Forms, Boxes, and Formats

In the introduction to this section we gave the 3-step process converting the input you type into output produced:

  • the input string is parsed to a FullForm S-Expression

  • The FullForm S-expression is evaluated giving another S-expression

  • The result is formatted to the kind of output desired. Formatting can cause additional evaluations to occur.

Here, we are going to go in more detail over the last step which broadly is formatting.


At the top-level, the kind of output you get is dictated by the Form specified either explicitly or implicitly. See Forms of Input and Output.

Formatting a “Form” can cause parts of the expression to be “boxed” as the S-expression is traversed. This is sort of like adding parenthesis around infix expressions at certain places, but we’ll explain in more detail below.

The Box Model

Ever since the typesetting system TeX was developed, most full-fledged formatters follow a two-level approach to formatting output. The current HTML has a box model that follows this approach.

In the Box-Formatting model, groups of items of a similar category are “boxed”, before final layout. In “Box”ing, object details which are not relevant for formatting are hidden. This is good because the objects themself can have wildly different attributes and properties.

On the other hand, common layout formatting properties, such as the box dimensions are exposed. Therefore layout is simplified, and we have the ability to move large collections of items around by simply positioning the enclosing box.

As with S-Expressions, Boxes can be sequenced and nested.

Within a box, once it is given its initial parameters such as the dimensions or style attributes, the layout is largely independent of other boxes in the system.

In the Python code then for each distinct kind of entity, like CompiledCode or a Graphics3D object, there will typically be another class with the the same name but with word Box added at the end. This convention ties to an entity to methods for how to Box the entity. For example, the CompiledCodeBox handles boxing for CompiledCode objects; Graphics3DBox handles boxing for Graphics3D objects.

How an object is boxed may be influenced by context in which the expression appears formatted. For example, if an an image is used inside of a list, we might choose to show the image as a thumbnail rather than a full-size image.

Output Formats

For each Box type, such as a CircleBox there can be a formatter for the kind of output format you want, such as SVG or Asymptote.

If a Box is not defined for a particular entity, there will be an enclosing entity which does define a Boxing routine, and that will need to be be to handle all of the objects inside it whether or not those objects are boxed.