[Haskell-cafe] Deduce issue.

[Magicloud Magiclouds]

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.
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