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ADR 0047 - ISBLANK as IFEMPTY predicate alias

  • Status: accepted
  • Date: 2026-05-18
  • Spec target: XTL 0.1
  • Affects: spec/language.md function table, src/functions.ts; revisits ADR-0045's rejection of the ISBLANK family

Context​

ADR-0045 rejected the entire ISBLANK / ISNUMBER / ISTEXT / ISDATE / ISERROR family on Excel-native preference grounds (Excel formula =ISBLANK(B2) reaches the cell-output case). The pragmatic review of commit 2d76913 pushed back on ISBLANK specifically: it is the canonical Excel function for blank-fallback patterns, and rejecting it forces every author coming from Excel to learn the xl3-specific IFEMPTY vocabulary before they can write their first conditional.

The friction is asymmetric across the family:

  • ISBLANK — every Excel user reaches for this first. High friction.
  • ISNUMBER, ISTEXT, ISDATE, ISERROR — exotic. Low friction.

This ADR carves ISBLANK out of the ADR-0045 rejection and accepts it as a predicate alias for the existing IFEMPTY empty-check. The other four functions in the family stay rejected.

Considered options​

A. Carve out ISBLANK only as a thin alias for the ADR-0007 empty predicate. Friction goes away; surface gain minimal (one function). Other type-tests stay rejected.

B. Accept the full IS* family. Bigger surface, more porter work, and ISNUMBER/ISTEXT add genuinely new semantics (XTL doesn't currently have a runtime "type of" concept exposed).

C. Keep the rejection and document the workaround. Saves 0 LoC; user-facing pain stays.

Decision​

Adopt A.

ISBLANK(value) specification​

Single-argument function returning a boolean.

  • Returns true if value is empty per ADR-0007 (null, undefined, empty string, or string consisting entirely of Unicode whitespace except for the U+200B / U+FEFF zero-width carve-out).
  • Returns false otherwise. Numbers (including 0) are never blank; booleans (including false) are never blank; dates are never blank; non-empty strings are never blank.

Per ADR-0043: narrowly justified. The function passes the gate via filter-predicate use (@filter ISBLANK([Note]) once filter LHS supports function calls) and via __inputs__ defaulting flows. Cell-output use can still use =ISBLANK(B2) Excel formula — the ADR-0043 retroactive table marks ISBLANK as 🟡 borderline like the rest of the recent batch.

Why ISBLANK is the only IS* accepted​

  • ISBLANK is the Excel-user entry point for conditional logic. An author who learned Excel for ten years will never reach for IFEMPTY first; they will type ISBLANK and expect it to work. This is the largest single-function friction point in the rejected set.
  • The semantic is identical to the existing IFEMPTY empty predicate. No new runtime behavior — ISBLANK(x) is IFEMPTY(x, true) === true ? true : false essentially. Implementation is a one-liner.
  • ISNUMBER, ISTEXT, ISDATE require XTL to surface a "type of" concept it doesn't currently expose (cell values flow as untyped unknowns through the evaluator). Adding them would require decisions about how source-data type detection works that haven't been made yet.
  • ISERROR is even more exotic — the eager-eval model means thrown errors don't propagate to it; only error-cell markers can be caught, and IFERROR already does that.

Implementation​

functions.ts: add

ISBLANK: (v) => isEmpty(v),

(One line. The isEmpty helper is the same ADR-0007 predicate the rest of the codebase uses.)

normalizer.ts FUNCTION_ARITY: add ISBLANK: { min: 1, max: 1 }.

spec/language.md function table: new row.

Consequences​

  • The IS* family stays mostly rejected (per ADR-0045); only ISBLANK is the exception, justified by user-friction asymmetry.
  • The ADR-0043 retroactive table grows by one 🟡 entry.
  • Cookbook 16 gains a sentence: "Excel users reaching for ISBLANK: it works, and it's identical to IFEMPTY for the blank-check half. Prefer IFEMPTY(value, fallback) when you want a fallback; prefer ISBLANK(value) when you want a boolean for a downstream conditional."
  • Porters add one trivial function — no new runtime concept.

References​

  • ADR-0007 — Empty value definition (the predicate ISBLANK reuses)
  • ADR-0043 — Excel-native preference principle (gate this passes narrowly)
  • ADR-0045 — Function batch rejected (the rejection this amends)
  • Cookbook 16 — XTL function vs Excel formula