mxcli check accepts `declare list = empty` but Studio Pro rejects it (CE0053/CE0038)

[ako]

Summary

declare $list list of Module.Entity = empty; parses and passes mxcli check, but Studio Pro's mx check rejects it with CE0053 ("type not allowed") and CE0038 ("value required"). In Mendix, declare maps to a Create Variable activity, which is not allowed for list data types (empty lists must use a Create List activity). So mxcli green-lights logic that crashes the project on open / mx check.

This is the concrete instance behind the broader "mxcli check vs mx check divergence" feedback.

Reproduction

create microflow MyFirstModule.TestEmptyList()
begin
  declare $Items list of MyFirstModule.Cars = empty;
end

$ ./bin/mxcli check /tmp/t.mdl
✓ Syntax OK (1 statements)
Check passed!

…then opening the project in Studio Pro → CE0053 + CE0038.

Why it slips through today

• Lint rule MDL002 only flags an empty-list variable when it is used as a LOOP source (mdl/executor/validate_microflow.go ~line 139), not the declare … = empty itself.
• The skills/docs present it as valid syntax with no warning:
  • .claude/skills/mendix/write-microflows.md:137
  • .claude/skills/mendix/check-syntax.md:21
  • .claude/skills/mendix/patterns-data-processing.md:217,261,300
  • CLAUDE.md:469 even claims violations are "caught by mxcli check" (aspirational).

Proposed fix (options)

1. Validation: mxcli check should error (or warn) on list-typed declare … = empty, pointing users to: pass the list as a microflow parameter, populate it immediately with retrieve, or use a Create List construct.
2. Or make it valid: emit a Create List activity for list-typed declare, so the output is accepted by Studio Pro.
3. Docs: update the skills/CLAUDE.md so the accumulation pattern shown is actually Studio-Pro-valid.

Reported via external user feedback (AI-agent MDL generation).

[github-actions]

Issue Triage

Summary:
The issue reports that mxcli check incorrectly accepts a declare … list = empty statement, while Mendix Studio Pro rejects it with errors CE0053/CE0038.

Checklist for bug report completeness

Required info	Present?	Details
mxcli version	❌	Not mentioned
Mendix version	❌	Not mentioned
Scenario (what the user was trying to do)	✅	MDL script with declare $Items list of MyFirstModule.Cars = empty; and running mxcli check
Expected output	✅	mxcli check should error/warn on list‑typed declare … = empty
Experienced output	✅	mxcli check passes; Studio Pro shows CE0053/CE0038
AI bug report (mxcli diag --bundle or session log)	❌	Not provided

Missing information:
Please add the following to help us reproduce and fix the issue:

1. The version of mxcli you used (mxcli --version or commit/build info).
2. The Mendix version of the project you are checking (e.g., 10.6, 9.24).
3. If possible, run mxcli diag --bundle and attach the resulting archive, or at least share the relevant session log.

Once those details are added, we’ll be able to investigate further. Thank you for bringing this to our attention!

---

Automated triage via OpenRouter — workflow source

[ako]

Recurring in customer feedback batch (2026-06, reporter Miwa, severity 4/5): DECLARE $Columns List of ExcelImporter.Column = empty; passes mxcli check but fails mx check with CE0053.

Investigation note at 297e0b0a: mxcli now appears to reject list-typed DECLARE via rule MDL040 (mdl/executor/validate_microflow.go:126-131: "declare creates a list variable, but Mendix does not allow Create Variable to produce a list (CE0053/CE0038)"), with a test in validate_microflow_declare_list_test.go. Worth confirming whether this fully covers the reported case (esp. the bare Module.Entity form, which parses as TypeEnumeration per the buildDataType ambiguity) or whether a gap remains.

[ako]

This is now fully addressed across validation, tests, and docs — closing.

Validation

mxcli check rejects list-typed declare … = empty via rule MDL040 (mdl/executor/validate_microflow.go), e.g.:

✗ declare '$Items' creates a list variable, but Mendix does not allow the Create Variable
  activity to produce a list (CE0053/CE0038). Pass the list as a microflow parameter,
  populate it with retrieve, or use create list. [MDL040]

Verified it fires for every reported form:

Form	Result
declare $Items list of MyFirstModule.Cars = empty; (original repro)	✅ MDL040
declare $Columns List of ExcelImporter.Column = empty; (capitalized, comment)	✅ MDL040
declare $xs list of String = empty; (list of primitive)	✅ MDL040
declare $ys list of M.SomeEntity = empty;	✅ MDL040

Re the open question about the bare Module.Entity form parsing as TypeEnumeration: that's moot here — MDL040 keys off the outer Type.Kind == TypeListOf, not the element type, so the entity/enum ambiguity never reaches it. All forms are caught.

Tests & docs

• Unit test: mdl/executor/validate_microflow_declare_list_test.go
• Negative regression test: mdl-examples/bug-tests/607-declare-list-rejected.fail.mdl (part of make check-mdl)
• Docs corrected to present the pattern as invalid: write-microflows.md (now WRONG examples citing CE0053/CE0038/MDL040), check-syntax.md and patterns-data-processing.md (pattern removed), and CLAUDE.md ("always wrong").

The two suggested directions (validate, or emit a Create List) — we went with validate: fail fast at check with an actionable message pointing to parameter / retrieve / create list, rather than silently generating logic Studio Pro rejects.