数,该函数在解析失败时引发异常。