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<H2>Issue PRINT-CIRCLE-STRUCTURE Writeup</H2>
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<PRE><B>Status:</B> Passed, Jan 89 X3J13, as amended<P>
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<B>Issue:</B> <A HREF="iss275.htm">PRINT-CIRCLE-STRUCTURE</A><P>
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<B>References:</B> pp. 370-371<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> Version 4, Masinter, 17-Mar-89 (as amended)<P>
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Version 3, Masinter 10/8/88 (minor cleanup)<P>
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Version 2, Chris McConnell 10/05/88<P>
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Version 1, Chris McConnell 09/20/88<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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When a lisp object is printed that points to a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> with a user<P>
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defined print-function and there is a pointer back to the containing<P>
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object, the printer will recurse infinitely even when *print-circle*<P>
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is set to T. This prevents printing circular structures that point to<P>
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objects that cannot be printed and prevents the development of new<P>
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printed representations that can be read by the reader.<P>
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<P>
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<B>Proposal PRINT-CIRCLE-STRUCTURE:USER-FUNCTIONS-WORK: <P>
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</B><P>
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When *print-circle* is T, a user defined print-function can print<P>
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objects to the supplied stream using <A REL=DEFINITION HREF="../Body/f_wr_pr.htm#write"><B>WRITE</B></A>, <A REL=DEFINITION HREF="../Body/f_wr_pr.htm#prin1"><B>PRIN1</B></A>, <A REL=DEFINITION HREF="../Body/f_wr_pr.htm#princ"><B>PRINC</B></A>, or <A REL=DEFINITION HREF="../Body/f_format.htm#format"><B>FORMAT</B></A><P>
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and expect circularities to be detected and printed using #n# syntax.<P>
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If a user defined print-function prints to a stream other than the one<P>
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that was supplied, then circularity detection starts over for that<P>
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stream. This applies to methods on <A REL=DEFINITION HREF="../Body/f_pr_obj.htm#print-object"><B>PRINT-OBJECT</B></A> in addition to<P>
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:PRINT-FUNCTION options.<P>
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<P>
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<B>Test Cases/Examples:<P>
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</B><P>
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;;;<P>
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;;; Define a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> that can be circular and that has a slot with a<P>
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;;; value that cannot be printed.<P>
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;;;<P>
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(<A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A> (TEST (:print-function print-test))<P>
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ptr<P>
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(<A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>function</B></A> #'(<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> (x) <P>
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(<A REL=DEFINITION HREF="../Body/a_error.htm#error"><B>error</B></A> "No <A REL=DEFINITION HREF="../Body/a_fn.htm#function"><B>function</B></A> is defined."))))<P>
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<P>
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;;;<P>
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;;; This print function generates a machine readable printed<P>
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;;; representation for a <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> with a slot that cannot be printed.<P>
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;;;<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>defun</B></A> PRINT-TEST (<A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> <A REL=DEFINITION HREF="../Body/t_stream.htm#stream"><B>stream</B></A> depth)<P>
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(<A REL=DEFINITION HREF="../Body/f_format.htm#format"><B>format</B></A> <A REL=DEFINITION HREF="../Body/t_stream.htm#stream"><B>stream</B></A> "#S(TEST PTR ~S)" (test-ptr <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A>)))<P>
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<P>
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;;;<P>
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;;; Define two structures that point to each other. If this<P>
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;;; expression successfully evaluates at the top level, then the<P>
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;;; printed result should look like:<P>
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;;; #1=#S(TEST PTR #S(TEST PTR #1#))<P>
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;;;<P>
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;;; If it does not work then it will generate an infinite printed<P>
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;;; representation. <P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> <A REL=DEFINITION HREF="../Body/v_pr_cir.htm#STprint-circleST"><B>*print-circle*</B></A> t<P>
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*a (make-test)<P>
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*b (make-test)<P>
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(test-ptr *a) *b<P>
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(test-ptr *b) *a)<P>
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<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Many structures are circular and point to complex data structures that<P>
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may include things like closures that cannot be printed. It should be<P>
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possible to define a way to print these data structures such that they<P>
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can be read back in. Common LISP provides two mechanisms for these<P>
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problems (*print-circle* and the :print-function option to <A REL=DEFINITION HREF="../Body/m_defstr.htm#defstruct"><B>defstruct</B></A>),<P>
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but they do not currently work together in most implementations.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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Lucid 3.0 and the Genera do work on the test case. (Previous versions<P>
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did not.) KCL, Coral and Franz do not work.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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Lucid and Symbolics have done it, so they could give an idea of the<P>
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difficulty. Possible techniques include passing the printer state<P>
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information by dynamic binding rather than by explicit parameters or<P>
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by supplying an internal stream to the user print function.<P>
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<P>
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Cost to Users: None<P>
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<P>
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<B>Cost of non-adoption: <P>
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</B><P>
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Currently this problem causes an infinite recursion whenever a user<P>
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print-function prints a lisp object that contains the <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> that<P>
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is being printed by the user print function. If nothing is done, this<P>
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error will remain and the whole effort to <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> a portable printed<P>
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representation of LISP structures is of minimal usefulness. In almost<P>
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any real application, there are circular structures with non printable<P>
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objects such as closures and hash tables that need to be printed. In<P>
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addition the development of printers for new reader macros becomes<P>
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much more of an effort then it should be since it requires<P>
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reimplementing the complete circularity detection mechanism.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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If the proposal is adopted, then it becomes easier to define new<P>
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printed representations that are compact and that still capture the<P>
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information needed to rebuild data <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> instances. It allows a<P>
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printed representation to hide the actual details of how a data<P>
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<A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> is implemented in terms of underlying LISP data structures.<P>
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<P>
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<B>Esthetics: <P>
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</B><P>
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This proposal increases the uniformity of the language by making<P>
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*print-circle* work in all cases including where a user has defined a<P>
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new print function.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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At least three members of the cleanup committee read this and think<P>
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it looks good.<P>
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</PRE>
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