Archive for junho \27\UTC 2008

Rank 2 Types

27/6/2008 (sexta-feira)

I’ve always seen the forall a types in some GHC messages or in other person’s code, but I couldn’t get the point of it until I needed to use it myself. I was writing a Gtk2hs application with some windows, and I noticed that for each created window, I was doing the same steps. So I created a function that does what is needed for each function:


basic :: WidgetClass widget => String -> IO (Widget, (GObject -> Widget) -> String -> IO widget)

basic gladeFile =
  do
    (windowGlade :: GladeXML) <- getGlade gladeFile
    let
      windowGet :: Get
      windowGet = xmlGetWidget windowGlade
    (window :: Widget) <- windowGet castToWidget "window"
    (close  :: Button) <- windowGet castToButton "close"
    onClicked close $ widgetDestroy window
    modifyIORef windows (window :)
    return (window, windowGet)

Notice that this code uses GHC extension PatternSignatures, which I like a lot.

The problem was that the returned function, windowGet was not generalized enough, so I couldn’t use it with more than one type, even it being very general:

windowGet :: WidgetClass widget => (GObject -> widget) -> String -> IO widget

If I used it with, say, windowGet castToButton "ok" and WindowGet castToSpinButton "value", it would give, in the seconde line, the type error: Couldn’t match expected type `Button’ against inferred type `SpinButton’.

After asking in #haskell, and reading a little bit of the GHC User’s Guide, I got the point. This was only possible with Rank 2 Types. windowGet must be:

windowGet :: forall widget. WidgetClass widget => (GObject -> widget) -> String -> IO widget

So I changed the type signature for basic, and added Rank2Types to the LANGUAGE pragma, and it worked fine.

basic :: String -> IO (Widget, forall widget. WidgetClass widget => (GObject -> Widget) -> String -> IO widget)

Glade message dialog image …

27/6/2008 (sexta-feira)

In a message dialog, when I click in the … of the Image field, I get a tree of Objects. I don’t understand hat this means, and I don’t know where to get documentation from it. This is Glade 3.4.5-3 in Debian lenny.

Glade-3 comboBox Active Item

27/6/2008 (sexta-feira)

I don’t know if this is a known issue, but it seems strange to me that when I change the Active item value in the proprierties of a Combo Box in Glade 3, from -1 to any value, like 0 or 1, I have this message as soon as the glade file is loaded:

(test:5012): Gtk-CRITICAL **: gtk_tree_row_reference_new: assertion `GTK_IS_TREE_MODEL (model)' failed
(test:5012): Gtk-CRITICAL **: gtk_cell_view_set_displayed_row: assertion `GTK_IS_TREE_MODEL (cell_view->priv->model)' failed

And besides it, the Active Item keeps empty, and is not the first or second on the item list.

I’m using Glade-3 3.4.5 with libglade 2.6.2 in debian lenny.

Glade 3 with Message Dialog

26/6/2008 (quinta-feira)

When a Message Dialog is created in Glade 3.4.5-3 (Debian sid and lenny), saved and re-opened, it crashes. More details: Bug in debian .

libwnck package for sid

20/6/2008 (sexta-feira)

I’ve done a package for the correction of the libwnck bug to sid. It’s available at Debian Mentors and at my Home Page.

chroot locale

20/6/2008 (sexta-feira)

I’ve done a sid chroot in my lenny system, and to get UTF-8 working in it, I just had to install locales package and reconfigure it with dpkg-reconfigure.

libwnck

16/6/2008 (segunda-feira)

When XMonad was used with GNOME, through EwmhDesktops and the last window was destroyed, it would not be deleted from the window list and from the workspace switcher. This bug report explains the situation. To correct it, I’ve made a patch to libwnck’s screen.c, which was applied in the SVN version of this file. I’ve done a package for Ubuntu Hardy Heron and other for Debian Etch with a corrected version of this library.