Mutator tags

A mutator tag is of the form <mutator|body|cmd>, where body is the visible body of the tag and cmd a secure Scheme script which is called periodically and which is allowed to modify the body of the mutator (or even any other part of the document). During the execution of cmd the function mutator-path yields the position of the body in the document tree.

Mutator tags are a particularly interesting feature of TeXmacs for producing highly interactive documents. For instance, inside computer algebra sessions, the output is retrieved inside a mutator tag (which automatically removes itself when the output is complete). In the future, mutators might be used for live conferencing or in interfaces with proof systems.

Remark 1. Mutator tags only work properly when the explicitly occur in the document; they will not work if they merely occur in the body of a macro, even if the body of the mutator remains accessible.

Indeed, the current implementation of TeXmacs searches for mutator tags in all documents after a small period of inactivity. Documents which are known not to contain mutator tags are ignored during this research. Of course, this implementation is both quite efficient and incompatible with the macro system. So there is room for future improvements.

Remark 2. For efficiency reasons, it is recommended that mutator tags mainly modify there own bodies and not other parts of the document, except at really exceptional occasions.

The mutator plug-in

A very simple example with two types of mutators is provided in the mutator plug-in. It provides the user with two keyboard shortcuts C-F11 and C-F12, which respectively insert the current time and some blinking text. The plug-in consists of the single file

    mutator/progs/init-mutator.scm

The C-F11 key simply inserts the continuously updated time into the main text:

(kbd-map ("C-F11" (insert '(mutator "" "(mutate-date)"))))

The secure Scheme routine mutate-date is defined as follows:

The tm-assign-diff command is convenient, because it only modifies the document if a real change occurred.

The insertion of blinking content is slightly more complex, since it also takes into account the current content of the mutator tag. The C-F12 key inserts a blinking text into the main text and puts the cursor after the text in the body of the mutator:

(kbd-map ("C-F12" (insert-go-to '(mutator "text" "(mutate-blink)")                                   '(0 4))))

The secure Scheme routine mutate-blink is defined as follows:

Remark 3. Notice that the above examples are only meant to illustrate the use of mutators. Ideally speaking, dates and blinking content should not make use of mutators, since mutators continuously modify the document (think about undoing changes, for instance). In the future, we plan to add primitives like animations and movies to TeXmacs, for which the content remains fixed, but whose presentation changes over time.