You can use exisential types to do what you'd like. From memory, so there are probably errors: newtype ServerCommand = forall a. ServerCommandClass a => ServerCommand a instance ServerCommandClass ServerCommand where toString (ServerCommand c) = toString c commands :: [ServerCommand] commands = [ServerCommand $ DashCommand ..., ServerCommand $ MoveCommand ...] Also, for a less type-safe approach, see Data.Dynamic. On Sun, 19 Sep 2004 13:48:53 -0400, Andrew Harris wrote:

Hi -

I have another question. I am still working on a soccer server and thought it would be neat to create command objects that had a "toString" method. Then, I was going to keep a list of these command objects and at the right time stringify them and send them to the server. So I created a class with a toString method:

class ServerCommandClass a where toString :: a -> String

And then a few instances:

-- dash command data DashCommand = DashCommand { dashpower :: Double }

instance ServerCommandClass DashCommand where toString c = "(dash " ++ show (dashpower c) ++ ")\n"

-- move command data MoveCommand = MoveCommand { x :: Double, y :: Double }

instance ServerCommandClass MoveCommand where toString c = "(move " ++ show (x c) ++ " " ++ show (y c) ++ ")\n"

The problem is, I am not quite sure how to describe a *list* of command objects where the list could have both DashCommands and MoveCommands in it. Ideally the list could contain both, and then for each item in the list I could call the toString method.

I was reading Simon Thompson's Haskell: The Craft of Functional Programming and I read that Haskell 98 does not support dynamic binding, which (it seems) is what I'm trying to do. Does anyone have a suggestion on an alternative approach?

thanks, -andrew _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe