ADR 0070 — Derived columns (operator-authored computed group keys)

• Status: proposed
• Date: 2026-06-10
• Spec target: XTL 1.x (additive; backward-compatible, deferred from the 0.1 → 1.0 freeze per STABILITY.md "intentionally deferred")
• Affects: language.md (Group Keys, new "Derived Columns" section); evaluation.md (reserved sheets, render phases); impl (parser, grouper/pre-pass, renderer naming); new reserved sheet + new error codes
• Issue: #54

Context

Operators want output sheet/file names driven by a lookup — e.g. a 거래처/지점 code mapped to a human display name, or one output per entry of an operator-maintained table. Two hard limits block this today:

1. Sheet-name patterns cannot contain [ or ] (language.md "Group Keys"). The pattern lives in the worksheet tab name, and Excel forbids [/] in tab names — so {{ [Col] }} or {{ XLOOKUP([code], Map[code], Map[name]) }} is impossible to even author there. Sheet names accept only bare group keys (sheet group key → __inputs__ → __config__, per ADR-0054).
2. Group keys are column names. grouper.ts partitions by reading row[key] directly; a key must be a real column on the source row. There is no way to group by, or name by, a value that is computed (looked up, concatenated, formatted) rather than stored.

__lists__ does not help: it is a membership array (name → string[]), valid only as the RHS of @filter … in/!in (ADR-0057), and the normalizer rejects it everywhere else (xl3/lists/invalid-use). It is not a key → value map and cannot be referenced in a name.

The current spec-clean workaround is to materialize the display value as a real source column — authored upstream, or via a source_table Excel formula column whose cached result is read as data (ADR-0046, fixtures 014/036) — then group by that column. This works but forces the mapping into the data. The xl3 positioning is operator-authored rules + dev-owned engine: the operator should be able to declare the mapping in the template, not require it baked into every data export.

Decision

Introduce derived columns: named expressions, authored by the operator in the template, evaluated once per source row and materialized onto the row before grouping and rendering. A derived column is indistinguishable from a real column downstream — it can be a group key (hence usable bare in sheet and file names), read in a cell as {{ [Name] }}, etc.

This sidesteps limit (1) entirely: the lookup expression is authored where brackets are legal (a reserved cell), and the tab name references only the bare derived name. It dissolves limit (2): grouping/naming keep reading row[key] — the engine just populated that key first.

Declaration surface (RECOMMENDED — see Alternatives)

A new reserved sheet __derived__, parallel to __lists__/__sources__:

name	expression
RegionName	XLOOKUP([region_code], Regions[code], Regions[name])
Period	TEXT(TODAY(), "YYYY-MM")

• Row 1 = headers name, expression (fixed).
• Each subsequent row declares one derived column.
• The expression cell is an ordinary cell → brackets and function calls are legal (the whole point — it is NOT a tab name).
• Evaluated per source row with the row context + reserved sheets (__sources__, __config__, __inputs__) in scope, exactly like a data-cell expression. XLOOKUP(Source[…]) is therefore the map primitive — no new key→value map structure is introduced; a lookup table is just a declared source (ADR-0013).

Semantics

• Evaluation order: after sources are read and @join applied, before @filter / grouping. A derived column MAY reference real columns of its own row and other reserved sheets; it MUST NOT reference another derived column (no forward/inter-derived refs in 1.x — keeps evaluation a single pass; revisit if needed).
• Materialization: row[name] = eval(expression, rowCtx). The name occupies the same namespace as source columns. A derived name MUST be unique across the whole bare-resolution surface, enforced at declaration time (xl3/derived/name-collision) — see Name collision below.
• Naming and reference surface: no change to the naming or reference layer. A derived name is referenced exactly like a source column — bare in sheet/file-name patterns (extractGroupKeys already picks it up; grouping reads the materialized value) and {{ [Name] }} in cells / directives. There is no __derived__[Name] reference form: once declaration-time uniqueness (below) holds, a bare/[Name] reference is already unambiguous, so a qualifier would add a second spelling with zero correctness gain — and could not be applied in sheet names anyway ([/] forbidden), producing inconsistent spellings for one key. Keeping references identical to source columns preserves this ADR's "indistinguishable from a real column" property (see Alternatives 5).
• Errors: a derived expression that throws surfaces with the derived column's name and the offending row context.

Name collision (declaration-time uniqueness)

(Scope and identity below resolved in #54 review.)

Because a derived name is referenced bare, and sheet-tab names cannot carry a qualifier ([/] forbidden — limit (1) above), a name clash cannot be disambiguated at the reference site. A derived name materialized as a group key sits first in the bare sheet-name chain (group key → __inputs__ → __config__, ADR-0054), so a clash would silently shadow the other binding rather than error. Namespacing the reference therefore cannot be the safety mechanism; the only sound guard is to make the name unique at declaration time, mirroring evaluation.md's "Authors MUST NOT reuse system key names".

