
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