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Once the structure for your language is defined, you will probably go and create means to allow developers to build ASTs with it. Manipulating the ASTs directly would not be very intuitive nor productive. To hide the AST and offer the user comfortable and intuitive interaction is the role for language editors.

Editor Overview

An editor for a node serves as its view as well as its controller. An editor displays the node and lets the user modify, replace, delete it and so on. Nodes of different concepts have different editors. A language designer should create an editor for every concept in his/her language.

In MPS, an editor consists of cells, which themselves contain other cells, some text, or a UI component. Each editor has its concept for which it is specified. A concept may have no more than one editor declaration (or can have none). If a concept does not have an editor declaration, its instances will be edited with an editor for the concept's nearest ancestor that has an editor declaration.

To describe an editor for a certain concept (i.e. which cells have to appear in an editor for nodes of this concept), a language designer will use a dedicated language simply called editor language. You see, MPS applies the Language Oriented Programming principles to itself.

The description of an editor consists of descriptions for cells it holds. We call such descriptions "cell models." For instance, if you want your editor to consist of a unique cell with unmodifiable text, you create in your editor description a constant cell model and specify that text. If you want your editor to consist of several cells, you create a collection cell model and then, inside it, you specify cell models for its elements. And so on.

Types Of Cell Models

Constant cell

This model describes a cell which will always contain the same text. Constant cells typically mirror "keywords" in text-based progamming languages.

Collection cell

A cell which contains other cells. Can be horizontal (cells in a collection are arranged in a row), vertical (cells are on top of each other) or have so-called "indent layout" (cells are arranged horizontally but if a line is too long it is wrapped like text to the next line, with indent before each next line).

Property cell

This cell model describes a cell which will show a value of a certain property of a node. The value of a property can be edited in a property cell, therefore, a property cell serves not only as a view also but as a controller. In an inspector, you can specify whether the property cell will be read-only or will allow its property value to be edited.

Child cell

This cell model contains a reference to a certain link declaration in a node's concept. The resulting cell will contain an editor for the link's target (almost always for the child, not the referent). For example if you have a binary operation, say " + ", with two children, "leftOperand" and "rightOperand", an editor model for your operation will be the following: a indent collection cell containing a referenced node cell for the left operand, a constant cell with " + ", and a referenced node cell for the right operand. It will be rendered as an editor for the right operand, then a cell with " + ", and then an editor for the left operand, arranged in a row. As we have seen, and as follows from its name, this type of cell model is typically used to show editors for children.

Referent cell

Used mainly to show reference targets. The main difference between a referent cell and a child cell is that we don't need, or don't want, to show the whole editor for a reference target. For example, when a certain node, say, a class type, has a reference to a java class, we don't want to show the whole editor for that class with its methods, fields, etc - we just want to show its name. Therefore child cells cannot be used for such a purpose. One should use referent cells.
Referent cell allows you to show a different inlined editor for a reference target, instead of using target's own editor. In most cases it's very simple: a cell for a reference target usually consists only of a property cell with the target's name.

Child list cell

This cell is a collection containing multiple child cells for a node's children of the same role. For instance, an editor for a method call will contain a child list cell for rendering its actual arguments. Child list can be indent (text like), horizontal or vertical.
The cell generated from this cell model supports insertion and deletion of the children of the role given, thus serving both as a view and as a controller. The default keys for insertion are Insert and Enter (to insert a child before or after the selected one, respectively), and the default key for deletion is Delete. You also can specify a separator for your list.
A separator is a character which will be shown in constant cells between cells for the children. When you are inside the cell list and you press a key with this character, a new child will be inserted after the selected child. For instance, a separator for a list representing actual parameters in a method call is a comma.
In inspector, you can specify whether the resulting cell list will use folding or not, and whether it will use braces or not. Folding allows your cell list to contract into a single cell (fold) and to expand from it (unfold) when necessary. It is useful for a programmer writing in your language when editing a large root: he/she is able to fold some cells and hide all the information in editor that is not necessary for the current task at the moment. For instance, when editing a large class, one can fold all method bodies except the method he/she is editing at the moment.

Indent cell

An indent cell model will be generated into a non-selectable constant cell containing a whitespace. The main difference between a cell generated from an indent cell and one generated from a constant cell model containing whitespaces as its text is that the width of an indent cell will vary according to user-defined global editor settings. For instance, if a user defines an indent to be 4 spaces long, then every indent cell will occupy a space of 4 characters; if 2 spaces long, then every indent cell will be 2 characters.