A derived name is a declaration error (xl3/derived/name-collision) when it equals any of:

1. a source column in the post-@join combined row schema (wider than a single source's columns — the row a derived expression sees);
2. another derived name (__derived__ internal duplicate);
3. an __inputs__ key or a __config__ key (the rest of the bare sheet-name chain);
4. an engine-reserved context identifier injected into the row/name context — Rows, __rownum, __activeSource__, __joinedRow__. (Rows in particular is spread onto the row context after the row's own keys, so a derived Rows would be silently clobbered, not errored — hence a declaration guard.) Reserved sheet names (__config__, __lists__, …) are already rejected by xl3/sheet/reserved-name.

__sources__ and __lists__ names are not in this set: a source name appears only as the Source[col] prefix (distinct syntax from a bare/ [Name] reference) and a list name only as the @filter … in __lists__[X] RHS (xl3/lists/invalid-use blocks it elsewhere, ADR-0057) — neither lies on any bare-resolution path, so a derived name cannot shadow them.

Identity for the comparison: trimmed, byte-exact (case-sensitive, no NFC folding) — the same identity every resolver in the engine uses for column / group-key / __config__-key lookup (ctx[column], row[key], sheetKey.values[name.trim()]). NFC normalization is applied only to filename↔template matching (match_pattern), not to name resolution, so the guard must not fold case or normalize either, on pain of false collisions or a gap versus the resolver. (Engine-wide case/Unicode folding, if ever wanted, is a separate decision, not this ADR's.)

Scope

• In scope (this ADR, the #54(A) slice): derived columns + their use as group keys / in names / in cells.
• Out of scope (deferred to a follow-up, #54(B)): list/table-driven fan-out — emitting an output per declared entry even when the data has zero matching rows, and ordering outputs by a declared list. That changes the output-generation axis (grouper/renderer emit outputs only for groups that exist in the data today) and warrants its own ADR.

Consequences

• Operators express mapping/derivation in the template, matching the product positioning. Lookups reuse XLOOKUP + a source — one mental model, no new map type.
• Implementation is contained and low-risk: a pre-pass that stamps derived values onto rows; grouping, naming, and cell eval are unchanged because they already operate on row[col].
• Files: parser.ts (read __derived__, validate the declaration-time uniqueness guard over the full bare-resolution surface, collect into the template model), a grouper/render pre-pass (materialize per row), normalizer/extractColumnRefs (derived names resolve as columns — no new reference syntax), types.ts, new error codes, spec text, and stage-1 conformance fixtures (derived-key sheet name, derived-key filename, lookup via source, and name-collision errors covering each bucket: source column, duplicate derived, __inputs__/__config__ key, reserved identifier such as Rows).
• Additive only: no existing template changes behavior. No public API break; new reserved sheet + new error codes (catalog addition, not a G3-resetting change — additions are allowed).

Alternatives considered

1. Declare in __config__ rows (derived.RegionName = …). Rejected: __config__ is flat fixed-key metadata; overloading it with an open-ended name → expression map is muddier than a dedicated sheet.
2. A @derive Name = expr directive in-sheet. Viable, but couples a sheet-level construct to a workbook-global concern (the same derived key is wanted across sheets/files) and complicates per-sheet scoping. A reserved sheet is workbook-global by nature.
3. A new key→value map sheet (__maps__) + a bare-free LOOKUP function. Rejected as redundant: XLOOKUP against a source already is the map; adding a second lookup mechanism + data type is surface we do not need.
4. Allow brackets in sheet-name patterns. Impossible: the constraint is Excel's (tab names cannot contain [/]), not ours.
5. A __derived__[Name] reference form (an explicit qualifier in bracket-legal contexts, for self-documentation). Rejected: with declaration-time uniqueness (Name collision) a bare / [Name] reference is already unambiguous, so the qualifier adds no correctness; it cannot appear in sheet names ([/] forbidden), so a single key would be spelled two different ways across contexts; and a second reference spelling is precisely the surplus surface the small-surface thesis (ADR-0043, README "boring runtime") avoids. Derived columns therefore stay referenced identically to source columns, preserving the "indistinguishable from a real column" property. (Debuggability — telling a derived {{ [X] }} from a source one — is served by the single __derived__ declaration site plus the uniqueness guard, not by a reference marker.)

Open questions (for review before implementation)

• Reserved sheet name: __derived__ vs __columns__ vs __keys__.
• Whether to permit a derived column to reference an aggregate (SUM(...)) — proposed no for 1.x (per-row only), to keep the pre-pass row-local.