{- This program displays a list box (initially empty).  The user adds new items   by typing a line into the keyboard, and deletes items by selecting them in   the list box. -}module Main whereimport Control.Concurrentimport Computationimport Eventsimport Focusimport DialogWinimport SimpleListBoximport HTkhandleSelections :: SimpleListBox String -> Event [SimpleListBoxItem String]   -> Event ()handleSelections simpleListBox event =   do      selections <- event      always (mapM         (deleteItem simpleListBox)         selections         )      handleSelections simpleListBox eventhandleAdditions :: SimpleListBox String -> IO ()handleAdditions simpleListBox =   do      nextItem <- getLine      addItemAtEnd simpleListBox nextItem      handleAdditions simpleListBoxmain =    do      main <- initHTk [text "Simple List Box"]      simpleListBox <- newSimpleListBox main id [size (10,15)]      (event,terminator) <- bindSelection simpleListBox      spawnEvent (handleSelections simpleListBox event)      pack simpleListBox []      handleAdditions simpleListBox