[Haskell-cafe] Lists of Existential DT

[Miguel Mitrofanov]

Sorry, no luck with that.

But you can, probably, define some "customized comma":

data DrawPair a b = DrawPair a b
(<,>) :: a -> b -> DrawPair a b
(<,>) = DrawPair
instance (Drawable a, Drawable b) => Drawable (DrawPair a b) where ...
drawMany :: Drawable a => Window -> a -> IO ()
...
drawMany window $ image <,> text <,> otherImage

On 28 Feb 2010, at 17:31, Yves Parès wrote:

>
> Hello!
>
> I have a class Drawable, and some datatypes which are instances of  
> it, and I
> would like to be able to draw them all at once!
> drawMany window [image, text, otherImage]
>
> I think the type of the function drawMany would be:
> drawMany :: Window -> [forall a. (Drawable a) => a] -> IO ()
>
> However it doesn't work.
> I know one solution is to make a new datatype (e.g. DrawableObj)  
> which will
> be:
> data DrawableObj = forall a (Drawable a) =>
>                              DrawableObj a
>
> And then declare drawMany as:
> drawMany :: Window -> [DrawableObj] -> IO ()
>
> But to use it I have to wrap up myself every drawable in a  
> DrawableObj:
> drawMany window [DrawableObj image, DrawableObj text, DrawableObj
> otherImage]
>
> Is there another more suitable way to handle a list of drawables?
>
> -----
> Yves Parès
>
> Live long and prosper
> -- 
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