Writing MPS BaseLanguage with smodel / collections / closures

Use this skill when authoring code inside an MPS model where BaseLanguage is mixed with the smodel, collections, and closures extensions: behavior methods, checking/typesystem/dataflow rules, generator query functions, intentions, constraints, scope/canBe callbacks. The mps_mcp_parse_java_and_insert parser understands only plain Java, so most non-trivial code requires hand-built node blueprints — this skill is the catalog.

To enumerate nodes/instances/usages across a scope (a whole project, module, model, or repository) rather than navigating from one root, add the jetbrains.mps.lang.smodel.query language — it supplies #instances / #usages / #nodes / #references / #models / #modules and a with (<scope>) { … } wrapper, the same queries the MPS Console runs. Its results compose with the smodel/collections/closures operations catalogued here. See the mps-console skill.

Critical Directives

• Node equality uses :eq: and :ne:, never == or .equals(). Use NPEEqualsExpression / NPENotEqualsExpression. See references/node-equality.md.
• Every InferredClosureParameterDeclaration needs a type child, even though MPS infers the type. Use jetbrains.mps.baseLanguage.structure.UndefinedType as placeholder. Omitting it cascades into misleading "different parameter numbers" / "out of search scope" / "operation is not applicable to null" errors. See references/closures-catalog.md.
• The Java parser cannot produce smodel/collection types in method return types, cannot parse :Concept casts, .link accesses, .behaviorMethod() calls, or MetaAdapterFactory.getContainmentLink inline. Use LINKS.xxx/CONCEPTS.xxx constants and replace types post-parse. See references/java-parser-capabilities.md.
• Mutations need command { … }, reads from non-UI code need read action { … }. There is no separate "write action". Behavior methods and rule bodies already hold the right lock. See references/concurrent-access.md.
• Pick the right cardinality: SLinkAccess for 0..1 / 1; SLinkListAccess for 0..n / 1..n. One pair serves both reference links and containment children (there is no SChildAccess/SChildListAccess). Wrong choice triggers "out of search scope" / "access to link X is not expected here". See references/dot-expression-basics.md.
• foreach has two concepts: collections ForEachStatement for sequence/list/set, BaseLanguage ForeachStatement for Java arrays/iterables. Wrong choice → type error. See references/foreach-statements.md.
• Used-language imports: NF_* operations require jetbrains.mps.lang.actions in the model's used languages; without it, add new initialized(...) etc. fail. See references/property-and-mutation-ops.md and references/openapi-and-dependencies.md.
• Never recommend deprecated operations: .role (use .containingLink), .linkDeclaration (use .link), .role on SReference (use .link.name), .concept Node_GetConceptOperation (use node.concept via typesystem).

Common-path workflow

1. Identify the source-of-truth layer. If the change is to existing logic in a rule/method body, inspect the current node (shallow mps_mcp_print_node) before editing.
2. Prefer parse_java_and_insert for plain Java method bodies, expression replacements, and field declarations on LINKS/CONCEPTS inner classes. See references/java-parser-capabilities.md.
3. For smodel/collections/closures constructs not parseable as Java, build the node blueprint by hand. See:
   • references/dot-expression-basics.md — the universal DotExpression(operand, operation) chaining pattern, closure literal blueprint, type-cast wrapping rules.
   • references/property-and-mutation-ops.md — property reads/writes, add new(C), replace with new(C), sibling inserts.
   • references/references-and-queries.md — .ancestor<C>, .descendants<C>, .reference/C:role/, .target, .isInstanceOf, sibling queries.
   • references/attribute-access.md — the .@ operator: read/set/remove an attribute (annotation) attached to a node and reach the children/refs/properties it grafts on (generator macros, comments, requirement traces).
4. For variable declarations and method return types of MPS collection / smodel types, build the type child explicitly — parse_java_and_insert produces a wrong ClassifierType for these in return-type position. See references/variable-declarations.md.
5. Validate with mps_mcp_check_root_node_problems(nodeReference, onlyNodesWithProblems=true). Re-read generated source_gen/...java only to confirm runtime shape — never to apply fixes. See references/workflows.md.
6. If imports/dependencies break, add a used language or model dependency via mps_mcp_model_used_language / mps_mcp_model_dependency / mps_mcp_module_dependency. See references/openapi-and-dependencies.md.

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

Related Skills

• mps-baselanguage — host BaseLanguage statements, expressions, classes; this skill is the smodel/collections/closures overlay.
• mps-quotations — < … > quotations and antiquotations for assembling node trees inline; use instead of long blueprint chains where they apply.
• mps-aspect-behavior — per-concept method authoring; behavior bodies are the most common host for this skill.
• mps-aspect-generator — generator query bodies, $IF$/$MAP_SRC$.mapperFunction, where this skill provides the smodel idioms.
• mps-aspect-typesystem — inference and checking rule bodies; uses the same idioms.
• mps-aspect-constraints — getScope, canBe*, property validators; same idiom set.
• mps-node-editing — programmatic node creation and editing from outside model code (the MCP-tool side rather than smodel-language side).
• mps-console — the MPS Console and the jetbrains.mps.lang.smodel.query language (with-statement, #instances/#usages/#nodes/#models/#modules, scopes); use it to run or generate the smodel code in this skill across a project/repository scope, or to insert console commands via mps_mcp_insert_console_command_from_json.

