[Gtk2hs-users] changes to implementation of object hierarchy

[Duncan C. <dun...@wo...>]

All,

I've made the change to the TypeGenerator program so people who track
the cvs version should probably cvs update, rebuild and make sure I've
not broken anything. You'll need to 
$ touch gtk2hs/tools/hierarchyGen/hierarchy.list
(or make clean) as the gtk directory only depends on hierarchy.list, not
the TypeGenerator program as well (this is perhaps something we should
fix).

The only compatibility problem that I can foresee is if in any programs
you've written
import Gtk (Button(..))
and expect to get fromButton & toButton. Since these are not class
members anymore you'd have to change that to
import Gtk (Button, fromButton, toButton)

I've not removed the from<type> functions. If we are agreed that it
would be a good idea to do so then I'll do that.

As an alternative, we could keep the functions but make them typesafe so
that they can be safely exported to the user.

A quick recap; the from<type> functions (eg fromButton) perform
downcasts in the object hierarchy.
fromButton :: ButtonClass obj => Button -> obj

This is not a typesafe operation. A solution as in OO languages is to
perform a dynamic type check to make sure that the cast is permitted.
That is what the castTo<type> functions do, though they have a different
type signature:
castToButton :: GObjectClass obj => obj -> Button

The second form casts from any class to a Button while the first form
casts a Button to any derived class.

We cannot implement the first form with the current object hierarchy
infrastructure because we cannot get a representation of the type for an
arbitrary object type. If we want to be able to safely perform the first
kind of cast we can add a method to the GObject class that gets us the
glib type representation for a given object type. Instances for all
derived classes can be generated by the TypeGenerator program.

It would look like so:

class GObjectClass o where
  toGObject   :: o -> GObject
  fromGObject :: GObject -> o
  getGType    :: o -> GType

instance GObjectClass Button where
  toGObject   = ...
  fromGObject = ...
  getGType    = {#call fun unsafe gtk_button_get_type#}

Duncan