I'm writing a Gtk2hs app, and I have several custom widgets that are
composite objects represented by records, one field of which is a
container widget. I am trying to write a replacement for gtk2hs's
boxPackStart
boxPackStart :: (BoxClass self, WidgetClass child) => self -> child ->
Packing -> Int -> IO ()
that will accept either a gtk widget or one of my custom widgets to
place in the box, and do the right thing. Here's my attempt at it;
what I want to know is why the commented out bit didn't work and I had
to individually add instances of widgets instead:
---------------------------------------------
-- packable objects
class WidgetClass w => Packable a w | a -> w where
widgetOf :: a -> w
--instance WidgetClass w => Packable w w where
-- widgetOf = id
instance Packable Button Button where
widgetOf = id
instance Packable Entry Entry where
widgetOf = id
instance Packable Label Label where
widgetOf = id
instance Packable Notebook Notebook where
widgetOf = id
instance Packable HBox HBox where
widgetOf = id
-- add widget to box
boxPackS :: (BoxClass b, WidgetClass w, Packable a w) => b -> a ->
Packing -> Int -> IO ()
boxPackS box child p i = boxPackStart box (widgetOf child) p i
---------------------------------------------
If I try to use
instance WidgetClass w => Packable w w where
widgetOf = id
instead, I get the compilation error
Editor.hs:23:10:
Functional dependencies conflict between instance declarations:
instance Packable PairBox VBox -- Defined at Editor.hs:23:10-30
instance WidgetClass w => Packable w w
-- Defined at GuiUtils.hs:13:10-38
even though PairBox does not belong to WidgetClass.
martin
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