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
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