MPS Intentions Aspect

Intentions are user-invoked transformations shown in the Alt+Enter popup on a node. They are the canonical way to offer optional refactorings (wrap, convert, introduce, preview) alongside quick-fixes. Authoring lives in <lang>/languageModels/intentions.mps, language jetbrains.mps.lang.intentions.

Critical Directives

• Edit intention roots 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 serialized .mps files.
• The intentions model must list every language used in any block body as a used language. In particular, the add new initialized(...) surface syntax requires jetbrains.mps.lang.actions — add it with mps_mcp_model_used_language(kind="language"). Plain (non-NF_*) variants like parent.role.add new(C) do not need this import.
• Use n as C (MPS SNodeTypeCastExpression with asCast=true) — never a Java (C) n cast.
• Use :eq: (NPEEqualsExpression) instead of == whenever either operand might be null.
• The execute body runs on the EDT inside an MPS write action automatically — do not spawn threads or open dialogs that block.
• descriptionFunction runs every time the Alt+Enter popup opens — keep it cheap.
• After editing, validate with mps_mcp_check_root_node_problems on the intention root and rebuild the language before testing.

Common-Path Workflow

1. Ensure an intentions model exists in the language (create with mps_mcp_create_model and modelName: "<lang>.intentions" — aspect ID intentions, case-sensitive, no @ suffix; see aspect-model-stereotypes.md). Used languages: jetbrains.mps.lang.intentions; add jetbrains.mps.lang.actions if you will use factory-initialized splicing.
2. Insert an IntentionDeclaration root with mps_mcp_insert_root_node_from_json (see blueprint in references/json-blueprints.md). Set name, forConcept, and isAvailableInChildNodes if the popup should bubble from descendants.
3. Fill descriptionFunction (returns a short label String), executeFunction (the transformation), and optionally isApplicableFunction (gate predicate) and childFilterFunction (per-descendant filter when isAvailableInChildNodes=true).
4. For multiple menu entries per blueprint use ParameterizedIntentionDeclaration with paramType + queryFunction; use IntentionParameter wherever you want "the current value" inside other blocks.
5. For wrap-the-selection transforms use SurroundWithIntentionDeclaration and read editorContext.getSelectedNodes().
6. Validate (mps_mcp_check_root_node_problems), rebuild the language, exercise via Alt+Enter on a sandbox instance. Headlessly: after MAKE, call mps_mcp_list_node_intentions on a sandbox node to confirm the intention is registered and shows the expected description, then mps_mcp_apply_intention(nodeReference = <the entry's targetNode>, intentionId = <the entry's id>) to smoke-test executeFunction (for parameterized declarations also pass the row description). Surround-with intentions are not covered by these tools.

Implicit parameters available in every block

Related Skills

• mps-aspect-actions — when the intention needs factory-initialized children, the NodeFactories live there.
• mps-aspect-constraints — for gating that should apply everywhere (not just Alt+Enter), prefer can-be rules over isApplicable.
• mps-aspect-editor-menus-and-keymaps — when the action belongs on a substitute/transformation menu or a keystroke, not in the Alt+Enter popup.
• mps-model-manipulation — full smodel/baseLanguage/collections reference; covers the NF_* family and the dual IsEmptyOperation.
• mps-node-editing — MCP recipes for inserting children, updating references, and staged construction.
• mps-aspect-structure-concepts — when adding the concept that forConcept targets.
• mps-quotations — when the execute body builds a complex subtree literal.
• mps-tests → EditorTestCase — for testing that an intention is offered and produces the expected tree.

Reference Index

• Open references/blocks-and-parameters.md when wiring descriptionFunction, isApplicableFunction, executeFunction, or childFilterFunction, or when you need to look up the implicit parameter concept FQNs for any of them.
• Open references/execute-idioms.md when writing an executeFunction body — typical AST-editing operations (insert sibling, isInstanceOf, asCast, list add, select-in-editor), the verbatim WrapInParens and AddOnEntry examples, the attach-an-annotation (node attribute) idiom (forConcept = BaseConcept + context-gated isApplicable, .@role.add(...) vs .@Mark = ...), and the SelectInEditorOperation JSON shape live there.
• Open references/factory-initialized.md when an executeFunction allocates new nodes and you want the concept's NodeFactory to seed defaults — the NF_* family table, the add new initialized(<default>) semantics, and the used-language requirement live there.
• Open references/parameterized-intentions.md when authoring or debugging a ParameterizedIntentionDeclaration — paramType + queryFunction semantics, IntentionParameter usage, the per-value isApplicableFunction filter, and the full SetCardinalityTo example and JSON skeleton are there.
• Open references/surround-and-child-filter.md when authoring a SurroundWithIntentionDeclaration or when configuring isAvailableInChildNodes + a ChildFilterFunction — verbatim WrapInParens and AddOnEntry.childFilterFunction walk-throughs are there.
• Open references/json-blueprints.md when inserting an intention root via mps_mcp_insert_root_node_from_json — minimal blueprints for IntentionDeclaration, ParameterizedIntentionDeclaration, and SurroundWithIntentionDeclaration and the validated concept-reference table.
• Open references/common-failures.md when an intention does not appear, fires on the wrong node, leaves the caret in the wrong place, or shows duplicates — the symptom/cause/fix table is there.