Sigs (short for "signatures") are the basic building block of any model in Forge. They represent the types of the system being modeled. To declare one, use the sig keyword.

sig <name> {}

A sig can also have one or more fields, which define relationships between members of that sig other atoms. The definition above has no fields because the braces are empty. In contrast, this sig definition would have many fields:

sig <name> {

Syntax Note

Ensure that there is a comma after every field except for the last one. This is a common source of compilation errors when first defining a model!


Fields allow us to define relationships between a given sigs and other components of our model. Each field in a sig has:

  • a name for the field;
  • a multiplicity (one, lone, pfunc, func, or, in Relational or Temporal Forge, set);
  • a type (a -> separated list of sig names, including the built-in sig Int).

Example sig definition

Here is a sig that defines the Person type from the overview.

sig Person {
    bestFriend: lone Person

The lone multiplicity says that the field may contain at most one atom. (Note that this example has yet to express the constraint that everyone has a friend!)

More Examples

Let's look at a few more examples.

Example: Sig with one field

Basic Sig with Fields (Linked List):

A model of a circularly-linked list might have a sig called Node. Node might then have a field next: one Node to represent the contents of every Node's next reference. We use one here since every Node always has exactly one successor in a circularly linked list.

sig Node {
    next: one Node

Example: Sig with multiple fields

Basic Sig with Fields (Binary Tree):

A model of a binary tree might have a sig called Node. Node might then have three fields:

  • left: lone Node and right: lone Node to represent the Node's children. We use lone here since the left/right child fields can either be empty or contain exactly one Node.
  • val: one Int to represent the value of each Node, where we have decided that every Node should have an Integer value. We use one here because each Node should have exactly one value.
sig Node {
    left: lone Node,
    right: lone Node,
    val: one Int

Int is a built-in sig provided by Forge. To learn more about how Forge handles integers, see Integers in Forge.

Example: Sig with No Fields

Example - Basic Sig without Fields:

Not every sig in a model needs to have fields to be a useful part of the model! sigs with no fields are often used in conjunction with other sigs that reference them. One such example might look like this:

sig Student {}
sig Group {
    member: set Student

Note that the set multiplicity is only available in Relational and Temporal Forge, not Froglet.

Field names must be unique

You cannot use the same field name within two different sigs in a model. This is because field names are globally available for writing constraints.