MPS Generator Aspect

A generator transforms models written in the source language into models of one or more target languages (usually BaseLanguage or another DSL). It is a separate MPS module — a generator module — owned by the language and driven by templates: target-language code snippets annotated with macros.

Generator architecture (read first)

A generator translates from the problem domain (source language) toward the implementation domain, often as a cascade: each generator lowers the abstraction level by producing its target language, which becomes the next generator's input, until a base language (usually BaseLanguage) is reached and TextGen emits plain text.

A generator definition has a stable part and a variable part:

• the stable part does not change with the model being generated — engines, base classes, helpers. Provide it once in a runtime solution (an MPS Solution the language declares as a runtime module — as MPS-authored source or a bundled JAR), not as a template.
• the variable part is the templates + macros, which react to the input model and choose different output.

Idiomatic generators keep the stable part out of the templates and emit thin code that calls into the runtime solution. Before adding rules, decide how much is stable: see the architecture ladder in references/cookbook.md and the stable-vs-variable split + wiring in mps-aspect-accessories/references/runtime-solutions.md (worked example: the Kaja language + its JavaKaja runtime).

Critical Directives

• Edit the generator, not source_gen/. If a bug appears in generated Java, the fix almost always belongs in a template, macro body, or concept behavior. Patch generated output only when explicitly told to.
• applicableConcept must be a structure-model node ref (r:<modelUUID>(<lang>.structure)/<nodeId>), never the c:<langUUID>/<conceptId> form. Wrong form → silent Unresolved reference: c:.... See references/rule-consequences.md.
• A TemplateDeclaration used as a reduction target requires at least one TemplateFragment on the produced subtree, or the rule fires and emits nothing.
• Module dependencies vs. model used-languages are two distinct layers (see references/module-structure.md). Adding a class to the wrong one yields "cannot resolve" in generated Java or unusable concepts in templates.
• Macros attach via the smodelAttribute child role, and co-located macros are chained in child order — order is semantics. $IF$ before $LOOP$ gates the whole loop in the outer context; $LOOP$ before $IF$ evaluates the condition per iteration. The $LOOP$ + $COPY_SRC$ pair is order-insensitive only because $COPY_SRC$ is terminal — don't generalize. See references/macro-catalog.md.
• Reductions can self-loop when they produce their own concept — break out with DismissTopMappingRule, a marker subconcept, or $COPY_SRC$.
• Cross-rule/cross-template references go through mapping labels. Direct references survive only within a single fragment.

Common-path workflow

1. Locate or create the generator module. List languages with mps_mcp_get_project_structure(moduleKind="Language") and find yours by name: each language entry carries a generators array (unconditional — not gated on includeDependencies) listing every owned generator's name and reference. Prefer this over startingPoint="<language-name>" — a language and its owned generator share a base name, so a name starting point can resolve to the generator module instead, and then the generators array is never emitted. (If you already hold the language's persistent reference, startingPoint="<reference>" resolves by id and is also unambiguous.) To go the other direction, each generator entry carries a sourceLanguage field (only when includeDependencies=true) with the owning language's name and reference; follow it with mps_mcp_get_project_structure(startingPoint="<sourceLanguage.reference>", includeModels=true). If no generator exists yet: mps_mcp_create_module(type="generator", parentLanguage="<lang>").
2. In template/main@generator.mps (the generator model stereotype — name@generator; see aspect-model-stereotypes.md for all model identifiers), add the used languages you target (jetbrains.mps.devkit.templates is the quickest umbrella) and ensure the target language is also a module generate-into dependency on the language .mpl (see references/module-structure.md).
3. Create or edit the MappingConfiguration root (mps_mcp_create_root_node with the FQN in references/concept-fqns.md).
4. Add the rule(s):
   • Root_MappingRule for whole-root mapping;
   • Reduction_MappingRule for in-place rewrites (inline RuleConsequence or TemplateDeclarationReference);
   • Weaving_MappingRule to add content into an already-generated container.
5. Author target-language templates; attach macros as smodelAttribute children. Mark the produced subtree with TemplateFragment (or RootTemplateAnnotation on a target root).
   • A RootTemplateAnnotation alone does NOT register a root mapping. It only marks a target root as a template; a Root_MappingRule (step 4) whose template reference points to that annotated root is also required. Without the rule, generation silently produces no output for that root — MAKE still reports success.
6. Declare any mapping labels on the MappingConfiguration; tag writers (labelDeclaration ref or $LABEL$ macro); read with genContext.get/pick output <label> for (<input>) inside $REF$ or other queries.
7. Validate with mps_mcp_check_root_node_problems on the MappingConfiguration and every template (re-run with onlyNodesWithProblems = false if siblings look "missing").
8. mps_mcp_alter_nodes MAKE over the generator and a sample model; read the generated text (see Reading generator output below). Use the Generator Tracer / $TRACE$ macros to bisect misgenerated fragments.

