Lesson 35 · Undo - Build a Text Editor

Undo

Now the history pays off. Today you pop the last snapshot and restore it, taking the editor back to exactly where it was before the most recent edit.

The goal

Undo the last edit by popping the undo stack and restoring that snapshot, with an empty stack as a no-op.

Start here - the target

TO DO

Scenario: Undoing the last edit

Givenan editor over "abc", cursor moved to Row 0, Col 3, then InsertChar('d') makes "abcd" with the cursor at Col 4

WhenUndo is called

ThenText() is "abc" again and the cursor is back at Row 0, Col 3 (exactly the pre-edit state)

Andcalling Undo again with an empty stack does nothing and leaves the text "abc"

Background

Undo is the reward for all that recording: pop the most recent snapshot off the undo stack and Restore it. Because a snapshot carries both the piece list and the cursor, undo brings back not just the text but the exact position you were at before the edit - the cursor lands where it would have been, which is what makes undo feel like stepping backward through time rather than just reverting text. Typing d onto "abc" and undoing returns both the "abc" and the cursor at column 3.

The edge that must be safe is an empty stack: with no history, undo has nothing to restore and must quietly do nothing rather than pop off the end of an empty slice and crash. Notice how little code this takes - the snapshot and restore machinery did the heavy lifting, so undo is just stack discipline on top. What it cannot do yet is take you forward again after you have undone too far; that is redo, next.

Make it work

func (e *Editor) Undo() {
  if len(e.undo) == 0 { return }        // nothing to undo
  last := e.undo[len(e.undo)-1]
  e.undo = e.undo[:len(e.undo)-1]       // pop
  e.Restore(last)                       // back to the pre-edit state
}

CheckpointDONE

Undo returns the editor to the state before the last edit. Commit and stop here.