74 lines
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74 lines
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<TITLE>CLHS: Issue PRINT-CIRCLE-SHARED Writeup</TITLE>
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<H2>Issue PRINT-CIRCLE-SHARED Writeup</H2>
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<PRE><B>Status:</B> proposal RESPECT-PRINT-CIRCLE passed, as amended, Jun 89 X3J13<P>
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(another proposal NEW-VALUE was passed <A REL=DEFINITION HREF="../Body/a_and.htm#and"><B>and</B></A> then reconsidered.)<P>
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<P>
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<B>Issue:</B> <A HREF="iss274.htm">PRINT-CIRCLE-SHARED</A><P>
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<B>Edit history:</B> Version 1, KMP 3/30/89<P>
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Version 2, Masinter, 2 Jul 89, as amended Jun 89 X3J13<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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A label defined with #n= is valid for the rest of <P>
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the top-level call to <A REL=DEFINITION HREF="../Body/f_rd_rd.htm#read"><B>READ</B></A> on input, permitting<P>
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'(#1=(A B) #1#)<P>
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to designate ((A B) (A B)), where the two lists<P>
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(A B) are <A REL=DEFINITION HREF="../Body/f_eq.htm#eq"><B>EQ</B></A>. However, on output the implementations<P>
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are only required to detect circularities, not sharing,<P>
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when <A REL=DEFINITION HREF="../Body/v_pr_cir.htm#STprint-circleST"><B>*PRINT-CIRCLE*</B></A> is T. That is<P>
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(<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> '(#1=(A #1#) #1#))<P>
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may print as<P>
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(#1=(A #1#) #2=(A #2#))<P>
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or (#1=(A #1#) #1#)<P>
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<P>
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<B>Proposal (PRINT-CIRCLE-SHARED:RESPECT-PRINT-CIRCLE):<P>
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</B><P>
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Require the printer to use #n# to identify sharing<P>
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of <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> objects that would not <A REL=DEFINITION HREF="../Body/f_rd_rd.htm#read"><B>READ</B></A> to be <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A><P>
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when <A REL=DEFINITION HREF="../Body/v_pr_cir.htm#STprint-circleST"><B>*PRINT-CIRCLE*</B></A> is true, and not if false.<P>
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<P>
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<B>Proposal (PRINT-CIRCLE-SHARED:NEW-VARIABLE):<P>
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</B><P>
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Introduce *PRINT-SHARED*. When <A REL=DEFINITION HREF="../Body/v_pr_cir.htm#STprint-circleST"><B>*PRINT-CIRCLE*</B></A><P>
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and *PRINT-SHARED* are both true [rationale: <P>
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check one variable first for efficiency], identify sharing <P>
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with #n# in the printer. Otherwise, don't.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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Storage needed for detecting shared <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> is slightly more.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Several implementations implement RESPECT-PRINT-CIRCLE.<P>
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<P>
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<P>
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</PRE>
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