MPS Constraints Aspect

The constraints aspect controls runtime rules about nodes: what values properties may hold, how a property is stored when set, what nodes a reference may point to, and where a node of a concept is allowed to appear in the AST. It lives in the language's constraints model (<lang>/languageModels/constraints.mps) and uses the language jetbrains.mps.lang.constraints.

Constraints are enforced at edit time and also consulted by the typesystem/editor. They are not the place for structural cardinalities (that belongs in structure) or for types (that belongs in typesystem).

Critical Directives

• One ConceptConstraints root per concept. Set its concept reference; everything else is child blocks.
• Pick the right body for the job:
  • Validator (propertyValidator) — pure acceptance check; returns boolean; never mutates state.
  • Setter (propertySetter) — normalises and must explicitly assign node.<prop> = propertyValue (a body that forgets this silently drops the value).
  • Getter (propertyGetter) — only when the displayed value differs from the stored value (derived URL, aliased name).
  • set <read-only> means no setter at all — direct assignment to the property is rejected by MPS. An empty-body setter is almost always a bug.
• canBeChild / canBeParent / canBeAncestor are inherited by sub-concepts; per-property and per-reference constraints are not — duplicate the ConceptConstraints root or hoist the rule up.
• Pick the right scope shape:
  • Inherited via InheritedNodeScopeFactory + ScopeProvider.getScope (behavior) — preferred whenever the scope naturally belongs to an ancestor (block, declaration, program, module). Composes cleanly along the containment chain with parent scope.
  • Imperative ConstraintFunction_ReferentSearchScope_Scope — only when the scope is tiny, highly local, and has no natural ancestor owner.
• referentSetHandler performs side effects (auto-rename referrer, copy fields, keeps original reference). Never overload the scope to do this.
• defaultConcreteConcept on ConceptConstraints chooses which concrete subconcept replaces an abstract pick — it is a single reference, not a child block.
• Surface forms (come from, parent scope, kind.isSubConceptOf(...), link/C : role/, concept/C/) are not BaseLanguage. Each is a dedicated jetbrains.mps.lang.scopes / jetbrains.mps.lang.smodel concept — construct those concepts, not strings (see references/scope-fqn-reference.md).
• Concept_IsSubConceptOfOperation matches a concept and its sub-concepts; Concept_IsExactlyOperation excludes sub-concepts. Use isExactly for guards that must not fire on specialisations.
• Edit constraint models through MPS MCP (mps_mcp_insert_root_node_from_json, mps_mcp_update_node, mps_mcp_parse_java_and_insert). Do not hand-edit .mps files. Validate with mps_mcp_check_root_node_problems and rebuild the language; if the new constraint is invisible at runtime, run mps_mcp_reload_all.

Common-Path Workflow

1. Ensure the language has a constraints model; create it with mps_mcp_create_model (modelName: "<lang>.constraints" — aspect ID constraints, case-sensitive, no @ suffix; see aspect-model-stereotypes.md) if absent.
2. Add jetbrains.mps.lang.constraints to the model's used languages. For scope bodies also add jetbrains.mps.lang.smodel, jetbrains.mps.lang.scopes, jetbrains.mps.lang.behavior, jetbrains.mps.baseLanguage; import models jetbrains.mps.scope and jetbrains.mps.lang.core.behavior.
   • Module-level dependency for scope classes: the language module must have a Default dependency on solution jetbrains.mps.kernel (which ships model jetbrains.mps.scope). mps_mcp_model_dependency adds it automatically when importing jetbrains.mps.scope; mps_mcp_create_module type=language does not. See references/scope-fqn-reference.md → "Required module-level dependency".
3. For each concept to constrain, create one ConceptConstraints root (mps_mcp_create_root_node); set concept ref. Minimal blueprint and validated FQN/c-ref in references/concept-roots.md.
4. Add NodePropertyConstraint / NodeReferentConstraint / canBe* children as needed. For inherited scopes, also add a getScope method to the owning ancestor's ConceptBehavior (see mps-aspect-behavior).
5. Write bodies in BaseLanguage + smodel — follow the mps-model-manipulation skill for smodel / collections / closures syntax. Prefer mps_mcp_parse_java_and_insert for the BaseLanguage skeleton, then drop in scope / constraint-specific concepts via MCP.
6. Validate with mps_mcp_check_root_node_problems and rebuild the language.

