drawMany is just sequence dressed up a bit: I assume you have this class: class Drawable a where draw :: Drawable a => a -> Window -> IO () So, the key is to remember that functions and IO actions are first-class values! data Box = ... instance Drawable Box where ... box :: Box box = ... data Circle = ... instance Drawable Circle where ... circle :: Circle circle = ... instance Drawable String where ... with :: b -> [b -> a] -> [a] with w = map ($ w) manyWith :: Monad m => w -> [w -> m a] -> m () manyWith w = sequence . with w sample :: Window -> IO () sample w = manyWith w $ [draw box, draw circle, draw "hello"] On Sun, Feb 28, 2010 at 6:31 AM, Yves Parès <limestrael@gmail.com> 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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