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