Undone

A monk asked master Kaimu, “What is the ideal behavior of ‘undo’?”

Kaimu answered, “To undo the last significant action of the user, then the action before that, and so on to the beginning of time.”

The monk asked, “What about the resizing of a window?”

Kaimu answered, “Ideally, that would be undone.”

The monk asked, “The saving of a file?”

Kaimu answered, “Also undone.”

The monk asked, “The sending of email?”

Kaimu answered, “Also undone.”

The monk laughed and said, “Absurd! How could anyone implement such behavior?”

The master launched an application on this desktop. “Here is an editor of my own devising, which implements the Ideal Undo.”

The monk entered a line of text. Then he deleted a word, resized the window, saved the file, and sent it as an email. When the email appeared on his phone, he hit the editor’s Undo key four times.

The monk said, “No effect.”

Kaimu answered, “Null. Time itself was reset to as it was immediately after you entered your line of text. At which point you—lacking all memory of the prior iteration—again deleted the word, resized the window, saved the file, and sent the email.”

The monk pointed triumphantly at the master. “Null to your Null! For in the alleged second iteration I would have subsequently hit Undo four times, just as I did in the first iteration...thus creating an inescapable loop!”

Kaimu folded his arms and with satisfaction answered, “The actions of the second iteration were not significant, since they were fated to occur as a consequence of resetting the clock; thus they are not candidates for the Ideal Undo. Only your four Undo key-presses in the first iteration—the ones which initiated the second iteration—were significant.”

The monk asked, “So what became of these four Undos?”

Kaimu answered, “Thanks to the four Undos in the second iteration... also undone.”