Specifically, if *print-readably* is true, printing proceeds as if *print-escape*, *print-array*, and *print-gensym* were also true, and as if *print-length*, *print-level*, and *print-lines* were false.
If *print-readably* is false, the normal rules for printing and the normal interpretations of other printer control variables are in effect.
Individual methods for print-object, including user-defined methods, are responsible for implementing these requirements.
If *read-eval* is false and *print-readably* is true, any such method that would output a reference to the "#." reader macro will either output something else or will signal an error (as described above).
(let ((x (list "a" '\a (gensym) '((a (b (c))) d e f g)))
(*print-escape* nil)
(*print-gensym* nil)
(*print-level* 3)
(*print-length* 3))
(write x)
(let ((*print-readably* t))
(terpri)
(write x)
:done))
(a a G4581 ((A #) D E ...))
("a" |a| #:G4581 ((A (B (C))) D E F G))
:DONE
;; This is setup code is shared between the examples
;; of three hypothetical implementations which follow.
(setq table (make-hash-table)) #<HASH-TABLE EQL 0/120 32005763>
(setf (gethash table 1) 'one) ONE
(setf (gethash table 2) 'two) TWO
;; Implementation A
(let ((*print-readably* t)) (print table))
Error: Can't print #<HASH-TABLE EQL 0/120 32005763> readably.
;; Implementation B
;; No standardized #S notation for hash tables is defined,
;; but there might be an implementation-defined notation.
(let ((*print-readably* t)) (print table))
#S(HASH-TABLE :TEST EQL :SIZE 120 :CONTENTS (1 ONE 2 TWO))
#<HASH-TABLE EQL 0/120 32005763>
;; Implementation C
;; Note that #. notation can only be used if *READ-EVAL* is true.
;; If *READ-EVAL* were false, this same implementation might have to
;; signal an error unless it had yet another printing strategy to fall
;; back on.
(let ((*print-readably* t)) (print table))
#.(LET ((HASH-TABLE (MAKE-HASH-TABLE)))
(SETF (GETHASH 1 HASH-TABLE) ONE)
(SETF (GETHASH 2 HASH-TABLE) TWO)
HASH-TABLE)
#<HASH-TABLE EQL 0/120 32005763>