What is the difference between `readMay` and `readMaybe`?

Question

The two functions readMay and readMaybe have the same signature Read a => String -> Maybe a.

Is there any difference between them? If so, what are they? Which of the two function should be preferred?

Answer 1

There is no difference. Here's how readMay's defined:

-- | 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

And here is 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

They differ in the intermediate error message (readEitherSafe shows the input), but the result will be same.

---

readMay from Safe predates readMaybe from Text.Read. Unless you're on a base version less than 4.6.0.0, use readMaybe from Text.Read as it does not need another package.