Application: Undo/Redo

Text editors use persistent structures for undo/redo:

class Editor:
    versions: list of document states
    current: int // current version index

    function edit(operation)
        // Discard future versions (can't redo after new edit)
        versions := versions[0..current + 1]
        newDoc := apply(operation, versions[current])
        versions.append(newDoc)
        current := current + 1

    function undo()
        if current > 0 then
            current := current - 1

    function redo()
        if current < len(versions) - 1 then
            current := current + 1

With persistent structures, "discard future versions" is optional. You can keep all branches.