haskell - `readMay` 和 `readMaybe` 有什么区别？

Question

readMay 和 readMaybe 这两个函数具有相同的签名 Read a => String -> Maybe a。

它们之间有什么区别吗？如果是这样，它们是什么？这两个功能应该首选哪个？

Answer 1

没有区别。以下是 readMay 的定义方式:

-- | This function provides a more precise error message than 'readEither' from 'base'.
readEitherSafe :: Read a => String -> Either String a
readEitherSafe s = case [x | (x,t) <- reads s, ("","") <- lex t] of
        [x] -> Right x
        []  -> Left $ "no parse on " ++ prefix
        _   -> Left $ "ambiguous parse on " ++ prefix
    where
        maxLength = 15
        prefix = '\"' : a ++ if length s <= maxLength then b ++ "\"" else "...\""
            where (a,b) = splitAt (maxLength - 3) s

readMay :: Read a => String -> Maybe a
readMay = eitherToMaybe . readEitherSafe

这里是 readMaybe:

-- | Parse a string using the 'Read' instance.
-- Succeeds if there is exactly one valid result.
-- A 'Left' value indicates a parse error.
--
-- @since 4.6.0.0
readEither :: Read a => String -> Either String a
readEither s =
  case [ x | (x,"") <- readPrec_to_S read' minPrec s ] of
    [x] -> Right x
    []  -> Left "Prelude.read: no parse"
    _   -> Left "Prelude.read: ambiguous parse"
 where
  read' =
    do x <- readPrec
       lift P.skipSpaces
       return x

-- | Parse a string using the 'Read' instance.
-- Succeeds if there is exactly one valid result.
--
-- @since 4.6.0.0
readMaybe :: Read a => String -> Maybe a
readMaybe s = case readEither s of
                Left _  -> Nothing
                Right a -> Just a

它们在中间错误消息上有所不同(readEitherSafe 显示了输入)，但结果是相同的。

---

Safe 中的

readMay 早于 Text.Read 中的 readMaybe。除非您使用的 base 版本低于 4.6.0.0，否则请使用 Text.Read 中的 readMaybe，因为它不需要其他包。