If MPS MCP tools are unavailable, do not hand-edit serialized .mps files unless explicitly asked — inspect only and report.

Reading generator output

After a MAKE, generators write Java (and TextGen artifacts like .xml, .scxml) to the owning module's source_gen/ directory. No mps_mcp_* tool reads this output — but the MPS MCP server exposes the same generic IDE file tools, so generated text is readable over MCP today. Don't drop to a raw shell find/cat.

Path convention. Output lands at <module-dir>/source_gen/<model-namespace>/<java-package>/<File>.java, where the model namespace's dots become directory separators. Example: model Kaja.sandbox in solution Kajak.sandbox → samples/robot_Kaja/solutions/Kajak.sandbox/source_gen/Kaja/sandbox/sandbox/Karel.java (trailing sandbox/ is the Java package). TextGen artifacts (.scxml, etc.) follow the same layout. A module may override its output root, so if source_gen/ is not beside the module descriptor, check the module's output path.

Tools. Use the generic IDE file tools (exposed by the MPS MCP server, also by IDEA's; pass projectPath to disambiguate) — not mps_mcp_*:

• find_files_by_glob — discover what was generated, e.g. project-root-relative **/Kajak.sandbox/source_gen/**/*.java. Note this matches only .java; use list_directory_tree to also see TextGen artifacts (.scxml, trace.info, …).
• list_directory_tree — browse the output directory.
• read_file — read a specific generated file (output is line-capped; use its max_lines/start_line args for large files).

Staleness. On-disk source_gen/ reflects the last MAKE. Re-run MAKE before reading so the output matches the current model, otherwise you may read stale text.

Related Skills

• mps-aspect-generation-plan — priorities, checkpoints, plan contributions; pair this with priority-ordering questions.
• mps-aspect-textgen — converting the final model to plain text (last pipeline stage).
• mps-aspect-behavior — put non-trivial per-concept logic in behavior methods called from macros.
• mps-model-manipulation — smodel-access idioms (SPropertyAccess, SLinkAccess, Node_GetChildrenOperation) inside query bodies.
• mps-quotations — quotation/anti-quotation syntax for building nodes inside $INSERT$ and $MAP_SRC$.mapperFunction.
• mps-baselanguage — host language for macro query bodies and RulesFunctions_* helpers.

Reference Index

• Generator module structure (descriptor deps, model used-languages, MappingConfiguration roles): references/module-structure.md — read when setting up a generator or fixing "cannot resolve"/missing-concept issues.
• RuleConsequence catalog (TemplateDeclarationReference, Inline*, AbandonInput, DismissTop) + the critical applicableConcept ref-form gotcha: references/rule-consequences.md.
• Macro catalog and JSON blueprints for COPY_SRC/COPY_SRCL/PROPERTY/IF/LOOP/CALL: references/macro-catalog.md — read before attaching any macro.
• $MAP_SRC$ in depth (replace input, mapperFunction, postMapperFunction, label-on-site): references/macro-map-src.md.
• $REF$ / ReferenceMacro in depth (label-based and name-based forms, return types): references/macro-reference.md.
• Less-common macros — $WEAVE$, $VAR$, $TRACE$, $LABEL$, $INSERT$: references/macros-rare.md.
• Query-body subconcepts and their implicit parameters (IfMacro_Condition, SourceSubstituteMacro_*, etc.): references/query-bodies.md.
• Mapping labels — declare/write/read pattern, get vs pick vs get list: references/mapping-labels.md.
• Weaving rules — contextNodeQuery, when to use vs. reductions: references/weaving-rules.md.
• Template switches — when to switch, extends across generator modules: references/template-switches.md.
• Pattern reduction rules — structural-pattern matching with typed holes: references/pattern-rules.md.
• Mapping scripts (pre / post) — bulk passes and modifiesModel: references/mapping-scripts.md.
• Utility models, template functions, fragments — where to place non-trivial generator-time helpers: references/utility-models.md.
• genContext operations catalog (every GenerationContextOp_* with editor alias and purpose): references/generation-context.md.
• Generation algorithm — pipeline stages (selection → implicit grouping/ordering → explicit priorities → step-by-step transformation → TextGen), micro-steps, weaving vs. reduction ordering: references/algorithm.md.
• Cookbook recipes — multi-root, cross-ref, unique IDs, extensible generators, loop-extraction refactor: references/cookbook.md.
• Common failures — symptom-to-cause-to-fix table: references/common-failures.md.
• Validated concept FQNs and minimal MappingConfiguration blueprint: references/concept-fqns.md.
• Calling from/into the generator — behaviors, RulesFunctions_*, utility models: references/calling-into-generator.md.