ADR 0058 — @subtotal row composition: same-row level binding

• Status: accepted
• Date: 2026-05-22
• Spec target: XTL 0.1
• Affects: language.md § "Group + Subtotal", ADR-0038

Context

ADR-0038 § "Subtotal" pinned the per-row nesting-level inference rule but left two boundary shapes implicit:

1. Multiple @subtotal expressions on the same row. ADR-0038 says "a @subtotal row is a row that contains one or more {{ @subtotal <aggregate> }} expressions." Multiple expressions in different cells of the same row is the common shape (label cell + value cell, or value cells for several aggregated columns). But: are all those expressions at the same nesting level (the level the row binds to)? Or could they bind to different levels via cell position? The ADR is silent on the latter; the reference impl picks "all at the row's level" implicitly.

2. Heterogeneous aggregate kinds on the same row. Can a single row carry {{ @subtotal SUM([Amount]) }} in one cell and {{ @subtotal COUNT() }} in another? The aggregate-supported list in ADR-0038 § "Supported aggregates" permits each individually, but the per-row composition is not pinned.

The reference impl (src/grouper.ts, src/renderer.ts) does the expected thing — every @subtotal expression on a row evaluates at the row's nesting level, against the same group's row set, with the aggregate computed independently per expression. But "the expected thing" is the silent-fallthrough red flag.

Considered Options

A. Pin: all @subtotal cells on a row share the row's nesting level; each aggregate is computed independently against the same row set. Matches reference impl; minimal new surface. Adopted.

B. Allow per-cell explicit level binding via syntax extension (e.g., {{ @subtotal SUM([X]) on [Region] }}). ADR-0038 already flagged this as a future ADR. Defer.

C. Reject multiple @subtotal cells on the same row. Forces one aggregate per row, which would multiply the template's row count. Rejected — not what authors want.

Decision

Adopt A.

Normative rules (added to language.md § "Group + Subtotal")

A @subtotal row MAY contain any number of {{ @subtotal <aggregate> }} expressions, each in its own cell. All of these expressions:

1. Share the row's single nesting-level binding (per ADR-0038 row-order rule). The row binds to one level — innermost for the first @subtotal row in source order, the next outer level for the second, etc.

2. Evaluate against the same group row set for the bound level at each group boundary.

3. May use different aggregate functions and column references. Permitted combinations include:

   Row cells	Bound level
   "Subtotal:" and {{ @subtotal SUM([Amount]) }}	innermost
   {{ @subtotal SUM([Amount]) }}, {{ @subtotal COUNT() }}, {{ @subtotal AVERAGE([Amount]) }}	innermost
   {{ @subtotal SUM([Sales]) }}, {{ @subtotal SUM([Returns]) }}	innermost

4. Are emitted together — when a group boundary fires, every cell on the bound subtotal row renders simultaneously (the row is a row, not a stream of independent cells).

What is still rejected (per ADR-0038)

• A current-row column reference outside an aggregate on a @subtotal row (xl3/expression/unknown-name-class error).
• A @subtotal body that is not one of the five supported aggregates (xl3/subtotal/bad-aggregate).
• More @subtotal rows than @group keys (xl3/subtotal/outside- group).

Explicit per-cell level binding — still deferred

The syntax {{ @subtotal SUM([X]) on [Region] }} remains deferred per ADR-0038. Same-row cells cannot bind to different group levels in XTL 0.1.

Same-row mixed-level subtotals: explicitly unsupported

A @subtotal row binds to exactly one group level (the row's position in the source order determines that level per ADR-0038). An author who places two @subtotal expressions on the same visual row hoping each binds to a different level (e.g., one to [Customer]-level totals next to one to [Region]-level totals) will NOT get that behavior — both expressions bind to the same level (the row's level). The row emits each time the bound level's group boundary fires; the "other" level's aggregate is computed against the SAME-level row set, which produces the same value both times.

This is the documented limitation, not a bug. Authors who need two-level subtotals on the same visual row of the rendered output either:

1. Compose them across two source rows (the standard pattern).
2. Use a future ADR's explicit binding form once it lands.

Implementations MAY emit a warning when a @subtotal row contains multiple aggregates targeting columns that differ from the row's implicit binding level (heuristic detection), but the warning is not normative and detection rules are implementation-defined.

Consequences

• No spec behavior change; ADR-0038's implicit composition is now explicit.
• Reference impl change: none.
• Conformance fixture additions:
  • 161-subtotal-multi-aggregate-same-row — a single inner-level @subtotal row carrying SUM + COUNT + AVERAGE; all three render at the same boundary against the same group set.
  • 162-subtotal-mixed-column-refs — two SUM aggregates over different columns on the same row.
• No new error code.

References

• ADR-0038 — @group + @subtotal directives
• ADR-0015 — Stable error codes
• language.md § "Group + Subtotal"