feat: support ellipsis placeholder expression

[hjotha]

Summary

Adds ... as a microflow expression placeholder for intentionally unbound code-action parameters. This lets describe output preserve empty BasicCodeActionParameterValue bindings and lets exec write them back without inventing synthetic expression values.

Part of #332.

Changes

• Adds ELLIPSIS to the MDL grammar and preserves it as a source expression.
• Maps Java action arguments written as ... back to empty basic code-action parameter values.
• Documents the syntax in the quick reference, proposal index, and Mendix microflow skill.
• Adds a doctype example and parser/builder regression coverage.

Validation

• make build
• make lint-go
• make test
• ./bin/mxcli check mdl-examples/doctype-tests/ellipsis_placeholder.test.mdl

[github-actions]

AI Code Review

Critical Issues

None found.

Moderate Issues

None found.

Minor Issues

• Visitor logic not explicitly shown: While the implementation appears correct (tests pass and documentation states the visitor produces ast.SourceExpr{Source: "..."} for ELLIPSIS), the diff doesn't show explicit visitor changes. Given the truncation note, this may be in the omitted sections, but it would be good to confirm the visitor correctly bridges the ELLIPSIS token to the SourceExpr AST node.

What Looks Good

• Full-stack implementation: The feature is properly implemented through the entire pipeline:

  • Grammar: Added ELLIPSIS token and rule
  • AST: Added SourceExpr to preserve source text
  • Executor: Handles placeholder detection and conversion to empty BasicCodeActionParameterValue
  • Tests: Includes unit test verifying BSON preservation and doctype example
  • Documentation: Updated quick reference, skills doc, and added detailed proposal

• Correct semantics: The implementation correctly preserves empty Argument values in BSON for round-trip compatibility, addressing the core motivation of maintaining symmetry in describe → exec → describe cycles.

• Clear documentation: The proposal and accompanying documentation clearly explain that ... is round-trip-only and should not be used in newly authored scripts, preventing misuse.

• Robust handling: The implementation correctly handles whitespace variations (e.g., " ... ") through trimming in the placeholder detection function.

• Test coverage: Includes both unit test for the builder logic and a doctype test for end-to-end validation.

• Minimal scope: Focused solely on adding the ellipsis placeholder for Java action parameters without attempting to over-extend to other contexts prematurely.

Recommendation

Approve. The PR thoroughly implements the ellipsis placeholder feature following all project guidelines, with proper documentation, test coverage, and full-stack implementation. The minor concern about visitor logic is likely addressed in the generated or omitted code sections given the passing tests and explicit documentation of the AST behavior.

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Automated review via OpenRouter (Nemotron Super 120B) — workflow source

[github-actions]

AI Code Review

Critical Issues

None found.

Moderate Issues

None found.

Minor Issues

• LSP wiring not verified: While the PR mentions that LSP wiring should be checked if the feature adds formatting, diagnostics, or navigation targets, this expression placeholder doesn't appear to require LSP changes. However, it would be good to confirm that the existing LSP implementation correctly handles the new SourceExpr AST node for features like hover/tooltips.

• VS Code extension not verified: Similar to LSP, since this doesn't add new LSP capabilities, VS Code extension updates may not be needed, but it's worth confirming the extension correctly tokenizes and highlights the new ... syntax.

What Looks Good

1. Complete full-stack implementation: The PR properly implements the feature through all required layers:

   • Grammar (MDLLexer.g4, MDLParser.g4)
   • AST (ast_expression.go with SourceExpr type)
   • Executor handling (cmd_microflows_builder_calls.go with isPlaceholderExpression)
   • Tests covering the functionality (cmd_microflows_builder_java_action_test.go)
   • Documentation (skills file, quick reference, proposal)
   • Doctype test example (ellipsis_placeholder.test.mdl)

2. Correct semantics: The implementation properly distinguishes between:

   • Java action arguments (using BasicCodeActionParameterValue with empty Argument)
   • Microflow/nanoflow arguments (using MicroflowParameterValue with empty Microflow)
   • Only allows the placeholder in appropriate contexts

3. Round-trip focus: The feature is correctly positioned as a round-trip-only placeholder, with clear documentation advising against its use in authored scripts.

4. Test coverage: Includes unit tests verifying:

   • Preservation of empty BasicCodeActionParameterValue.Argument for Java actions
   • Proper handling of microflow-type parameters with MicroflowParameterValue
   • Whitespace tolerance in the placeholder (" ... ")

5. Documentation quality:

   • Clear explanation of the motivation and problem being solved
   • Proper warning about round-trip-only usage
   • Examples showing correct usage in context
   • Updated quick reference with appropriate categorization

Recommendation

Approve the PR. The implementation correctly addresses the need to preserve round-trip symmetry for unbound Java action parameters, follows all required architectural patterns, includes comprehensive tests, and provides appropriate documentation. The minor concerns about LSP/VS Code extension are unlikely to impact functionality given the nature of the feature as an expression placeholder that doesn't introduce new language constructs requiring special editor support.

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Automated review via OpenRouter (Nemotron Super 120B) — workflow source