On Sun, Feb 28, 2010 at 6:31 AM, Yves Parès
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?
Luke Palmer has an article on his blog that proposes a different solution to the problem that I believe you're trying to solve: http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-t... You might give his approach a try and see if it's simpler and easier to work with. Good luck! Jason