UI component cell

This cell model allows a language designer to insert an arbitrary UI component inside an editor for a node. A language designer should write a function that returns a JComponent, and that component will be inserted into the generated cell. Note that such a component will be re-created every time an editor is rebuilt, so don't try to keep any state inside your component. Every state should be taken from and written into a model (i.e. node, its properties and references) - not a view (your component).
A good use case for such a cell model is when you keep a path to some file in a property, and your component is a button which activates a modal file chooser. The default selected path in a file chooser is read from the above-mentioned property, and the file path chosen by the user is written to that property.

Model access

A model access cell model is a generalization of a property cell and, therefore, is more flexible. While a property cell simply shows the value of a property and allows the user to change that value, a model access cell may show an arbitrary text based on the node's state and modify the node in an arbitrary way based on what changes the user has made to the cell's text.
While making a property cell work requires you only to specify a property to access via that cell, making a model access cell work requires a language designer to write three methods: "get," "set," and "validate." The latter two are somewhat optional.
A "get" method takes a node and should return a String, which will be shown as the cell's text. A "set" method takes a String - the cell's text - and should modify a node according to this String, if necessary. A "validate" method takes the cell's text and returns whether it is valid or not. If a text in a cell becomes invalid after a user change, then it is marked red and is not passed to the "set" method.
If a "validate" method is not specified, a cell will always be valid. If a "set" method is not specified, no changes in a cell's text will affect its node itself.

Custom cell

If other cell models are not enough for a language designer to create the editor he/she wants, there's one more option left for him/her: to create a cell provider which will return an arbitrary custom cell. The only restriction is that it should implement an "EditorCell" interface.

Editor Components and editor component cells

Sometimes two or more editor declarations for different concepts have a common part, which is duplicated in each of those editors. To avoid redundancy, there's a mechanism called editor components. You specify a concept for which an editor component is created and create a cell model, just as in concept editor declaration. When written, the component could then be used in editor declarations for any of the specified concept's descendants. To use an editor component inside your editor declarations, one will create a specific cell model: editor component cell model, and set your editor component declaration as the target of this cell model's reference.

Cell layouts

Each collection cell has property "cell layout", which describes how child nodes will be placed. There is several layouts:

  • indent layout - places cells like text.
  • horizontal layout - places cells horizontally in row.
  • vertical layout - places cells vertically.


Each cell model has some appearance settings that determine the cell's presentation. They are, for instance, font color, font style, whether a cell is selectable, and some others. Those settings are combined into an entity called stylesheet. A stylesheet could be either inline, i.e. be described together with a particular cell model, or it could be declared separately and used in many cell models.

It is good practice to declare a few stylesheets for different purposes. Another good practice is to have a style guideline in mind when developing an editor for your language, as well as when developing extensions for your language. For example, in Base Language there are styles for keywords (applied to those constant cells in Base Language editor which correspond to keywords in Java), static fields (applied to static field declarations and static field references), instance fields, numeric literals, string literals, and so forth. When developing an extension to Base Language, you should apply keyword style to new keywords, field style to new types of fields, and so forth.

A stylesheet is quite similar to CSS stylesheets; it consists of a list of style classes, in which the values for some style properties are specified. A style class may extend another style class: if some style property value is unspecified in a descendant class, its value will be inherited from the extended class.

Style properties

Boolean style properties

  • Is selectable - whether the cell can be selected. True by default.
  • Is editable - whether one can modify text in a cell or not. By default is false for constant cell models, true for other cell models.
  • Draw border - whether border will be drawn around a cell
  • Draw brackets - whether brackets will be drawn around a cell
  • First position allowed/last position allowed - for text-containing cells, specifies whether it is allowed that a caret is on the first/last position (i.e. before/after the whole text of a cell)
    You can either choose a property value from a completion menu or specify a query i.e. a function which returns a boolean value.

Padding properties.

  • Left/right padding. It's a floating point number which specifies the padding of a text cell, i.e. how much space will be between cell's text and cell's left and right sides, respectively.

Punctuation properties.

All cells in collection are separated with one space by default. Sometimes we need cells placed together.

  • Punctuation left - if this property is true, space from left side of the cell is deleted and first position in cell becomes not allowed.
  • Punctuation right - if this property is true, space from right side of the cell is deleted and last position in cell becomes not allowed.
  • Horizontal gap - specifies gap size between cells in collection. Default value is 1 space.

For example in code

we don't want spaces between "(" and "1", and between "1" and ")". So we should add property punctuation-right to the cell "(", and property
punctuation-left to the cell ")".

