Undo and restore

Status

Variants

Freeze or copy a snapshot

Keep a static copy of a sheet or column, with computed cells evaluated to plain values.

Undo or redo a change

Step back through recent changes, or forward again.

What other tools call it

Meridian's classification; an independent blind rating agreed on 86.2% of 312 rows (exact class, version 2).

Background

When to use it

Some actions are not stored in the undo list and so cannot be undone: saving a file is not undoable, and scrolling or selection is usually not stored.^[1] The number of previous actions that can be undone varies by program, version, and hardware or software capabilities.^[1] Making a new edit usually clears the redo list.^[1] In OpenRefine, if you move back one or more states and then perform a new operation, the later actions are erased and cannot be re-applied.^[2] Before trying a new step from an earlier state, make sure you will not want the later steps again, or keep a copy of them first. (Meridian's view)

In VisiData, U undoes the most recent modification and R redoes the most recent undo, both requiring options.undo to be enabled.^[3] In VisiData, undoing a search does not clear the highlighting of its matches; the highlight-clear command does that.^[3] In VisiData, commands whose names begin with commit, open-file or reload-sheet are not undoable.^[4]

In VisiData, the rows of a duplicated sheet, other than a deep copy, are references to rows on the source sheet, so edits to them are reflected in the original rows.^[3] In VisiData, g' opens a frozen copy of the current sheet with all visible columns evaluated, which freezes the sheet's contents in a deliberate copy.^[3] When a copy is meant to keep the data as it stood, make it a deliberate frozen copy rather than a duplicate whose rows are shared with the source. (Meridian's view)

In VisiData, the commands of a session are kept in a CommandLog, or cmdlog, which can be saved and replayed on the command line with vd -p.^[5][3] A VisiData cmdlog records the exact filename and sheet names from the original session, so reusing it on a different input file means making it filename-independent first.^[5] The change history of each OpenRefine project is saved with the project's data, so quitting OpenRefine does not erase the steps taken.^[2]

The method

Undo can be implemented through different patterns, the most common being the command pattern and the memento pattern.^[1] The command pattern is a software design pattern that stores every action in a command object, with an execute operation and, for undo, an unexecute operation that undoes its effect.^[1] In the memento pattern, the internal state of an object is stored in a memento, and the undo mechanism keeps the mementos safe but never changes their contents.^[1]

Linear undo is implemented with a stack, a last in first out data structure storing a history of all executed commands, so only the last executed command can be undone.^[1] Non-linear undo lets the user undo executed commands in an arbitrary order, choosing a command from the list rather than only the most recent one.^[1]

A full checkpoint saves the complete state after each command, which is the easiest implementation but is not highly efficient and so not often used.^[1] A complete rerun saves the initial state and reaches any state in the history by starting from it and redoing all commands from the beginning.^[1] A partial checkpoint, the most used strategy, saves the changed part of the application state and sets that part back on undo.^[1]

OpenRefine shows a project's history as a list of changes in order, the first being the creation of the project itself, which cannot be undone.^[2] In OpenRefine, clicking an earlier action in the history undoes the changes after it, and clicking a greyed-out change redoes everything up to and including it.^[2] OpenRefine can extract a project's operations up to the current state as JSON and apply them to another project, though edits to a single cell cannot be extracted.^[2] In VisiData, redo replays the most recently undone command from the sheet's list of undone commands.^[4]

The File Retrieval and Editing System, developed starting in 1968 at Brown University, is reported to be the first computer-based system to have had an undo feature.^[1] Multi-level undo commands were introduced in the 1980s, allowing users to take back a series of actions, not just the most recent one.^[1]

Compared with the alternatives

In database technologies, a rollback is an operation which returns the database to some previous state.^[6] In Structured Query Language (SQL), ROLLBACK discards all data changes since the last START TRANSACTION or BEGIN, so the data is as it was before those changes were made.^[6] Reach for undo when exploring a table step by step, and for a transaction when a batch of changes must land whole or not at all. (Meridian's view)

Sources

1. [1]Undo - Wikipediasecondary source
2. [2]Running OpenRefine | OpenRefinedocumentation
3. [3]Quick Reference Guide | Docs | VisiDatadocumentation
4. [4]visidata/visidata/undo.py at 7609212ec2febafa9caba7183cbebfaf49627909 · saulpw/visidatadocumentation
5. [5]Save and Restore | Docs | VisiDatadocumentation
6. [6]Rollback (data management) - Wikipediasecondary source