As an example, the two expressions evaluated below have the same "printed" representation, namely ((A) (A)), but they have very different internal structure. Here the structure is shared:
* (let ((item '(a))) (list item item))
((A) (A))
Here each (a) is a distinct entity:
* '((a) (a))
((A) (A))
Specifically, the value of the first expression is a two-element list were the two elements are identical; the value of the second expression is also a two-element list, but its two elements are distinct (i.e. not identical).
How can I represent these two values in a way that makes this difference in their internal structures more apparent?
What you are looking for is *print-circle*:
* (setq *print-circle* t)
T
* '((a) (a))
((A) (A))
* (let ((item '(a))) (list item item))
(#1=(A) #1#)
If you want "print -
read consistency", you should investigate
*print-readably* and
with-standard-io-syntax:
(equal (read-from-string (with-standard-io-syntax (write-to-string x)))
x)
should either return T or signal an error of type
print-not-readable.