[Haskell-cafe] Lists of Existential DT

[Eugene Kirpichov]

Or like this, with the benefit of using lists.
data DrawableObj a = forall a.Drawable a => DrawableObj a
a <,> b = DrawableObj a : b
drawMany (a<,>b<,>c<,>[])

2010/2/28 Miguel Mitrofanov <miguelimo38 at yandex.ru>:
> 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
>> --
>> View this message in context:
>> http://old.nabble.com/Lists-of-Existential-DT-tp27735354p27735354.html
>> Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
>>
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-- 
Eugene Kirpichov
Senior Developer, JetBrains