22.1.1 Overview of The Lisp Printer

Common Lisp provides a representation of most objects in the form of printed text called the printed representation. Functions such as print take an object and send the characters of its printed representation to a stream. The collection of routines that does this is known as the (Common Lisp) printer.

Reading a printed representation typically produces an object that is equal to the originally printed object.

22.1.1.1 Multiple Possible Textual Representations

Most objects have more than one possible textual representation. For example, the positive integer with a magnitude of twenty-seven can be textually expressed in any of these ways:

 27    27.    #o33    #x1B    #b11011    #.(* 3 3 3)    81/3

A list containing the two symbols A and B can also be textually expressed in a variety of ways:

 (A B)    (a b)    (  a  b )    (\A |B|)
(|\A|
  B
)

In general, from the point of view of the Lisp reader, wherever whitespace is permissible in a textual representation, any number of spaces and newlines can appear in standard syntax.

When a function such as print produces a printed representation, it must choose from among many possible textual representations. In most cases, it chooses a program readable representation, but in certain cases it might use a more compact notation that is not program-readable.

A number of option variables, called printer control variables, are provided to permit control of individual aspects of the printed representation of objects. The next figure shows the standardized printer control variables; there might also be implementation-defined printer control variables.

In addition to the printer control variables, the following additional defined names relate to or affect the behavior of the Lisp printer:

22.1.1.1.1 Printer Escaping

The variable *print-escape* controls whether the Lisp printer tries to produce notations such as escape characters and package prefixes.

The variable *print-readably* can be used to override many of the individual aspects controlled by the other printer control variables when program-readable output is especially important.

One of the many effects of making the value of *print-readably* be true is that the Lisp printer behaves as if *print-escape* were also true. For notational convenience, we say that if the value of either *print-readably* or *print-escape* is true, then printer escaping is “enabled”; and we say that if the values of both *print-readably* and *print-escape* are false, then printer escaping is “disabled”.