Re: [Gtk2hs-users] A problem not too much related to gtk2hs.

[John L. <jw...@gm...>]

What do you find ugly about it?  I think it's pretty elegant myself.
Anyway, there is a third option, that isn't quite full existential
types.

data GridWidget = GwLabel Label
  | GwTextView TextView

gridWidgetNew :: String -> IO GridWidget
gridWidgetNew str = case str of
  "GwLabel" -> liftM GwLabel $ labelNew Nothing
  "GwTextView" -> liftM GwTextView textViewNew
  _ -> error "Invalid GridWidget"

Rather than matching on strings, you could use another type to
represent the names and pattern match on that.
John

On Mon, Aug 24, 2009 at 9:02 PM, Magicloud
Magiclouds<mag...@gm...> wrote:
> I thought about this before, in another case, but still, I cannot
> belive I should do it this ugly....
>
> On Tue, Aug 25, 2009 at 12:15 AM, John Lato<jw...@gm...> wrote:
>> Hello,
>>
>> For this, I don't think you'd need existentials.  First of all, I
>> would use different names for the GridWidget constructors since Label
>> and TextView are already used by gtk.
>>
>> data GridWidget = GwLabel
>>               | GwTextView
>>
>> and I'd probably derive Eq, Read, and Enum as well.
>>
>> Next point, you can't have gridWidgetNew return a polymorphic type
>> like this.  See http://www.haskell.org/haskellwiki/OOP_vs_type_classes
>> for more detail.  Even if you use existential types, you'll still need
>> to wrap it in another constructor.  If you don't need the child other
>> than simply adding it to the parent widget, I would use the following
>> function:
>>
>> addGridWidget :: ContainerClass c => c -> GridWidget -> IO c
>> addGridWidget self GwLabel = do
>>  child <- labelNew Nothing
>>  set self [ containerChild := child ]
>>  return self
>> addGridWidget self GwTextView = do
>>  child <- textViewNew
>>  set self [ containerChild := child ]
>>  return self
>>
>> This will work without existential types because the type of the child
>> widget doesn't escape the function.
>>
>> Cheers,
>> John
>>
>>> From: Magicloud Magiclouds <mag...@gm...>
>>>
>>> I have read the wiki, and I think gtk2hs's way is enough for me.
>>> The other part of my code would be like this:
>>>
>>> myWidgetNew gridWidget = do
>>>  self <- someWidgetNew
>>>  child <- gridWidgetNew gridWidget
>>>  set self [ containerChild := child ]
>>>  return self
>>>
>>> Well, it failed compiling on gridWidgetNew definition.
>>>
>>> On Mon, Aug 24, 2009 at 8:19 PM, Bulat
>>> Ziganshin<bul...@gm...> wrote:
>>>> Hello Magicloud,
>>>>
>>>> Monday, August 24, 2009, 1:39:48 PM, you wrote:
>>>>
>>>>> ? My total purpose is to create a widget with an argument that what
>>>>> child widget the user wants to have. So I pass GridWidget to the
>>>>> widget init function, and it calls gridWidgetNew to see what new
>>>>> function it actually has to call.
>>>>
>>>> this depends on how you plan to use the widget later. Haskell provides
>>>> existential types for such things, but Gtk2Hs own mechanisms may be
>>>> also useful
>>>>
>>>> read about existentials here: http://haskell.org/haskellwiki/OOP_vs_type_classes
>>>>
>>
>
>
>
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