12. History, branches and merging

Names are the only mutable thing in a codebase, so history is a log of name changes.

12.1 Reviewing changes: ucm diff

ucm diff reviews changes by meaning: what the last command changed, or ucm diff <id> since a history entry, ucm diff <branch> against another branch, ucm diff <a> <b> between two branches.

• Edits are shown as line diffs, with each side's own names.
• Definitions re-hashed only because a dependency changed are listed separately, with what triggered them.
• Renames and aliases are recognized as such.
• Renaming locals or reformatting doesn't count as a change, because it doesn't change the hash.

12.2 History and undo

• ucm history [n] shows the reflog of name changes on this branch.
• ucm reset <id> undoes back to an entry. Code is never deleted, so this is always safe; in an app it's also how you roll back a deploy (production).

12.3 Branches

ucm branch [name] lists or creates branches; ucm switch <name> changes branch. A branch is a set of names; the definitions themselves are shared.

12.4 Merging

ucm merge <branch> is a three-way merge of names, against the other branch as of the last merge, sync or fork.

• A name changed on one side takes that side.
• A name changed differently on both sides becomes a conflict, listed by ucm todo and settled with ucm resolve <name> <#hash>.
• A definition only re-hashed because a dependency moved doesn't count as a change.
• Each one-sided update is a patch, so the other side's code is re-derived against it. What no longer typechecks goes to todo.

12.5 ucm todo

Conflicts, plus dependents that stopped typechecking after an update or merge. ucm edit them, fix, and ucm update.

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