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Overview of the LionWeb Model

At the core of the LionWeb systems are nodes, which are organized into trees, and grouped into partitions.

Collections of partitions can be informally referred to as models.

Trees and Partitions​

In LionWeb, data is organized into trees of nodes. Each individual tree is called a partition, which:

  • Has exactly one root node
  • Each node can contain zero or more children, organized through named containments
  • The tree can grow arbitrarily deep, with no limit on the number of children or levels of nesting

Each containment defines a specific slot under which children may appear.

Nodes​

Each node in LionWeb has several important characteristics:

  • It has a globally unique ID, allowing it to be referenced uniquely.
  • It may contain a set of properties, which are named primitive values (e.g., strings, integers, booleans).
  • It may contain children, under several containments
  • It can hold references to other nodes, allowing for graph-like interconnections across trees.
  • It is an instance of a Concept, which defines its structure. This means that all nodes which are instances of the same Concept* will have the same properties, containments, and references, even if they could be empty for some instances.

Properties​

Properties represent the primitive values associated with a node. These are defined by the Concept and can include values such as:

  • name: "MyClass"
  • visibility: public
  • isAbstract: false

The Concept determines:

  • Which properties are available
  • The type of each property
  • Whether a property is required or **optional&&

References​

Nodes may contain references to other nodes, forming non-tree links between elements. Each reference can hold zero, one, or more reference values. Each reference value has:

  • A resolveInfo field: a string representing the logical name or identifier to be used during resolution (e.g., "B" in class A extends B)
  • An optional referenced field: this contains the ID of the node that the reference actually points to, once resolved

This dual mechanism allows references to exist before they are resolved, supporting workflows like parsing incomplete models or deferring resolution to later phases.

For example:

class A extends B { }

In this case:

  • The reference to B would have resolveInfo = "B"
  • If the class B is defined elsewhere and known, then referenced will point to the node ID for B

Concepts and Structure​

Each node is an instance of a Concept, and the Concept defines the nodess shape:

  • Properties: what primitive values the node can or must carry
  • Containments: what children it can have and in which slots
  • References: what links it can maintain to other nodes

Properties, containments, and references are all features.

Concepts enforce both the structure and constraints of the model, much like classes do in object-oriented programming.

To learn more about how languages define Concepts and other elements, see Introduction to Languages.