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ADR 0046 - Cell formula preservation contract

  • Status: accepted
  • Date: 2026-05-18
  • Spec target: XTL 0.1
  • Affects: ADR-0036 (preservation matrix), ADR-0040 (range PE), ADR-0043 (the Excel-native preference principle relies on this ADR's contract being normative)

Context​

ADR-0043 (Excel-native preference principle) advises authors to put computations in cell formulas when the value only matters at workbook-open time. The principle relies on the reference impl's behavior of preserving template cell formulas through render. That behavior was never normatively pinned — it was implicit in ADR-0036's "preserve verbatim" matrix but the matrix didn't single out formulas, and no conformance fixture exercises them.

A porter reading ADR-0043 needs a precise contract: what survives, what gets adjusted, what gets lost. Without this ADR, the Excel-native preference principle is asking porters to honor an unspecified behavior.

What the contract is (in OOXML terms)​

Two observations, stated in OOXML element terms so any port matches the contract regardless of which XLSX library it uses:

  1. The <f> element text is preserved verbatim across @repeat row expansion. A template cell with <f>B2*2</f> is cloned into every row produced by @repeat; the <f> text is copied byte-for-byte.

  2. References inside the <f> element text are NOT adjusted. When the @repeat block expands from 1 template row to N rendered rows, each cloned cell's <f> text keeps the original reference. A formula <f>B2*2</f> on the template row 2 stays <f>B2*2</f> on rendered rows 2, 3, 4, … — every row references row 2's B, not its own row's B.

    Similarly, a footer formula <f>SUM(B2:B5)</f> placed below a @repeat block that expands from 4 rows to 10 rows stays <f>SUM(B2:B5)</f> — the range is not extended.

How specific impls satisfy the OOXML contract​

The contract is OOXML-element-level. Implementations may use any library, but the I/O round-trip must produce equivalent <f> / <v> element text. Two example mappings:

  • ExcelJS (xl3-js): template cells with shape { formula: "B2*2" } round-trip to <f>B2*2</f> in OOXML output. Cached results, if present in source { formula, result }, emit a <v> element on the template row; cloned rows omit <v> (Excel re-evaluates at open time).
  • openpyxl (xl3-py): template cells with shape cell.value = "=B2*2" plus cell.data_type = 'f' round-trip identically. Cloned cells set cell.value = "=B2*2" and do not set a cached result attribute.

Implementations using library-specific shapes (ExcelJS objects, openpyxl attributes, OOXML element types in Rust crates, …) MUST translate to/from the OOXML element form at their I/O boundary. The normative form is the OOXML XML; library shapes are convenience.

Decision​

What is normative (preserved verbatim) — MUST​

Per ADR-0036 plus this ADR's amendment, stated in OOXML element terms:

  1. <f> element text is preserved through convert(), byte-for-byte. A formula cell in the template appears with the same <f> text in every rendered row that the cell participates in (the template row itself when no @repeat touches it; every cloned row when @repeat does).

  2. <v> element (cached result) is preserved on the template row only. Cloned rows MAY omit <v> (Excel re-evaluates at open time). Whether cloned rows omit or copy <v> is implementation-defined; impls MUST NOT compute new <v> values for cloned rows from the source data — that would silently diverge from the formula's intent.

  3. Cross-sheet <f> references (e.g., <f>Data!B2</f>) are preserved verbatim, including the sheet-name quoting form the author wrote.

  4. Defined-name references inside <f> text are preserved verbatim. Whether the defined name still resolves correctly depends on whether the defined name was preserved (ADR-0036 matrix item 4: yes, verbatim).

  5. Structured-table references (e.g., <f>Table1[Amount]</f>) are preserved verbatim. Whether the underlying table is preserved is governed by ADR-0036's pivot/ListObject row (still undefined as of 2026-05-18; see G12 in ROADMAP).

