ulis, 2002-07-21: the goal is to overwrite the chars of an entry, one by one.

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  # creating the entry widget
  pack [entry .e]
  # focusing the entry widget
  focus -force .e
  # binding to key press
  # the following break avoid the interpretation 
  # by the Entry class associated routine
  # (defined inside entry.tcl)
  bind .e <KeyPress> { overwrite %A; break }
  # overwriting proc
  proc overwrite {ch} \
  {
    if {[string is control $ch]} \
    {
      # for a control char, we need to bypass the break
      # so the standard routine is called to handle the char
      return -code return
    } \
    else \
    {
      # for a standard char
      # ::n is updated if needed
      set i [.e index insert]
      if {$i != $::n} { set ::n $i }
      # overwriting the char
      .e delete $::n
      .e insert $::n $ch
      # selecting the next char
      prepare
    }
  }
  # selecting next proc
  proc prepare {} \
  {
    incr ::n
    .e icursor $::n
    .e selection from $::n
    .e selection to [expr {$::n + 1}]
  }

----
  
  # the test
  
  # initial string
  .e insert 0 "the initial string"
  # selecting the first char
  set ::n -1
  prepare

Now, type ahead!

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