[SOLVED] How to do simple ComboBoxEntry in gtk2hs?

How to do simple ComboBoxEntry in gtk2hs?

I use the code below adapted from a tutorial. It shows the two options in the drop down, but when I select one, it gives the error at the console:

(combo:12158): Gtk-CRITICAL **: IA__gtk_entry_set_text: assertion `text != NULL' failed

import Graphics.UI.Gtk
import Graphics.UI.Gtk.Gdk.EventM
import Graphics.UI.Gtk.Gdk.GC

main = do
  initGUI
  window <- windowNew
  window `onDestroy` mainQuit
  windowSetDefaultSize window 800 600
  windowSetPosition window WinPosCenter

  store <- listStoreNew ["one", "two"]
  combo <- comboBoxEntryNewWithModel store
  ren <- cellRendererTextNew
  cellLayoutPackEnd combo ren False
  cellLayoutSetAttributes combo ren store
    (\txt -> [cellText := txt])
  containerAdd window combo

  widgetShowAll window
  mainGUI

Solution

You're trying to add a renderer and set it's attributes which is all fine. But you're not telling ComboBoxEntry where the text is that should be eventually edited. The Gtk+ developers have not provisioned for extracting this text using call-back functions which is what Gtk2Hs (and you in your example) does. Instead of callbacks, Gtk+ uses column number to refer to a specific datum in a row of data. Most special functions on Models use column numbers since it is much easier to operate with these in C than callback functions are. In Gtk2Hs you can add column numbers on top of any other attribute mapping. I've modified your example to declare a ColumnId constant (which can use any integer number you haven't yet for a model). Gtk2Hs always uses callback functions, so we have to associate the extraction function id with this column number. The third modification is to tell the ComboBoxEntry to use this column number as it's text source. I've kept the part of your code inserts the text renderer but set it's attribute to a constant. Thus, the text form your store and the constant "<-- your choice" will be displayed in each row.

import Graphics.UI.Gtk
import Graphics.UI.Gtk.Gdk.EventM
import Graphics.UI.Gtk.Gdk.GC

textColumn :: ColumnId String String
textColumn = makeColumnIdString 0

main = do
  initGUI
  window <- windowNew
  window `onDestroy` mainQuit
  windowSetDefaultSize window 800 600
  windowSetPosition window WinPosCenter

  store <- listStoreNew ["one", "two"]
  customStoreSetColumn store textColumn id -- set the extraction function
  combo <- comboBoxEntryNewWithModel store
  comboBoxEntrySetTextColumn combo textColumn -- set which column should be used
  ren <- cellRendererTextNew
  cellLayoutPackEnd combo ren False
  cellLayoutSetAttributes combo ren store
    (\txt -> [cellText := "<-- your choice"])
  containerAdd window combo

  widgetShowAll window
  mainGUI