The typesetting process

In order to unserstand the TeXmacs document format well, it is useful to have a basic understanding about how documents are typeset by the editor. The typesetter mainly rewrites logical TeXmacs trees into physical boxes, which can be displayed on the screen or on paper (notice that boxes actually contain more information than is necessary for their rendering, such as information about how to position the cursor inside the box or how to make selections).

The global typesetting process can be subdivided into two major parts (which are currently done at the same stage, but this may change in the future): evaluation of the TeXmacs tree using the stylesheet language, and the actual typesetting.

The typesetting primitives are designed to be very fast and they are built-in into the editor. For instance, one has typesetting primitives for horizontal concatenations (concat), page breaks (page-break), mathematical fractions (frac), hyperlinks (hlink), and so on. The precise rendering of many of the typesetting primitives may be customized through the built-in environment variables. For instance, the environment variable color specifies the current color of objects, par-left the current left margin of paragraphs, etc.

The stylesheet language allows the user to write new primitives (macros) on top of the built-in primitives. It contains primitives for definining macros, conditional statements, computations, delayed execution, etc. The stylesheet language also provides a special extern tag which offers you the full power of the Scheme extension language in order to write macros.

It should be noticed that user-defined macros have two aspects. On the one hand they usually perform simple rewritings. For instance, the macro

is a shortcut in order to produce sequences like a₁,…,a_n. When macros perform simple rewritings like in this example, the children var, from and to of the seq tag remain accessible from within the editor. In other words, you can position the cursor inside them and modify them. User defined macros also have a synthetic or computational aspect. For instance, the dots of a seq tag as above cannot be edited by the user. Similarly, the macro

serves an exclusively computational purpose. As a general rule, synthetic macros are sometimes easier to write, but the more accessability is preserved, the more natural it becomes for the user to edit the markup.

It should be noticed that TeXmacs also produces some auxiliary data as a byproduct of the typesetting product. For instance, the correct values of references and page numbers, as well as tables of contents, indexes, etc. are determined during the typesetting stage and memorized at a special place. Even though auxiliary data may be determined automatically from the document, it may be expensive to do so (one typically has to retypeset the document). When the auxiliary data are computed by an external plug-in, then it may even be impossible to perform the recomputations on certain systems. For these reasons, auxiliary data are carefully memorized and stored on disk when you save your work.