23.4.2 The Data Structure of the Mode Line
The mode line contents are controlled by a data structure called a mode line construct, made up of lists, strings, symbols, and numbers kept in buffer-local variables. Each data type has a specific meaning for the mode line appearance, as described below. The same data structure is used for constructing frame titles (see Frame Titles) and header lines (see Header Lines).
A mode line construct may be as simple as a fixed string of text, but it usually specifies how to combine fixed strings with variables’ values to construct the text. Many of these variables are themselves defined to have mode line constructs as their values.
Here are the meanings of various data types as mode line constructs:
string​
A string as a mode line construct appears verbatim except for %-constructs in it. These stand for substitution of other data; see %-Constructs.
If parts of the string have face properties, they control display of the text just as they would text in the buffer. Any characters which have no face properties are displayed, by default, in the face mode-line or mode-line-inactive (see Standard Faces in The GNU Emacs Manual). The help-echo and keymap properties in string have special meanings. See Properties in Mode.
symbol​
A symbol as a mode line construct stands for its value. The value of symbol is used as a mode line construct, in place of symbol. However, the symbols t and nil are ignored, as is any symbol whose value is void.
There is one exception: if the value of symbol is a string, it is displayed verbatim: the %-constructs are not recognized.
Unless symbol is marked as risky (i.e., it has a non-nil risky-local-variable property), all text properties specified in symbol’s value are ignored. This includes the text properties of strings in symbol’s value, as well as all :eval and :propertize forms in it. (The reason for this is security: non-risky variables could be set automatically from file variables without prompting the user.)
(string rest…)​
(list rest…)​
A list whose first element is a string or list means to process all the elements recursively and concatenate the results. This is the most common form of mode line construct.
(:eval form)​
A list whose first element is the symbol :eval says to evaluate form, and use the result as a string to display. Make sure this evaluation cannot load any files, as doing so could cause infinite recursion.
(:propertize elt props…)​
A list whose first element is the symbol :propertize says to process the mode line construct elt recursively, then add the text properties specified by props to the result. The argument props should consist of zero or more pairs text-property value. If elt is or produces a string with text properties, all the characters of that string should have the same properties, or else some of them might be removed by :propertize.
(symbol then else)​
A list whose first element is a symbol that is not a keyword specifies a conditional. Its meaning depends on the value of symbol. If symbol has a non-nil value, the second element, then, is processed recursively as a mode line construct. Otherwise, the third element, else, is processed recursively. You may omit else; then the mode line construct displays nothing if the value of symbol is nil or void.
(width rest…)​
A list whose first element is an integer specifies truncation or padding of the results of rest. The remaining elements rest are processed recursively as mode line constructs and concatenated together. When width is positive, the result is space filled on the right if its width is less than width. When width is negative, the result is truncated on the right to -width columns if its width exceeds -width.
For example, the usual way to show what percentage of a buffer is above the top of the window is to use a list like this: (-3 "%p").