[Haskell-cafe] no dynamic binding
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
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Re: [Haskell-cafe] no dynamic binding
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 <[EMAIL PROTECTED]> 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
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Re: [Haskell-cafe] no dynamic binding
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 <[EMAIL PROTECTED]> 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
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--
Georg Martius, Tel: (034297) 89434
--- http://www.flexman.homeip.net -
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Re: [Haskell-cafe] no dynamic binding
On Sun, Sep 19, 2004 at 08:54:57PM +0200, Georg Martius wrote: > Hi, > > it might be better to use a simple abstact data type for it like: You probably meant algebraic data type. Abstractness isn't essential here. Best regards, Tom -- .signature: Too many levels of symbolic links ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] no dynamic binding
On Sun, Sep 19, 2004 at 02:46:12PM -0400, Abraham Egnor wrote:
> 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
This can't be newtype, you must use 'data'.
> instance ServerCommandClass ServerCommand where
> toString (ServerCommand c) = toString c
>
> commands :: [ServerCommand]
> commands = [ServerCommand $ DashCommand ..., ServerCommand $ MoveCommand ...]
As it was pointed to me some time ago, you can also achieve this without
existential types, simply by keeping the partially applied methods in
your ServerCommand data type, eg.
data ServerCommand =
ServerCommand
{ scToString:: String
}
or, if you want to avoid memoizing the toString result in this case
data ServerCommand =
ServerCommand
{ scToString:: () -> String
}
Best regards,
Tom
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Re: [Haskell-cafe] no dynamic binding
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-misuse
#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
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Graham Klyne
For email:
http://www.ninebynine.org/#Contact
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