[Haskell-cafe] no dynamic binding

[Tomasz Zielonka]

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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