What is intentionally NOT yet adjusted — MUST NOT silently change​

Per the principle in this ADR: implementations MUST NOT silently rewrite formula references. Failed silently-correct behavior is worse than honestly-unchanged behavior because the author can see the unchanged formula and decide to use whole-column references or rewrite.

  1. Relative cell references inside formulas are NOT adjusted to match the cloned row's position. A template formula =B2*2 on a @repeat down row stays =B2*2 on every cloned row. Authors who want per-row arithmetic SHOULD either:

    • Use whole-column or absolute references that remain correct regardless of position (=$B$2*2 is still wrong; there is no good single-formula solution in this case).
    • Use an XTL expression ({{ [Column] * 2 }}) which evaluates per-row at render time and produces a value, not a formula.
    • Author the template such that the formula references its own row symbolically — currently not possible without impl-side reference adjustment.

  2. Range references in formulas are NOT extended when @repeat expands the block. A footer =SUM(B2:B5) placed below a 4-row template block that expands to 10 rows remains =SUM(B2:B5). Authors who want a sum across the expanded block SHOULD use whole-column references (=SUM(B:B) with a @filter upstream removing the header) or use an XTL aggregate ({{ SUM([Column]) }}).

Why no auto-adjustment yet​

Auto-adjusting formula references is non-trivial:

  • Excel's formula grammar is rich (relative B2, absolute $B$2, mixed $B2 / B$2, range B2:B5, structured table refs Table1[Amount], cross-sheet 'Sheet 1'!B2, defined names, array formulas).
  • The right "shift" depends on whether the formula is on a cloned row (per-row context) vs in a footer cell after the block (range-extension context).
  • Different implementations of XLSX writers have different formula AST representations; xl3-py (openpyxl) and xl3-js (ExcelJS) would need separate AST handling.

The principled response: document current behavior as the 0.x contract; a follow-up ADR (placeholder ADR-0047) defines the preserve-and-extend variant for formulas, parallel to ADR-0040 for CF/DV ranges.

How this relates to ADR-0043​

ADR-0043's "use the Excel-formula path for cell-output values" advice is sound for formulas that do not depend on per-row reference adjustment:

  • ✅ =UPPER("acme") (literal) — works.
  • ✅ =NOW() / =TODAY() — works.
  • ✅ =A1+1 referencing a fixed cell — works.
  • ✅ =HYPERLINK("https://...", "label") (static) — works.
  • ⚠️ =B2*2 referencing the row's own column — works on the first rendered row only; remaining cloned rows reference the wrong row.
  • ⚠️ =SUMIF(B:B, "VIP", C:C) whole-column — works (no row dependence).
  • ❌ =SUM(B2:B5) where 2-5 was the original template span — the range does not extend.

Cookbook 16 will be updated (next commit) to use ✅ / ⚠️ / ❌ markers per case so authors know which Excel-formula path is safe today.

Conformance fixture​

Fixture 129 (129-cell-formula-preservation) pins both the present-tense MUSTs (text preservation, cached-result on template row) and the present-tense MUST-NOTs (no silent reference adjustment). Two cell cases:

  • C2: { formula: "B2*2" } template — every rendered row carries the same B2*2.
  • B5 (post-block footer): { formula: "SUM(B2:B2)" } template — shifted to its post-expansion row position, formula text unchanged.

Consequences​

  • Porters have a precise contract: preserve text, do not adjust.
  • ADR-0043's advice surface narrows: cookbook 16 (this commit's follow-up) explicitly flags which formula shapes work.
  • Future ADR-0047 (placeholder) covers formula PE (preserve-and- extend). It pairs with ADR-0040 for CF/DV ranges — the same row-delta tracking machinery applies.

References​

  • ADR-0036 — Template feature preservation matrix (which this ADR sharpens for the cell-formula row)
  • ADR-0040 — Preservation extension (CF/DV range PE; this ADR defers the formula PE counterpart to ADR-0047)
  • ADR-0043 — Excel-native preference principle (which relies on this ADR's contract)
  • evaluation.md § "Styles and Workbook Structure" — adds formula-preservation row to the MUST list