Custom Visualization Basics

As seen in the Sterling Visualizer section, Forge uses an adapted version of the Sterling visualization tool. One of the advantages of using Sterling is that users can create their own custom visualization scripts.

Script View and Modes

When viewing an instance in Sterling, there are three options for viewing an instance: Graph, Table, and Script. To swap to script-view mode, click on Script. Most of the screen will be taken up by 3 new panes:

  • the visualization canvas (blank by default);
  • the script editor; and
  • a list of available variables that are available for scripts to use.

Where the "Next" button was in the graph visualizer, there will now be 4 more buttons:

  • Run executes the visualization script when clicked.
  • <div>, <canvas> and <svg> swap between visualization modes. The default is <svg>. This documentation focuses on the <svg> mode, because that is where most of the library support is.

A Basic Script

Try entering some visualization code in the script window and clicking "Run":

const stage = new Stage()
stage.add(new TextBox({
  text: 'Hello!', 
  coords: {x:100, y:100},
  color: 'black',
  fontSize: 16
stage.render(svg, document)

This will create a TextBox in the visualization area that says "Hello!".

Custom scripts are written in JavaScript. JavaScript constructs like looping, mapping over lists, etc. are all available for your use.


At the moment (February 2023), the script editor works only with "vanilla" JavaScript; Sterling will not run (e.g.) a TypeScript compiler on the code. Moreover, importing external libraries is somewhat restricted.

Working with an Instance

Sterling uses a number of JavaScript classes to represent an instance. The alloy-ts library documentation describes these in more detail, but you should be aware of the following:

The Instance

The instance variable provides access to the current instance. If temporal mode is active, instance will refer to the current state; to get the entire list of states, use instances instead (which is only available in temporal mode).

Accessing Sigs and Fields

You can use the .signature(name) method of an instance to obtain an object representing a sig in the instance. Likewise, .field(name) will yield an object representing a particular field.

A Basic Script

If you're viewing an instance of the binary search model from class, you can run this script directly. (If you use this model, uncomment the run at the bottom of the file!)

const stage = new Stage()
stage.add(new TextBox({
  text: `${instance.signature('IntArray')}`, 
  coords: {x:100, y:100},
  color: 'black',
  fontSize: 16
stage.render(svg, document)

This will create a TextBox in the visualization area whose text is a string representation of the Int sig in whatever instance you're viewing. The string won't be very useful yet; it will be something like "[IntArray]". Next, change instance.signature('IntArray') to instance.signature('IntArray').atoms()[0]---the first (and in this case, only) IntArray object in the instance. You'll see the sig name become an object id like [IntArray0].

All IntArray objects have an elements field. We can get access to the field and print its contents by joining the object to its field:

const stage = new Stage()
const theArray = instance.signature('IntArray').atoms()[0]
const elementsField = instance.field('elements')
stage.add(new TextBox({
  text: `${theArray.join(elementsField)}`, 
  coords: {x:100, y:100},
  color: 'black',
  fontSize: 16}))
stage.render(svg, document)

This should display something like "1, 7 2, 7 3, 7 4, 7 5, 7 6, 7 7, 7". Again, not very useful. Sterling is printing the contents of the array in index,value form, but separating elements with space. We can fix this, though!

Built-in Library Shapes

Scripts in <svg> mode use the D3 visualization library by default. However, D3 can be fairly complex to use, and so various built-in helpers are also available. We encourage using the helper library, although in more advanced cases you may wish to use D3 directly.

Potential Pitfalls and Debugging

Sterling presents a "full Forge" view of instances, and one that's closer to the way the solver works. All sigs and fields are represented as sets in the instance. Each set contains AlloyTuple objects, which themselves contain lists of AlloyAtoms. Because (at the moment) there is no support for types in the script editor, it can sometimes be troublesome to remember which kind of datum you're working with. The alloy-ts docs below provide much more detail, but in brief:

  • if the object has an .atoms() method, it's an AlloyTuple;
  • if the object has an .id() method, it's an AlloyAtom; and
  • signatures, fields, tuples, and atoms are all AlloySets and provide a .tuples() method.

In order to do a Froglet-style field access, you should use the .join method. (E.g., in the example above, we wrote theArray.join(elementsField) rather than theArray.elementsField or theArray.elements.)

We suggest using TextBoxes to visualize debugging information. As a fallback, you can also use console.log(...) as normally in JavaScript, but the web-developer console in Sterling can be somewhat congested. Printing raw AlloySet objects via console.log will also not be immediately useful, since Sterling uses a proxy to manage the difference between the set as Forge syntax and the set as a value in the instance.

Further Resources and Examples

Further chapters:

External resources:

Examples using D3:


Work on custom visualization in Sterling is an ongoing collaborative effort. We are grateful to Tristan Dyer for working to expand Sterling for Forge. We are also grateful for contributions from (in alphabetic order):

  • Ethan Bove
  • Sidney LeVine
  • Mia Santomauro