[Haskell-cafe] Deduce issue.

[Magicloud Magiclouds]

I should just use Widget I defined. Why not before....

But then I am confused on type cast again. How to extract the existential
type inside Widget?


On Tue, Jul 29, 2014 at 3:04 PM, Magicloud Magiclouds <
magicloud.magiclouds at gmail.com> wrote:

> I can see this is because of existential type used in Widget. But removing
> it would make a lot of other types using a type var for no reason.
>
>
> On Tue, Jul 29, 2014 at 2:31 PM, Magicloud Magiclouds <
> magicloud.magiclouds at gmail.com> wrote:
>
>> Sorry, missed a line of `class WidgetClass w`.
>>
>> ```
>> class WidgetClass w where
>>   widgetChildren :: w -> [Widget]
>> ```
>>
>>
>> On Tue, Jul 29, 2014 at 2:29 PM, Magicloud Magiclouds <
>> magicloud.magiclouds at gmail.com> wrote:
>>
>>> Hi,
>>>
>>>   I have code like follow. And compiling gave me error as the last part.
>>> What should I do?
>>>
>>> ```
>>> class WidgetClass w
>>> data Widget = forall a. WidgetClass a => Widget a
>>> instance WidgetClass Widget
>>>
>>> lookupWin :: (WidgetClass w) => WINDOW -> w -> Maybe w
>>> lookupWin xwin widget =
>>>   if widgetWindowId widget == xwin
>>>   then Just widget
>>>   else foldl (\a (Widget b) ->
>>>                maybe (lookupWin xwin b) Just a
>>>              ) Nothing $ widgetChildren widget
>>> ```
>>> ---
>>> ```
>>> Graphics/HUI/Widget.hs:15:23:
>>>     Could not deduce (w ~ a)
>>>     from the context (WidgetClass w)
>>>       bound by the type signature for
>>>                  lookupWin :: WidgetClass w => WINDOW -> w -> Maybe w
>>>       at Graphics/HUI/Widget.hs:10:14-54
>>>     or from (WidgetClass a)
>>>       bound by a pattern with constructor
>>>                  Widget :: forall a. WidgetClass a => a -> Widget,
>>>                in a lambda abstraction
>>>       at Graphics/HUI/Widget.hs:14:19-26
>>>       ‘w’ is a rigid type variable bound by
>>>           the type signature for
>>>             lookupWin :: WidgetClass w => WINDOW -> w -> Maybe w
>>>           at Graphics/HUI/Widget.hs:10:14
>>>       ‘a’ is a rigid type variable bound by
>>>           a pattern with constructor
>>>             Widget :: forall a. WidgetClass a => a -> Widget,
>>>           in a lambda abstraction
>>>           at Graphics/HUI/Widget.hs:14:19
>>>     Expected type: Maybe w
>>>       Actual type: Maybe a
>>>     Relevant bindings include
>>>       b :: a (bound at Graphics/HUI/Widget.hs:14:26)
>>>       a :: Maybe w (bound at Graphics/HUI/Widget.hs:14:16)
>>>       widget :: w (bound at Graphics/HUI/Widget.hs:11:16)
>>>       lookupWin :: WINDOW -> w -> Maybe w
>>>         (bound at Graphics/HUI/Widget.hs:11:1)
>>>     In the first argument of ‘maybe’, namely ‘(lookupWin xwin b)’
>>>     In the expression: maybe (lookupWin xwin b) Just a
>>> ```
>>> --
>>> 竹密岂妨流水过
>>> 山高哪阻野云飞
>>>
>>> And for G+, please use magiclouds#gmail.com.
>>>
>>
>>
>>
>> --
>> 竹密岂妨流水过
>> 山高哪阻野云飞
>>
>> And for G+, please use magiclouds#gmail.com.
>>
>
>
>
> --
> 竹密岂妨流水过
> 山高哪阻野云飞
>
> And for G+, please use magiclouds#gmail.com.
>



-- 
竹密岂妨流水过
山高哪阻野云飞

And for G+, please use magiclouds#gmail.com.
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