On Fri, Oct 17, 2008 at 7:21 AM, leledumbo
So, what's the solution? This one:
(l::[Ord]) <- readLn
doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements.
What you're after is not possible that easily. What you want to do is to read a bunch of elements, and based on what you read in, determine their type. This is contrary to static typing. A number of ways are possible, but they all involve some sort of enumeration of all the possible types to read in. Way #1: make a data type which represents any of the types of things you can accept, and define Ordering on it. data Thing = ThingInt Int | ThingString String | ThingList [Thing] | ... deriving Ord And then define a parser to get from a string to a Thing (pretty easy using ReadP or Parsec or something). Note that this would accept a heterogeneous list like 1, "foo", 2, [3,"bar",4], and it would sort it as 1, 2, "foo", [3,"bar",4], because of the way Ord deriving works (all Ints are less than all Strings are less than all Lists ...). You can customize this by not using "deriving", and instead manually declaring the instance. Way #2: use the parser from way #1 to parse the first one and determine what kind of thing you're reading, and then read a bunch of those. sortThings :: (Read a) => IO [a] main = do first <- getLine case first of ThingInt _ -> print =<< (sortThings :: IO [Int]) ThingString _ -> print =<< (sortThings :: IO [String]) ... There are some other more clever ways. But you will not be able to get Haskell to make the choice for you. In particular, there are some ambiguous choices, for example is read "42" an Int, an Integer, a Double ? So you need to write an algorithm which resolves the ambiguities. Luke