Immutable data structures are great for undo-redo. Sean Parent has a conference video out there about how undo in Photoshop is implemented by breaking the imag

Immutable data structures are great for undo-redo. Sean Parent has a conference video out there about how undo in Photoshop is implemented by breaking the image into tiles then making a list of the history of each tile. This allows the “History Brush” to reach into the past and paint old pixels over new ones.