Color style properties

  • Text foreground color - cell text's color (affect text cells only)
  • Text background color - cell text's background color (affects text cells only)
  • Background color - the background color of a cell. Affects any cell. If a text cell has non-zero padding and some text background color, the cell's background color will be the color of its margins.
    You can either choose a color from the completion menu or specify a query i.e. a function which returns a color.

Indent layout properties

  • Indent layout indented - all lines will be placed with indent. This property can be used for indent in code block.

  • Indent layout new line - after this cell will be placed new line.

  • Indent layout on new line - this cell will be placed on new line
  • Indent layout children new line - all children of collection will be placed on new line

  • Indent layout no wrap - line won't be wrapped before this cell

Other style properties

  • Font size
  • Font style. Can be either plain, bold, italic, or bold italic.
  • Layout constraint.
    • For flow layout
      • none - default behavior
      • punctation - means that previous item in flow layout should always be placed on the same line as the item which this constraint is assigned to.
      • noflow - excludes a cell from flow layout. Current line is finished and item is placed below it. After this item a new line is started and normal flow layout is applied. This style can be used to embed a picture inside of text.
  • Underlined. Can be either underlined, not underlined, or as is ('as is' means it depends on properties of the enclosing cell collection).

Cell actions

Every cell model may have some actions associated with it. Such actions are meant to improve usability of editing. You can specify them in an inspector of any cell model.

Key maps

You may specify a reference to a key map for your cell model. A key map is a root concept - a set of key map items each consisting of a keystroke and an action to perform. A cell generated from a cell model with a reference to a certain key map will execute appropriate actions on keystrokes.

In a key map you must specify a concept for which a key map is applicable. For instance, if you want to do some actions with an expression, you must specify Expression as an applicable concept; then you may specify such a key map only for those cell models which are contained inside editor declarations for descendants of Expression, otherwise it is a type error.

If a key map property "everyModel" is "true," then this key map behaves as if it is specified for every cell in the editor. It is useful when you have many descendants of a certain concept which have many different editors, and your key map is applicable to their ancestor. You need not specify such a key map in every editor if you mark it as an "every model" key map.

A key map item consists of the following features:

  • A function which is executed when a key map item is triggered (returns nothing)
  • A set of keystrokes which trigger this key map item
  • A boolean function which determines if a key map item is applicable here (if not specified, then it's always applicable). If a key map item is not applicable the moment it is triggered, then it will not perform an action.
  • You may specify caret policy for a key map item. Caret policy says where in a cell a caret should be located to make this key map item enabled. Caret policy may be either first position, last position, intermediate position, or any position. By default, caret policy is "any position." If a caret in a cell does not match the caret policy of a key map item the moment it is triggered, then this key map item will not perform an action.

Action maps

A cell model may contain a reference to an action map. An action map overrides some default cell actions (delete and right transform) for a certain concept. An action map consists of several action map items. In an action map, you must specify a concept for which the action map is applicable.

An action map item contains:

  • an action description which is a string,
  • and a function which performs an action (returns nothing).

An action map item may override one of two default actions: default delete action or right transform (see Actions). For instance, when you have a return statement without any action maps in its editor, and you press Delete on a cell with the keyword "return," the whole statement is deleted. But you may specify an action map containing a delete action map item, which instead of just deleting return statement replaces it with an expression statement containing the same expression as the deleted return statement.

action DELETE description : <no description>
              execute : (node, editorContext)->void {
                           node < ExpressionStatement > expressionStatement = node . replace with new ( ExpressionStatement ) ;
                           expressionStatement . expression . set ( node . expression ) ;

Cell menus

One may specify a custom completion menu for a certain cell. Open an inspector for your cell declaration, find a table named Common, find a row named menu, create a new cell menu descriptor. Cell menu descriptor consists of menu parts, which are of differend kinds, which are discussed below.

Property values menu part

This menu part is available on property cells, it specifies a list of property values for your property which will be shown in completion. One should write a function which returns a value of type list<String>.

Property postfix hints menu part

This menu part is available on property cells, it specifies a list of strings which serve as "good" postfixes for your property value. In such a menu part one should write a function which returns a value of type list<String>. Such a menu is useful if you want MPS to "guess" a good value for a property. For instance, one may decide that it will be a good variable name which is a variable type name but with the first letter being lowercased, or which ends with its type name: for a variable of type "Foo" good names will be "foo", "aFoo", "firstFoo", "goodFoo", etc. So one should write in a variable declaration's editor in a menu for property cell for variable name such a menu part:

property postfix hints
   postfixes : (scope, operationContext, node)->list<String> {
                  list < String > result ;
                  node < Type > nodeType = node . type ;
                  if ( nodeType != null ) {
                     result = MyUtil.splitByCamels( nodeType . getPresentation() );
                  } else {
                     result = new list < String > { empty } ;
                  return  result ;

where splitByCamels() will be a function which returns a list of postfixes of a string starting with capitals (for instance MyFooBar -> MyFooBar, FooBar, Bar).

