Hi, it might be better to use a simple abstact data type for it like: -- | Command Type has for all possible actions a contructor data Command = INIT String String Bool -- ^ Teamname, Version, Goalkeeper or not | RECONNECT String Integer -- ^ Teamname, Playernum (Unum) | CATCH Angle | CHANGEVIEW ViewWidth ViewQuality | DASH Integer -- ^ Power [-100, 100] | KICK Integer Angle -- ^ Power [-100,100], direction | MOVE Double Double -- ^ X, Y Coordinate | SAY String | REQUESTSENSEBODY | REQUESTSCORE | TURN Angle -- ^ Angle [-180, 180] see 'Direction' | TURNNECK Angle -- ^ Angle [-90,90] | BYE | NOACTION -- ^ this means we do nothing -- Show instance for Command to create syntactical correct command strings instance Show Command where show NOACTION = "" show c = "(" ++ (concat strlist) ++ ")" where strlist = case c of INIT team ver True -> ["init ", team, " (version ", ver, ") goalie"] INIT team ver False -> ["init ", team, " (version ", ver, ")"] RECONNECT team unum -> ["reconnect ", team, " ", show unum] CATCH dir -> ["catch ", show dir] CHANGEVIEW width quali -> ["change_view ", show width, show quali] DASH p -> ["dash ", show p] KICK p dir -> ["kick ", show p, " ", show dir] MOVE x y -> ["move ", show x, " ", show y] SAY msg -> ["say ", msg] REQUESTSENSEBODY -> ["sense_body"] REQUESTSCORE -> ["score"] TURN a -> ["turn ", show a] TURNNECK a -> ["turn_neck ", show a] BYE -> ["bye"] NOACTION -> [""] -- just for compiler you can just have a [Command] and use show to get the string representation. As you might guess I have implemented a socker client in haskell allready. I can mail you the code if you want. It is still in the alpha stage, but it can do quite a lot. Best Regards! Georg 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

-- ---- Georg Martius, Tel: (034297) 89434 ---- ------- http://www.flexman.homeip.net ---------

I think others have provided the immediate answers to your question, so I'll skip that. But, standing back, you seem to have fallen prey to a misunderstanding I made when learning Haskell, thinking that a "class" is directly analogous to a class in OO languages. While there are similarities, the Haskell notion of a class is much more restrictive, as you have found. I wrote a few more notes here: http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#type-class-mi... #g -- At 13:48 19/09/04 -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

------------ Graham Klyne For email: http://www.ninebynine.org/#Contact