From: julio@diegidio.name   
      
   On Sunday, 3 April 2022 at 13:16:14 UTC+2, Malcolm McLean wrote:   
   > To implement a simple undo redo, you need two stacks, and a copyable   
   representation of the program state. You then push to the undo stack, with   
   straight forwards extensions for redo.    
      
   Why two stacks?  I just see the need for one.  And why "program state", this   
   is usually about a specific set of properties along some tree hierarchy so   
   that each property is identified by a unique path... and then, in a basic   
   implementation, at any    
   change you just record the path together with the value before change.   
      
   That said, it's hardly the case that any diffing logic can do better than just   
   recording the old value in the case of "primitive" types: you can do better   
   for composite objects/arrays, by checking what properties have actually   
   changed (and extending a    
   little bit on the notion of changed property path).  Anyway this is a further   
   step...   
      
   Julio   
      
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