When *print-pretty* is not nil, printing is controlled by the ‘pprint dispatch table’ stored in the variable *print-pprint-dispatch*. The initial value of *print-pprint-dispatch* is implementation-dependent and causes traditional pretty printing of Lisp code. The last section of this chapter explains how the contents of this table can be changed.

A primary goal of pretty printing is to keep the output between a pair of margins. The left margin is set at the column where the output begins. If this cannot be determined, the left margin is set to zero.

When *print-right-margin* is not nil, it specifies the right margin to use when making layout decisions. When *print-right-margin* is nil (the initial value), the right margin is set at the maximum line length that can be displayed by the output stream without wraparound or truncation. If this cannot be determined, the right margin is set to an implementation-dependent value.

To allow for the possibility of variable-width fonts, *print-right-margin* is in units of ems—the width of an “m” in the font being used to display characters on the relevant output stream at the moment when the variables are consulted.

If *print-miser-width* is not nil, the pretty printer switches to a compact style of output (called miser style) whenever the width available for printing a substructure is less than or equal to *print-miser-width* ems. The initial value of *print-miser-width* is implementation-dependent.

When given a value other than its initial value of nil, *print-lines* limits the number of output lines produced when something is pretty printed. If an attempt is made to go beyond *print-lines* lines, “ ..” (a space and two periods) is printed at the end of the last line followed by all of the suffixes (closing delimiters) that are pending to be printed.

(The symbol “..” is printed out to ensure that a reader error will occur if the output is later read. A symbol different from “...” is used to indicate that a different kind of abbreviation has occurred.)