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ADR 0065 — @source default explicit reference

  • Status: accepted
  • Date: 2026-05-22
  • Spec target: XTL 0.1
  • Affects: evaluation.md § "External Data Sources", language.md § "Source"

Context​

evaluation.md § "External Data Sources" introduces the implicit default source:

The implicit default source — declared via source_sheet and source_table rows in __config__ — is always named default. It cannot be redeclared in __sources__.

And language.md § "Source":

Without @source, the active source is the default source_sheet configured in __config__.

Both spec the implicit case ("no @source → default"). Neither states whether the explicit form @source default is legal.

The reference impl handles @source default correctly — it resolves to the implicit source. But a porter reading the spec without testing could reasonably:

  • Reject @source default because "default" is a reserved name (the spec says it cannot be declared in __sources__; conflating "cannot declare" with "cannot reference").
  • Accept it (current impl).
  • Treat it as a no-op (the same as omitting @source).

Without a normative sentence, two ports could diverge.

A second, smaller question: case sensitivity. @source DEFAULT vs @source default vs @source Default. The directive-name parsing is case-insensitive per ADR-0029, but the source-name argument is the actual source identifier. Reference impl: source-name match is case-sensitive. So @source DEFAULT errors with xl3/source/undeclared. Not pinned.

Considered Options​

A. Accept @source default as a synonym for the implicit case. Adopted. Matches current impl; gives authors an explicit form when they want to be unambiguous about the source.

B. Reject @source default as a parse error. Pro: prevents authors from typing it when they don't need to (the spec already says "rarely written explicitly"). Con: too strict; valid code in a port that accepts it would error in another.

C. Treat as a warning ("you don't need this"). Half-measure. Rejected.

Decision​

Adopt A.

Normative rules (added to evaluation.md § "External Data Sources")​

The explicit form @source default is legal. It is equivalent to omitting the @source directive: both make the active source the implicit default source declared via source_sheet / source_table in __config__.

Authors MAY use @source default for clarity (e.g., inside a multi-source template where most blocks have explicit @source directives and the author wants the default-source blocks to also read explicitly).

Source-name case sensitivity​

Source-name arguments to directives (@source, @join) and inside Source[Column] references are case-sensitive. The implicit source is named default (lowercase) per ADR-0012; @source DEFAULT raises xl3/source/undeclared.

This matches the spec's general posture (column names are case- sensitive per evaluation.md § "Source Data Model"; the __config__ key match is case-insensitive but column / source identifiers are case-sensitive).

Author convention SHOULD use the same case as the declared __sources__ row's name. The reference impl does not warn on case mismatch — the failure mode is the xl3/source/undeclared error.

Why not reject the explicit form​

The spec already accepts @source SomeSource for any declared source. Rejecting default would create an asymmetry — "this directive accepts source names, except the one you most commonly mean." The explicit form costs nothing to support and makes the template more readable in multi-source contexts.

Consequences​

  • @source default is now spec-pinned as legal.
  • @source DEFAULT (or any non-default casing) raises xl3/source/undeclared. This was current impl behavior; spec now says so.
  • Conformance fixture additions:
    • 186-source-default-explicit-valid — @source default block behaves identically to a block with no @source.
    • 187-source-default-case-mismatch-error — @source DEFAULT raises xl3/source/undeclared.

References​

  • ADR-0012 — Multi-source data model (default source origin)
  • ADR-0014 — @join (source-name reference precedent)
  • ADR-0029 — Directive composition (function name case- insensitivity contrasted with source-name case sensitivity)
  • evaluation.md § "External Data Sources"
  • language.md § "Source"