62 lines
5.5 KiB
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62 lines
5.5 KiB
HTML
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<TITLE>CLHS: Variable *PRINT-CIRCLE*</TITLE>
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<A NAME="STprint-circleST"><I>Variable</I> <B>*PRINT-CIRCLE*</B></A> <P>
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<P>
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<P><B>Value Type:</B><P>
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<P>
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a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. <P>
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<P><B>Initial Value:</B><P>
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<P>
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<A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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Controls the attempt to detect circularity and sharing in an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> being printed. <P>
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If <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, the printing process merely proceeds by recursive descent without attempting to detect circularity and sharing. <P>
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If <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, the printer will endeavor to detect cycles and sharing in the structure to be printed, and to use <TT>#</TT><I>n</I><TT>=</TT> and <TT>#</TT><I>n</I><TT>#</TT> syntax to indicate the circularities or shared components. <P>
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If <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, a user-defined <A REL=DEFINITION HREF="f_pr_obj.htm#print-object"><B>print-object</B></A> <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> can print <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> to the supplied <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A> using <A REL=DEFINITION HREF="f_wr_pr.htm#write"><B>write</B></A>, <A REL=DEFINITION HREF="f_wr_pr.htm#prin1"><B>prin1</B></A>, <A REL=DEFINITION HREF="f_wr_pr.htm#princ"><B>princ</B></A>, or <A REL=DEFINITION HREF="f_format.htm#format"><B>format</B></A> and expect circularities and sharing to be detected and printed using the <TT>#</TT><I>n</I><TT>#</TT> syntax. If a user-defined <A REL=DEFINITION HREF="f_pr_obj.htm#print-object"><B>print-object</B></A> <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> prints to a <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A> other than the one that was supplied, then circularity detection starts over for that <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A>. <P>
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Note that implementations should not use <TT>#</TT><I>n</I><TT>#</TT> notation when the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A> would automatically assure sharing without it (e.g., as happens with <A REL=DEFINITION HREF="26_glo_i.htm#interned"><I>interned</I></A> <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbols</I></A>). <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(let ((a (list 1 2 3)))
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(setf (cdddr a) a)
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(let ((*print-circle* t))
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(write a)
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:done))
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>> #1=(1 2 3 . #1#)
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=> :DONE
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_wr_pr.htm#write"><B>write</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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An attempt to print a circular structure with <A REL=DEFINITION HREF="#STprint-circleST"><B>*print-circle*</B></A> set to <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> may lead to looping behavior and failure to terminate. <P>
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<P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss116.htm">DEFSTRUCT-PRINT-FUNCTION-AGAIN:X3J13-MAR-93</A><LI> <A REL=CHILD HREF="../Issues/iss275.htm">PRINT-CIRCLE-STRUCTURE:USER-FUNCTIONS-WORK</A><LI> <A REL=CHILD HREF="../Issues/iss274.htm">PRINT-CIRCLE-SHARED:RESPECT-PRINT-CIRCLE</A><P></UL><HR>
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