Round-trip editing and preservation laws

The output becomes a bounded input#

The round-trip profile adds a controlled path from generated artifacts back into persistent source knowledge. A human can edit a designated region, the system can recover that region using explicit conventions, and the next build can reinsert the adaptation.

The adjective bounded is essential. The system is not required to infer every possible semantic change from arbitrary edited output. It recognizes an admissible edit language and a set of regions with stable identities.

A precise domain#

Fix a source snapshot $D$ and a finite set $S_D$ of editable region identities. Let $\mathcal{M}_D$ contain canonical maps assigning a byte string to every region in $S_D$. This includes default content where necessary; a delta-only overlay needs a separate normalization that supplies defaults.

Define a partial renderer and extractor:

$$ \operatorname{put}_D:\mathcal{M}_D\rightharpoonup\mathcal{A}_D, \qquad \operatorname{get}_D:\mathcal{A}_D\rightharpoonup\mathcal{M}_D. $$

The admissible artifact domain requires unambiguous region markers, unique region identities, a valid ownership mapping, and content that does not violate the marker grammar. The renderer preserves the identity markers while placing each region's content into its assigned location.

Proposition 7 — extraction after rendering#

Assume each region is emitted exactly once, markers are parseable and unambiguous, content is preserved byte-for-byte within the declared encoding, and no later stage alters a preserved region. Then

$$ \operatorname{get}_D(\operatorname{put}_D(M))=M. $$

Proof. For each $s\in S_D$, unique markers identify precisely the byte interval written from $M(s)$. Extraction returns that interval under the same identity. Equality holds pointwise over the common domain $S_D$. $\square$

If a later stage rewrites tokens inside editorial content, the law must instead be stated over a canonicalized region representation or tested after a documented reverse transformation. Raw-byte preservation cannot be claimed simultaneously with an unspecified rewriting stage.

No-edit stability#

Let $\mu$ reconcile a canonical extracted region map with persistent memory and satisfy $\mu(M,M)=M$. Then

$$ \mu(M,\operatorname{get}_D(\operatorname{put}_D(M)))=M. $$

This is the formal no-edit round-trip law. It concerns authoritative editorial memory, not every timestamp or packaging byte in a build.

Admissible edit conservation#

Let $A'$ differ from $\operatorname{put}_D(M)$ only within admitted region bodies, without changing their identities or violating syntax. With $M'=\operatorname{get}_D(A')$, Proposition 7 gives

$$ \operatorname{get}_D(\operatorname{put}_D(M'))=\operatorname{get}_D(A'). $$

Thus the next rendering preserves those edited bodies. Unmarked changes outside the owned regions are not covered and may be regenerated from source.

What changes when the generator changes?#

When $D$ changes, the region set can change. A removed or renamed region cannot be silently assumed to have a destination. Reconciliation must apply an explicit migration, retain the record as an orphan requiring attention, or reject publication. This is a cross-epoch problem, not a consequence of the fixed-$D$ law above.

Relationship to bidirectional transformations#

The preservation laws are related to lens-style reasoning about updating a view and recovering source information. JCB's marker-and-fingerprint mechanism is a specialized operational realization, not evidence that it implements a general well-behaved lens calculus. The distinction is important because generated output often contains much information that was never editable source. R03

See reconciliation for merge policy and editorial recovery in JCB for the observed implementation.