Issue PRINT-CIRCLE-SHARED Writeup

Status: proposal RESPECT-PRINT-CIRCLE passed, as amended, Jun 89 X3J13

 (another proposal NEW-VALUE was passed and then reconsidered.)


Issue: PRINT-CIRCLE-SHARED

Edit history: Version 1, 		KMP 3/30/89

	      Version 2, Masinter, 2 Jul 89, as amended Jun 89 X3J13


Problem Description:


 A label defined with #n= is valid for the rest of 

 the top-level call to READ on input, permitting

    '(#1=(A B) #1#)

 to designate ((A B) (A B)), where the two lists

 (A B) are EQ.  However, on output the implementations

 are only required to detect circularities, not sharing,

 when *PRINT-CIRCLE* is T.  That is

    (PRINT '(#1=(A #1#) #1#))

 may print as

    (#1=(A #1#) #2=(A #2#))

 or (#1=(A #1#) #1#)


Proposal (PRINT-CIRCLE-SHARED:RESPECT-PRINT-CIRCLE):


 Require the printer to use #n# to identify sharing

 of EQL objects that would not READ to be EQL

 when *PRINT-CIRCLE* is true, and not if false.


Proposal (PRINT-CIRCLE-SHARED:NEW-VARIABLE):


 Introduce *PRINT-SHARED*.  When *PRINT-CIRCLE*

 and *PRINT-SHARED* are both true [rationale: 

 check one variable first for efficiency], identify sharing 

 with #n# in the printer. Otherwise, don't.


Cost to Users:


 Storage needed for detecting shared structure is slightly more.


Current Practice:


 Several implementations implement RESPECT-PRINT-CIRCLE.