MPS Editor — Technical Reference

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The jetbrains.mps.lang.editor language defines projectional editors as trees of cell models (display elements) with cell layouts and style items. This skill is the technical lookup: concept names, suffixes, style classes, and the rules that decide which cell model to use where.

Critical Directives

• Always use CellModel_RefNodeList for 0..n or 1..n children. CellModel_RefNode is for single cardinality only.
• Use IndentLayoutOnNewLineStyleClassItem (apply to the cell that should start on a new line) instead of the legacy IndentLayoutNewLineStyle (applied to the cell before the break) — the latter is easier to get wrong.
• Reference cells: set the internal CellModel_Property (e.g. name) inside an InlineEditorComponent to readOnly=true so the referenced target cannot be accidentally renamed through the editor.
• Smart references: no wrapping layout cell — just a single CellModel_RefCell whose editorComponent is an InlineEditorComponent with a CellModel_Property (relationDeclaration: name, readOnly: true).
• List empty state: a CellModel_RefNodeList already synthesizes an editable empty cell (caret lands there, Ctrl+Space works). Add an explicit emptyCellModel (e.g. a CellModel_Constant "<no items>") only to customize the empty cell — but a plain constant is read-only and loses the instant-typing, so to keep it, put the hint in the constant's nullText (not text) and add an EditableStyleClassItem (flag=true). See the Editable Empty-List Placeholder pattern in references/editor-patterns.md.
• Inheritance: components can be reused if defined for the concept, its super-concepts, or implemented interfaces.
• Componentize shared cells: when several concept editors repeat the same cell group, factor it into an EditorComponentDeclaration (bound to the most general concept or interface that owns the relations) and reuse it via CellModel_Component. See references/editor-components.md.
• Stylesheets: check for existing style classes in extended languages before creating new ones in a StyleSheet node.

Common Workflow

0. Check whether the editor model already exists — it almost always does. editor is one of a language's default aspects (alongside structure, constraints, behavior, typesystem), so any non-trivial language already has it. Confirm with mps_mcp_get_project_structure (it lists the module's models) before doing anything else. Only when a brand-new language genuinely has no editor model do you create one — with mps_mcp_create_model with moduleName: "<lang>" and modelName: "<lang>.editor" (aspect ID editor, case-sensitive, no @ suffix; see aspect-model-stereotypes.md). Action maps, keymaps, transformation/substitute menus live in the same editor model — there is no separate aspect ID for them.
1. Scaffold first — mps_mcp_scaffold_editor builds a default editor that wires properties/children/references to sensible cell models.
2. Componentize reusable cell groups into an EditorComponentDeclaration and embed them with CellModel_Component — see references/editor-components.md for the manual and mps_mcp_scaffold_editor (type="component", includeComponents, detectComponents) paths.
3. Refine cell choice and layout using the catalog below.
4. Inspect with mps_mcp_print_node (structural, textual or HTML projection).
5. Validate with mps_mcp_check_root_node_problems.

Node cell layout vs. inspected cell layout

A ConceptEditorDeclaration holds two editor sections:

• Node cell layout — the cellModel child (cardinality 1). The main editor, shown inline in the document where the node appears.
• Inspected cell layout — the inspectedCellModel child (cardinality 0..1). A secondary definition shown in the Inspector tool window when the caret is on this node in the main editor. Typically used for less-important or less-frequently read/modified properties so they stay out of the main layout.

Both roles accept any EditorCellModel and are built the same way (collections, property cells, ref cells, …). The inspected section is optional — omit inspectedCellModel when everything belongs in the main editor. See references/editor-patterns/inspected-cell-layout.md.

Related Skills

• mps-aspect-editor-menus-and-keymaps — for action maps, cell keymaps, transformation menus, substitute menus, side transforms, paste wrappers, copy-paste handlers, completion styling, and reference presentation. Use this skill alongside that one when authoring an editor end-to-end.
• mps-aspect-structure-concepts — concept declarations supply the relations (properties / children / references) the editor wires up.
• mps-language-aspects-overview — for the broader authoring order around editors.

Reference Index

Start here — most common case: laying out cells for one concept → read only references/cell-models.md; a known layout shape (child on its own line, indented block, vertical list) → only references/editor-patterns.md; a style item's accepted literals → only references/stylesheet-values.md.

• Open references/cell-models.md for the catalog of cell-model concepts (ConceptEditorDeclaration, EditorComponentDeclaration, CellModel_Collection, CellModel_Constant, CellModel_Property, CellModel_ReadOnlyModelAccessor, CellModel_RefNode, CellModel_RefNodeList, CellModel_RefCell, CellModel_Component, InlineEditorComponent) with their suffixes and purpose.
• Open references/indent-layout-styles.md for the style-item catalog (IndentLayoutOnNewLineStyleClassItem, IndentLayoutIndentStyleClassItem, IndentLayoutNewLineChildrenStyleClassItem, SelectableStyleSheetItem) used inside CellLayout_Indent.
• Open references/editor-patterns.md for ready-to-paste JSON / pseudo-code patterns: indented child on a new line, named concept with keyword header, transparent list, smart-reference cell, a minimal ConceptEditorDeclaration JSON template, a concept editor with an inspected cell layout, the empty-constant vertical-gap separator, the editable empty-list placeholder with a disappearing nullText hint (EditableStyleClassItem), and the annotation (node attribute) editor with CellModel_AttributedNodeCell (attributed node), guillemet labels, and keyboard-deletability.
• Open references/editor-components.md for how to factor editor design into reusable editor components: what an EditorComponentDeclaration is, the inheritance rule, and step-by-step manual (mps_mcp_insert_root_node_from_json + CellModel_Component) and scaffold (mps_mcp_scaffold_editor) recipes with a worked Kaja example.
• Open references/stylesheet-values.md for the enum literals accepted by stylesheet items: _Colors_Enum (darkBlue, gray, darkGreen, black, …) for ForegroundColorStyleClassItem.color and other color items, _FontStyle_Enum (PLAIN, BOLD, ITALIC, BOLD_ITALIC, QUERY) for FontStyleStyleClassItem.style, plus UnderlineStyle and FocusPolicy.