Primary replace child menu

It's a cell menu part which returns primary actions for child (those by default, as if no cell menu exists).

Primary choose referent menu

It's a cell menu part which returns primary actions for referent (those by default, as if no cell menu exists).

Replace node menu (custom node's concept)

This kind of cell menu parts allows to replace an edited node (i.e. node on which a completion menu is called) with instances of a certain specified concept and its subconcepts. Such a cell menu part is useful, for example, when you want a particular cell of your node's editor to be responsible for replacement of a whole node. For instance, consider an editor for binary operations. There's a common editor for all binary operations which consists of a cell for left operand, a cell for operation sign which is a cell for concept property "alias" and a cell for right operand.

[> % leftExpression % ^{{ alias }} % rightExpression % <]

It is natural to create a cell menu for a cell with operation sign, which will allow to replace an operation sign with another one, (by replacing a whole node of course). For such a purpose one will write in the cell for operation sign a replace node menu part:

replace node (custom node concept)
   replace with : BinaryOperation

The former left child and right child are added to newly created BinaryOperation according to Node Factories for BinaryOperation concept.

Replace child menu (custom child's concept)

Such a cell menu part is applicable to a cell for a certain child and specifies a specific concept which and subconcepts of which will be shown in completion menu (and instantiated when chosen and the instance will be set as a child). To specify that concept one should write a function which returns a value of a type node<ConceptDeclaration>.

Replace child menu (custom action).

This kind of cell menu parts is applicable to a cell for a certain child and allows one to customize not only child concept, but the whole replace child action: matching text (text which will be shown in completion menu), description text (a description of an action, shown in the right part of completion menu), and the function which creates a child node when the action is selected from completion menu. Hence, to write such a menu one should specify matching text, description text and write a function returning a node (this node should be an instance of a target concept specified in a respective child link).

Generic menu item

This kind of cell menu part allows one to make MPS perform an arbitrary action when a respective menu item will be selected in a completion menu. One should specify matching text for a menu item and write a function which does what one wants. For instance, one may not want to show a child list cell for class fields if no class fields exist. Hence one can't use its default actions to create a new field. Instead, one can create somewhere in a class' editor a generic menu item with matching text "create field" which creates a new field for a class.

generic item
   matching text : add field
   handler : (node, model, scope, operationContext)->void {
                node . field . add new ( <default> ) ;

Action groups

An action group is a cell menu part which returns a group of custom actions. At runtime, during the menu construction, several objects of a certain type, which are called parameter objects, are collected or created. For that parameter object type of an action group functions, which return their matching text and description text, are specified. A function which is triggered when a menu item with a parameter object is chosen is specified also.

Thus, an action group description consists of:

  • a parameter object type;
  • a function which returns a list of parameter objects of a specified type (takes an edited node, scope and operation context);
  • a function which takes a parameter object of a specified type and returns matching text (a text which will be shown in a completion menu);
  • a function which takes a parameter object of a specified type and returns description text for a parameter object;
  • a function which performs an action when parameter object is chosen in a completion menu.

A function which performs an action may be of different kinds, so there are three different kinds of cell action group menu parts:

  • Generic action group. Its action function, given a parameter object, performs an arbitrary action. Besides the parameter object, the function is provided with edited node, its model, scope and operation context.
  • Replace child group. It is applicable to child cells and its action function, given a parameter object, returns a new child, which must have a type specified in a respective child link declaration. Besides the parameter object, the function is provided with edited node, its model, current child(i.e. a child being replaced), scope and operation context.
  • Replace node group. Its action function, given a parameter object, returns a node. Usually it is some referent of an edited node (i.e. node on which a completion menu is called). Besides the parameter object, the function is provided with edited node, its model, scope and operation context.

Cell menu components

When some menu parts in different cells are equal one may want to extract them into a separate and unique entity, to avoid duplications. For such a purpose cell menu components are meant. A cell menu component consists of a cell menu descriptor (a container for cell menu parts) and a specification of an applicable feature. A specification of applicable feature contains a reference to a feature (i.e. child link declaration, reference link declaration or property declaration), to which a menu is applicable. For instance if your menu component will be used to replace some child its child link declaration should be specified here; etc.

When a cell menu component is created, it can be used in cell menus via cell menu component menu part, which is a cell menu part which contains a reference to a certain menu component.SModel language

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