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ADR 0069 — Per-block directive scoping

  • Status: accepted
  • Date: 2026-05-24
  • Spec target: XTL 0.1
  • Affects: language.md (Directives); impl (parser); 1 new error code

Context​

Through XTL 0.1 up to 0.7.x, transformation directives (@filter, @sort, @top, @source, @join, @group) attach to a sheet's single data block by construction — there's only one block, so attachment is unambiguous. Phase 2 enables multiple @block directives per sheet (ADRs 0067, 0068), making directive attachment a real design question:

  • When a sheet has two @blocks and one @filter, which block does the filter modify?
  • When a sheet has 3 blocks and a @source directive in a cell that overlaps two of the blocks' column ranges, which block binds the source?

Three credible models for the attachment rule:

  • Sheet-level (status quo) — directive applies to all blocks on the sheet. Simple, but breaks the multi-block use case (different @filter per block is a common need).
  • Explicit per-block syntax — extend @block to nest the transformations, e.g., {{ @block A:D filter=[Status]="VIP" sort=[Rev] desc }}. Reduces number of directive cells but reinvents the existing directive grammar inside @block.
  • Proximity-based per-block scoping (this ADR) — directives attach to the closest @block they apply to, by row position and column overlap.

Considered Options​

A. Proximity-based per-block scoping (this ADR). A directive cell D at position (r_D, c_D) attaches to the data block B on the sheet such that:

  1. r_D < B.startRow (directive is above the block); AND
  2. The column range [c_D..c_D] of the directive cell overlaps with [B.colStart..B.colEnd]; AND
  3. Among all blocks satisfying (1) and (2), B.startRow - r_D is minimized (the closest block below the directive in column- overlap zone wins).

If no block satisfies (1) and (2), the directive is orphan and parsing raises xl3/directive/orphan with the cell location.

B. Sheet-level (no scoping change). Directives apply to all blocks. Simple but doesn't support per-block filtering — the primary motivation for multi-block.

C. Explicit binding via @block arguments. Fold the transformation directives into the @block declaration: {{ @block A:D source=Customers filter=[Status]="VIP" }}. Reduces cell count but conflicts with ADR-0067's "small @block grammar" decision and duplicates existing directive syntax.

Option A chosen. Proximity scoping uses the same positional intuition Excel authors already apply when reading a template (a @filter cell sitting above and aligned with a block's columns is "that block's filter"). It requires zero new syntax, re-uses existing directive cells, and degrades to the sheet- level behavior naturally when there's only one block on the sheet (every directive trivially attaches to that block).

Decision​

The following becomes normative spec text in language.md's "Directives" section (extending existing prose):

Directive scoping (multi-block sheets). Each transformation directive (@filter, @sort, @top, @source, @join, @group, @repeat) attaches to exactly one data block on the sheet, determined by the following rule:

Let D be the directive's cell at position (r_D, c_D). The block B it attaches to is the one such that:

  1. The directive's row is strictly above the block's first row: r_D < B.startRow.
  2. The directive's column lies within the block's column range: B.colStart <= c_D <= B.colEnd.
  3. Among blocks satisfying (1) and (2), B.startRow - r_D is minimized.

If no block satisfies conditions (1) and (2), the directive is orphan and the parser raises xl3/directive/orphan at parse time with the cell location.

On single-block sheets (one block detected per ADR-0066/0068), the rule degenerates: every directive cell positioned above the block's first row and within the block's column range attaches to that one block. Directives outside the block's column range on single-block sheets ALSO raise xl3/directive/orphan — consistent with multi-block — even though there is only one block to attach to.

@subtotal is an exception: it sits inside a block (its row position determines which block contains it, ADR-0058 row binding rules apply within that block).

Worked example​

row 1: {{ @sort [Revenue] desc }}      {{ @source Vendors }}
row 2: {{ @filter [Status]="VIP" }}                                   {{ @top 5 }}
row 3: {{ @block A:D }}                {{ @block E:H }}
row 4: {{ [Account] }} {{ [Name] }} {{ [Rev] }} {{ [Status] }}  {{ [Sku] }} {{ [Price] }} {{ [Stock] }} {{ [Vendor] }}
  • @sort [Revenue] desc at A1 → col A is in block 1's col-range [A..D] AND r=1 < block 1 startRow=3 → attaches to block 1.
  • @source Vendors at E1 → col E in block 2's [E..H] AND r=1 < block 2 startRow=3 → attaches to block 2.
  • @filter [Status]="VIP" at A2 → block 1.
  • @top 5 at H2 → block 2.

Each block ends up with its own filter / sort / source / top without any explicit binding markup.

Single-block sheet behavior​

A directive cell at (r_D, c_D) on a sheet with only one block still requires c_D to lie within the block's column range to attach. If a single-block sheet has a @filter cell positioned to the right of the block's last column, the directive raises xl3/directive/orphan — same as multi-block. The benefit: authors writing single-block templates today get a clear error if they accidentally place a directive outside the block's column scope, instead of the previous "directive silently applies sheet-wide" behavior.

(This is a narrow behavior change: pre-0.8 templates with a directive outside the single block's col range previously had that directive applied; now it errors. In practice no real-world template was observed using this pattern intentionally — the out-of-scope placement was always an authoring mistake.)

Consequences​

One new error code — xl3/directive/orphan, raised at parse time when a directive cannot attach to any block on the sheet. The error message names the directive name and cell location. G3 clock reset compounds with ADRs 0067/0068.

Narrow behavior change on single-block sheets. A directive placed outside the block's column range previously applied sheet-wide; now it errors. Migration: move the directive into a column within the block's range (almost always what the author intended). Documented in 0.7.x → 0.8.0 migration notes.

Sheet-level directives effectively don't exist anymore. With proximity-based attachment, every directive belongs to exactly one block (or is orphan). There is no "applies to all blocks" semantics — authors who want the same filter on multiple blocks must declare the filter directive cell once per block (a small but justified cost of explicit scoping).

Directive order on a shared row is irrelevant. When multiple directives sit on the same row (e.g., {{ @sort ... }} at A1 and {{ @source ... }} at E1), each attaches independently per the rule. Evaluation order across directive types is unchanged (ADR-0029).

Conformance fixture coverage:

  • 153-directive-orphan-error — directive cell outside any block's column range raises xl3/directive/orphan.
  • 154-multi-block-per-block-filter — two blocks with different @filter directives, each block's output reflects only its own filter.
  • 155-multi-block-row-function-scope — ROW() returns the iteration index of the cell's containing block (carry-over from ADR-0066 single-block rule, now exercised in multi-block context).

References​

  • ADR-0029 — directive composition and evaluation order (unchanged by this ADR; ordering across directive types is the same).
  • ADR-0066 — column-scoped data block; provides the colStart / colEnd used by the column-overlap check.
  • ADR-0067 — @block directive grammar.
  • ADR-0068 — multi-block detection (strict mode); companion to this ADR.