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@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.