Mps Aspect Typesystem

MPS Typesystem and Checking Aspect

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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 references/variable-declarations.md in the mps-model-manipulation skill root after loading that companion skill from the same origin.
• 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.

Common-Path Workflow

1. Create a typesystem model (mps_mcp_create_model with moduleName: "<lang>" and modelName: "<lang>.typesystem" — aspect ID typesystem, case-sensitive, no @ suffix; see aspect-model-stereotypes.md) if absent.
2. Add used languages: jetbrains.mps.lang.typesystem, and any languages used in bodies (smodel, collections, closures, baseLanguage).
3. Add InferenceRule roots for concepts whose types you compute (see references/inference-rules.md).
4. Add SubtypingRule / ComparisonRule / InequationReplacementRule / SubstituteTypeRule roots for the type lattice (see references/lattice-rules.md).
5. Add NonTypesystemRule roots for checks that are not about types (see references/non-typesystem-checking.md).
6. Add TypesystemQuickFix roots for any fix you want to attach to reports; wire them via a helginsIntention/TypesystemIntention child of the warning/error/info statement (see references/quick-fixes.md).
7. Write bodies with BaseLanguage + smodel. Use when concrete (references/when-concrete.md) when you need resolved types before checking. For helper code, place a ClassConcept root directly in the typesystem model and call it from rules.
8. Validate: mps_mcp_check_root_node_problems plus compile the language and test on sample models. A helginsIntention-wired quick fix now surfaces in the check report's quickFixes array and can be applied with mps_mcp_apply_intention(nodeReference = <problem node>, intentionId = <fix id>); an auto-applicable fix runs under autoApplyQuickFixes=true.