How to avoid memory capture violation during addExitProc capture?

[DMisener]

Thanks... your explanation was indeed helpful. It now seems to work :-). The 
_reference casting_ I used to get the closure capture working seems a bit 
awkward. Any suggestions for improvement would be appreciated.
    
    
    import flatty, std/[exitprocs, os, times]
    
    type
      HistoryEntry = string
      History = seq[HistoryEntry]
    
    template reference(thing:untyped): untyped = cast[ref thing.type](addr 
thing)
    
    proc persistence(history: var History, filename: string) =
      proc save(history: ref History, filename: string) =
        writeFile(filename, history[].toFlatty)
      
      if fileExists filename: history = filename.readFile.fromFlatty History
      let historyRef = history.reference
      addExitProc( proc() = save(historyRef, filename) )
    
    proc show(history: History) =
      echo "History:"
      for entry in history: echo entry
    
    # Example usage
    var history: History
    
    history.persistence "history.his"
    show history
    history.add "Another entry: " & $now() # Add another entry on each run
    
    
    Run