Reference Index

• references/golden-rules-and-pitfalls.md — read first when an error looks weird (cascade UnknownDotCall, "out of search scope", "different parameter numbers", List<SNode> vs sequence<node<X>>); collected symptom→cause→fix table.
• references/java-parser-capabilities.md — open before calling mps_mcp_parse_java_and_insert. Lists what the parser handles, what it silently mis-types, and the canonical workarounds (LINKS/CONCEPTS pattern, post-parse replace_node_child, signature widening to avoid smodel casts in arguments).
• references/workflows.md — step-by-step recipes: adding a new helper method to a checking-rule class, replacing an expression inside a rule body, interacting with source_gen/ artifacts.
• references/concept-id-table.md — MetaAdapterFactory hex IDs for BaseLanguage concepts and links (ThrowStatement, CatchClause, TryCatchStatement, VariableReference, etc.) plus persistent node refs for Type, Expression, Classifier, SequenceType, ListType, SNodeType.
• references/smodel-concepts-catalog.md — every smodel concept (SNodeType, SLinkAccess, Node_GetAncestorOperation, OfConceptOperation, Concept_*Operation, EnumMember_*, …) with its ID and MPS surface notation. Open when you need to pick a concept and don't remember which one.
• references/baselanguage-key-concepts.md — the small set of BaseLanguage concepts that this skill keeps reaching for (DotExpression, ForeachStatement, BlockStatement, NPE*Expression, UndefinedType).
• references/concurrent-access.md — command, read action, execute command in EDT, undo-transparent command. Open when writing code that reads/mutates models outside a rule or behavior body.
• references/collections-catalog.md — every *Operation from jetbrains.mps.baseLanguage.collections (where/select/translate/any/take/sortBy/reduceLeft/toList/…), the type hierarchy (sequence ⊃ list/set; map), creator concepts, list/set/map mutators, iterator vs modifying_iterator, sorted variants, lazy-vs-eager rule, and the isEmpty/isNotEmpty collections-vs-baselanguage ambiguity.
• references/closures-catalog.md — ClosureLiteral parameters/body shape, InferredClosureParameterDeclaration requirements, FunctionType, InvokeFunctionOperation vs InvokeExpression (recursion), YieldStatement, SAM conversion, generated-Java compilation form, runtime jar dependency.
• references/dot-expression-basics.md — the universal DotExpression(operand, operation) chaining pattern; closure-literal blueprint (the reusable building block); cardinality cheat-sheet (SLinkAccess vs SLinkListAccess, one pair for both references and children); when to wrap an operand in SNodeTypeCastExpression; navigation blueprints (.link, .parent, .children, .ancestor<C>, .ancestors<C>, .descendants<C> including + include-self form); node:C vs node as C; expr/ (SemanticDowncastExpression) and downcast expr (DowncastExpression) for crossing into Java APIs; .isInstanceOf / .isNotNull / .isNull / .behaviorMethod(args); where / any / translate / list.add blueprints.
• references/property-and-mutation-ops.md — read for property reads/writes (SPropertyAccess, assignment, Property_RemoveOperation) and tree mutations (SNodeCreator, add new(C), set new(C), replace with new(C) / replace with(other), .copy, .detach, sibling inserts), including all NF_* factory-initialized variants and the required jetbrains.mps.lang.actions used-language import.
• references/references-and-queries.md — Node_GetReferenceOperation (.reference/C:role/), Node_GetReferencesOperation, Reference_GetTargetOperation etc.; plus query operations .siblings/.next-sibling/.prev-sibling*, .containingRoot/.containingLink/.model/.index, .isExactly(C), seq.ofConcept<C>, seq.ofAspect<a>, enum-member operations.
• references/attribute-access.md — the .@ operator (AttributeAccess + NodeAttributeQualifier / LinkAttributeQualifier / ChildAttributeQualifier / PropertyAttributeQualifier / AllAttributeQualifier): read, attach/replace (assignable l-value), or remove an attribute, and chain into the children/refs/properties it grafts onto a node. Pairs with mps-aspect-structure-concepts/references/attributes-and-annotations.md (the structure side).
• references/link-and-concept-literals.md — standalone link/C : role/ (LinkIdRefExpression, two required refs) and concept/C/ (ConceptIdRefExpression). Open when an API needs an SContainmentLink/SReferenceLink/SConcept argument with no preceding node. operand; covers the distinction from OperationParm_LinkQualifier and RefConcept_Reference.
• references/foreach-statements.md — the two foreach concepts (collections ForEachStatement + ForEachVariable / ForEachVariableReference vs BaseLanguage ForeachStatement + LocalVariableDeclaration / VariableReference). Open when iterating produces a typesystem error or when you need to reference the loop variable.
• references/node-equality.md — NPEEqualsExpression (:eq:) and NPENotEqualsExpression (:ne:) blueprints and when to use them.
• references/variable-declarations.md — type-child choices for locals and return types (node<C>, nlist<C>, node-ptr<C>, concept<C>, search scope, enummember<E>, sequence/list/set<node<C>>, boolean); creator blueprints for new arraylist<node<C>> / new hashset<node<C>> / etc.; the parse-time return-type fix-up blueprints for sequence<node<Type>> and list<node<Type>>.
• references/openapi-and-dependencies.md — org.jetbrains.mps.openapi.* interfaces and threading rules (when hand-written Java calls into models); module dependencies vs model dependencies vs used languages, with failure modes ("Concept not imported", "cannot resolve", "Class not found at runtime") and the matching MCP tool to call.