I don't know how other similar languages deal with GUI. It's always sort of hairy, obscure code when you look at it closely. Mainstream languages (like Java) usually give you GUI building tools which generate all the boilerplate stuff, but if you look at that generated code, it doesn't look so appealing either imho.

If you know the GUI libraries inside out, then the code becomes much easier to grasp though. The lablgtk library (which provides gtk support) is quite well written, providing both a modular interface and objects, but indeed there's a learning curve.

Lastly, you might find it more palatable with a little bit of formatting:

  let menu = GMenu.menu () in 
  let explain = GMenu.menu_item ~packing:menu#add ~label:"Explain this decision" () in
  let callback () = show_explanation impl in
  explain#connect#activate ~callback |> ignore; 
  menu#popup ~button:(B.button bev) ~time:(B.time bev)