Related Skills

• mps-aspect-behavior — ScopeProvider.getScope lives in the behavior aspect; this is the partner of every InheritedNodeScopeFactory constraint. Behavior is also where most validator/getter delegation lands.
• mps-aspect-structure-concepts — structural cardinalities belong in structure, not here. Add INamedConcept / IResolveInfo / ScopeProvider interfaces on the concept declaration before writing the matching constraint or scope.
• mps-aspect-editor-menus-and-keymaps — completion menus consume the scope built here; substitute / transformation menus are the place for actions that go beyond picking an existing target.
• mps-aspect-actions — when a constructor-style initialization is needed, use NodeFactory (actions aspect), not a constraint.
• mps-aspect-intentions — when a suggested fix belongs on a node, write an intention; constraints only accept or reject.
• mps-model-manipulation — full BaseLanguage / smodel / collections / closures reference for constraint and scope bodies. Indispensable for ancestor<concept = X, +>, selectMany, siblings.ofConcept<X>.all(...), anonymous-class scopes, and the smodel operation FQNs.
• mps-aspect-typesystem — type checks belong in typesystem; constraints are about value/scope/placement.
• mps-aspect-textgen and mps-aspect-constraints-co-located utility classes — see references/concept-roots.md for the BaseLanguage ClassConcept co-location pattern (used by XmlNameUtil).

Reference Index

• Open references/concept-roots.md when creating the ConceptConstraints root, setting defaultConcreteConcept to a concrete subconcept of an abstract concept, looking up the validated concept c-ref, or co-locating a BaseLanguage helper ClassConcept next to constraint roots.
• Open references/property-constraints.md when writing NodePropertyConstraint bodies — choosing validator vs. setter vs. getter, the {name} aliasing pattern with set <read-only>, derived-property getters (e.g. Element.details_url), sibling-uniqueness validators (State.isInitial), and full JSON blueprints for validator and getter bodies.
• Open references/referent-constraints.md when writing NodeReferentConstraint — both scope styles (imperative ConstraintFunction_ReferentSearchScope_Scope and ancestor-supplied InheritedNodeScopeFactory), the verbatim Calculator / StateChart / Kaja examples (SimpleRoleScope.forNamedElements, ListScope with anonymous getName, CompositeScope + addScope, model.rootsIncludingImported), the meta-level scope offering structure declarations (conceptNode.getAggregationLinkDeclarations() for LinkDeclaration referents), referentSetHandler patterns (auto-rename, multi-field copy, keeps original reference), and concept-level Default Scope.
• Open references/scope-helpers.md when picking among EmptyScope, ListScope, SimpleRoleScope, CompositeScope, FilteringScope, DelegatingScope, ModelsScope, ModelPlusImportedScope, Scopes.forConcepts, HidingByNameScope, or ListScope.forResolvableElements; or when caching with for model [...], visible roots [C], visible nodes [C]. Includes the verbatim StateChart Stateful_Behavior.getScope (ListScope + anonymous getName + HidingByNameScope over parent scope) idiom.
• Open references/scope-fqn-reference.md when constructing scope bodies via MCP — the full surface-syntax-to-FQN table (parent scope, come from, isSubConceptOf vs isExactly, RefConcept_Reference vs ConceptIdRefExpression, LinkIdRefExpression, Node_GetAncestorOperation + OperationParm_Concept, Node_GetDescendantsOperation, Node_GetModelOperation, Model_RootsIncludingImportedOperation, Node_GetParentOperation, Node_IsInstanceOfOperation), required used-languages and model imports, the validated ScopeProvider.getScope overriddenMethod ref, and JSON blueprints for the getScope skeleton, parent scope return, and the guarded local-scope if-branch.
• Open references/canbe-rules.md when authoring ConstraintFunction_CanBeAChild / CanBeAParent / CanBeAnAncestor / CanBeARoot — each block's parameter set (note canBeAnAncestor has only 3 parameters; no parentNode or link), the parentNode.parent.isInstanceOf(...) grandparent-traversal pattern (Kaja RoutineDefinition and Require), the node.isInRole(link/C : role/) pattern, the model parameter for canBeRoot, and the minimal ConstraintFunction_CanBeAChild blueprint.
• Open references/common-failures.md when a setter runs but stores nothing, a completion popup is empty, the editor reports "cannot be child", a constraint does not apply to a sub-concept, or constraint edits seem invisible at runtime.