Re: [Gtk2hs-users] Subclass a widget.

[Axel S. <Axe...@en...>]

On Apr 14, 2009, at 5:09, Magicloud Magiclouds wrote:

> Hi,
>   I just noticed a mail named "gtk2hs question - derive SettingsWindow
> from Window" in gtk2hs ML. As it said, I wonder, is it possible to
> subclass a widget? If I use newtype as the mail said, is the "newtype
> MyWindow" a widget, so I can use it like "containerAdd myWin"?
>

The use of the Haskell type class system to model the object sub- 
typing relationship is not really meant to be extended using Haskell  
values. In fact, the classes of most widgets contain no methods at  
all such that the compiler can optimize the class contexts away.

Creating a new Haskell type that contains a pointer to a Widget w and  
making it appear as if it were a value of type w is not easily  
possible in this setting. We could add the appropriate methods to the  
type classes but this would incur the overhead to each method that is  
of no use except for this special case. The benefit of being able to  
create a Haskell value that looks like a Gtk object is not that clear  
to me. Furthermore, if it seems important to extend a given Gtk  
object, a lightweight alternative is to simply add new fields to the  
object using 'objectCreateAttribute' in Glib.System.GObject:

type MyWidget = Label

extraData :: Attr MyWidget (Maybe String)
extraData = unsafePerformIO objectCreateAttribute

myWidgetGetData :: MyWidget -> IO String
myWidgetGetData w = do
   mData <- get w extraData
   case mData of
     Just v -> return v
     Nothing -> error "myWidgetGetData: extraData not present in Label"

myWidgetSetData :: MyWidget -> String -> IO ()
myWidgetSetData w s = set w [extraData := s]

This is obviously not quite what a true sub-type can give you since  
the type synonym MyWidget gives you no guarantee that the new field  
always exists (hence the need for the error message). But I think  
this is a good enough compromise between the full version that incurs  
a lot of overhead for all Gtk2Hs programs and the ability to extend  
an object.

Cheers,
Axel.