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ADR 0062 — __inputs__ default = "" semantics

  • Status: accepted
  • Date: 2026-05-22
  • Spec target: XTL 0.1
  • Affects: evaluation.md § "Inputs", ADR-0010, ADR-0050

Context​

evaluation.md § "Inputs" describes the default column:

If non-empty, used when the host omits the input. The default value is parsed by the input's type.

Two boundary shapes are not fully pinned:

  1. default cell is empty (blank). The input row has no default value; the input is required. Hosts MUST supply a value. This is implicit in "If non-empty, used" but not spelled out.

  2. default cell holds the literal empty string "". Per ADR-0028, a string literal "" is a valid XTL string. Per ADR-0050, the default cell is evaluated as XTL. The post- evaluation result is the empty string.

    Is the post-evaluation empty string the same as "no default" (required)? Or is it a default of "" (optional, default to empty string for type = text; error for type = number / type = date; selecting nothing for type = select)?

The reference impl (src/inputs.ts) treats both cases identically — "If non-empty after evaluation, used; otherwise required." The ADR-0007 empty-value rule (a string of zero length or only whitespace is empty) governs.

Two-real-world author paths cross this:

  • A required-string input where the host should always supply a value: leave default blank.
  • An optional-string input where blank is meaningful: there is currently NO way to declare "default = empty string, but it is optional." Both shapes (blank cell, "" literal) collapse to "required."

A separate, smaller question: when type = text with default = " " (whitespace-only literal, evaluated as "" per trim semantics), is the input required (empty) or has it a default of "" (literal empty string)?

Considered Options​

A. Unify: post-evaluation empty value (per ADR-0007) means required. Adopted. Matches current impl. Simple rule; no special case.

B. Distinguish blank cell from "" literal. Treat "" as "defaults to empty string, optional." Pro: gives authors an "optional empty string" tool. Con: introduces a non-empty-but- empty distinction; the ADR-0007 rule says they are the same. Spec churn for a marginal case.

C. Reject "" literal in default cells. Force authors to either leave it blank (required) or supply a non-empty default. Pro: removes ambiguity. Con: rejects a valid XTL expression in a specific cell, which is inconsistent with ADR-0050.

Decision​

Adopt A.

Normative rules (added to evaluation.md § "Inputs")​

After ADR-0050 XTL evaluation of the default cell:

  1. If the post-evaluation canonical-string-form value is empty per ADR-0007 (missing, "", or whitespace-only string), the input is required. Hosts MUST supply a value; omitting raises xl3/inputs/missing-required.
  2. Otherwise the post-evaluation string is the default. The default flows through the per-type coercion (number / date / select) per the existing rules.

The shapes "blank cell," default = "", default = " ", and default = {{ "" }} are all equivalent: they all mean "required."

Author intent of "default to empty string, optional" is NOT expressible in XTL 0.1. Authors who need that semantic use:

  • For type = text: a placeholder default (default = " " won't work — whitespace-only is also empty). Use a single non-printing character if absolutely necessary; the spec does not endorse this workaround.
  • For type = select: declare an options value like "|none|…" and treat empty selection upstream — but the spec already pins that select requires the value to match an option.

A future ADR may introduce an explicit "optional" flag (required = false column) if the corpus shows real demand.

type = select empty-string default​

Per ADR-0010, a type = select input with a non-empty default requires the default to match one of the declared options. With this ADR, a "default = empty" means required; the select-value validation still applies once the host supplies a value.

Consequences​

  • No behavior change in the reference impl; this ADR documents and pins the existing rule.
  • The "optional empty-string default" use case is explicitly unsupported. Cookbook 06 will note the limitation.
  • Conformance fixture additions:
    • 172-input-default-empty-literal-required — a row with default = "" and no host-supplied value raises xl3/inputs/missing-required.
    • 173-input-default-whitespace-only-required — default = " " behaves the same.
    • 174-input-default-evaluates-to-empty-required — default = {{ IF(TRUE, "", "fallback") }} post-evaluates to "" and behaves as required.

References​

  • ADR-0007 — Empty value definition (the canonical empty rule)
  • ADR-0010 — __inputs__ schema
  • ADR-0050 — Template inputs as XTL expressions (default cell evaluation)
  • evaluation.md § "Inputs"