Meta information and logical programming

Small software projects usually consist of a collection of routines and data. In a large software project, where a typical contributor has no complete overview of the program, it is a good practive to associate additional meta-information to the individual routines and data. This meta-information typically serves documentation purposes, but becomes even more interesting if it can be used in an automized fashion to implement more general additional functionality.

The tm-define macro supports several options for associating meta-information to Scheme functions and symbols. For instance, the :synopsis, :argument and :returns options allow you to associate short documentation strings to the function, its arguments and its return value:

This information is exploited by TeXmacs in several ways. For instance, the synopsis of the function can be retrieved by executing (help square). More interestingly, assuming that we defined square as above, typing M-x followed by square and return allows you to execute square in an interactive way: you will be prompted for “A number” on the footer. Moreover, after typing M-x, you will be able to use “tab-completion” in order to enter square: typing s q u tab will usually complete into square.

In a similar vein, the :interactive and :check-mark options allow you to specify that a given routine requires interactive user input or when it should give rise to a check-mark when used in a menu. For instance, the statement

in the source code of TeXmacs states that choose-file is an interactive command. As a consquence, the File→Load entry, which is defined by

will be followed by dots ... in the File menu. The interesting point here is that, although the command choose-file may be reused several times in different menu entries, we only have to specify once that it is an interactive command. Similarly, consider the definition

Given a menu item with (toggle-session-math-input) as its associated action, this definition specifies in particular that a check-mark should be displayed before the menu item whenever the session-math-input? predicate holds.

Another frequently used option is :secure, which specifies that a given routine is secure. For instance, the default implementation of the fold tag allows the user to click on the “∘” before the folded text so as to unfold the tag. When doing this, the scheme script (mouse-unfold) is launched. However, for this to work, the mouse-unfold function needs to be secure:

The :secure option is also needed in combination with other tags which depend on Scheme scripts, like extern and mutator.

In the future, the number of options for entering meta-information is likely to increase. TeXmacs also supports an additional mechanism for the automatic deduction of new meta-properties from existing meta-properties. This mechanism is based on a less general, but more efficient form of logical programming. However, since it is not fully stable yet, it will be documented only later.