mps-aspect-typesystem

MPS Typesystem and Checking Aspect

The typesystem aspect gives nodes types and reports semantic errors. It combines two related sub-aspects:

• Typesystem rules — compute types and constraints on them (InferenceRule, SubtypingRule, InequationReplacementRule, ComparisonRule, SubstituteTypeRule).
• Non-typesystem checking rules — produce errors/warnings/messages without contributing to type inference (NonTypesystemRule, a.k.a. "checking rule").

Lives in <lang>/languageModels/typesystem.mps, language jetbrains.mps.lang.typesystem. Rule bodies are BaseLanguage + smodel + collections + closures.

Critical Directives

• One InferenceRule (or other root rule) per concept whose type/check you compute. Multiple rules collectively constrain a node — keep each rule focused.
• The infer prefix makes an inequation soft (the solver tries to satisfy it, will not immediately error). Without infer, violating the inequation reports an error. Choose deliberately.
• In equation/inequation JSON, both leftExpression and rightExpression are TypeClause slots — always wrap the real Expression in a NormalTypeClause (normalType child holds the actual Expression). Do not put the Expression directly under leftExpression/rightExpression.
• TextGen-style dispatch caveat does not apply here; typesystem rules are inherited via concept hierarchy, but the overrides property on a rule suppresses inherited rules from superconcepts.
• TextGen / typesystem error messages: wrap smodel expressions that render types with <...> presentation: error "Expected " + <expectedType> + " but got " + <actualType> -> node;. Avoid raw toString.
• when concrete (typeof(expr) as v) { ... } defers a block until the type is fully resolved — use it before deciding whether to report an error or assign a result type.
• Quick fixes (TypesystemQuickFix) are roots, not executed automatically — the user triggers them via the UI. Wire them into a report through the helginsIntention slot (TypesystemIntention wrapper with quickFix ref + actualArguments). See references/quick-fixes.md.
• Reusable helper code (utility classes, shared algorithms) can live as a plain BaseLanguage ClassConcept root directly in the typesystem model. No separate utility module is required.
• For MPS-typed return types (sequence<node<X>>, list<node<X>>) the Java parser gives back List<SNode> — fix per mps-model-manipulation/references/variable-declarations.md.
• Edit typesystem models through MPS MCP tools (mps_mcp_insert_root_node_from_json, mps_mcp_update_node, mps_mcp_parse_java_and_insert). Do not hand-edit .mps files.
• After edits run mps_mcp_check_root_node_problems, compile the language, and test on sample models.