97 lines
8.9 KiB
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97 lines
8.9 KiB
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Variable *PRINT-READABLY*</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="v_pr_pre.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_printe.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="v_pr_rig.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="STprint-readablyST"><I>Variable</I> <B>*PRINT-READABLY*</B></A> <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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If <A REL=DEFINITION HREF="#STprint-readablyST"><B>*print-readably*</B></A> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, some special rules for printing <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> go into effect. Specifically, printing any <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> O1 produces a printed representation that, when seen by the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A> while the <A REL=DEFINITION HREF="26_glo_s.htm#standard_readtable"><I>standard readtable</I></A> is in effect, will produce an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> O2 that is <A REL=DEFINITION HREF="26_glo_s.htm#similar"><I>similar</I></A> to O1. The printed representation produced might or might not be the same as the printed representation produced when <A REL=DEFINITION HREF="#STprint-readablyST"><B>*print-readably*</B></A> is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>. If printing an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> <A REL=DEFINITION HREF="26_glo_r.htm#readably"><I>readably</I></A> is not possible, an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_pr_not.htm#print-not-readable"><B>print-not-readable</B></A> is signaled rather than using a syntax (e.g., the ``<TT>#<</TT>'' syntax) that would not be readable by the same <A REL=DEFINITION HREF="26_glo_i.htm#implementation"><I>implementation</I></A>. If the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of some other <A REL=DEFINITION HREF="26_glo_p.htm#printer_control_variable"><I>printer control variable</I></A> is such that these requirements would be violated, the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of that other <A REL=DEFINITION HREF="26_glo_v.htm#variable"><I>variable</I></A> is ignored. <P>
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Specifically, if <A REL=DEFINITION HREF="#STprint-readablyST"><B>*print-readably*</B></A> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, printing proceeds as if <A REL=DEFINITION HREF="v_pr_esc.htm#STprint-escapeST"><B>*print-escape*</B></A>, <A REL=DEFINITION HREF="v_pr_ar.htm#STprint-arrayST"><B>*print-array*</B></A>, and <A REL=DEFINITION HREF="v_pr_gen.htm#STprint-gensymST"><B>*print-gensym*</B></A> were also <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, and as if <A REL=DEFINITION HREF="v_pr_lev.htm#STprint-lengthST"><B>*print-length*</B></A>, <A REL=DEFINITION HREF="v_pr_lev.htm#STprint-levelST"><B>*print-level*</B></A>, and <A REL=DEFINITION HREF="v_pr_lin.htm#STprint-linesST"><B>*print-lines*</B></A> were <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>. <P>
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If <A REL=DEFINITION HREF="#STprint-readablyST"><B>*print-readably*</B></A> is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, the normal rules for printing and the normal interpretations of other <A REL=DEFINITION HREF="26_glo_p.htm#printer_control_variable"><I>printer control variables</I></A> are in effect. <P>
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Individual <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> for <A REL=DEFINITION HREF="f_pr_obj.htm#print-object"><B>print-object</B></A>, including user-defined <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A>, are responsible for implementing these requirements. <P>
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If <A REL=DEFINITION HREF="v_rd_eva.htm#STread-evalST"><B>*read-eval*</B></A> is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A> and <A REL=DEFINITION HREF="#STprint-readablyST"><B>*print-readably*</B></A> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, any such method that would output a reference to the ``<TT>#.</TT>'' <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro"><I>reader macro</I></A> will either output something else or will signal an error (as described above). <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 ((x (list "a" '\a (gensym) '((a (b (c))) d e f g)))
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(*print-escape* nil)
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(*print-gensym* nil)
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(*print-level* 3)
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(*print-length* 3))
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(write x)
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(let ((*print-readably* t))
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(terpri)
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(write x)
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:done))
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>> (a a G4581 ((A #) D E ...))
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>> ("a" |a| #:G4581 ((A (B (C))) D E F G))
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=> :DONE
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;; This is setup code is shared between the examples
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;; of three hypothetical implementations which follow.
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(setq table (make-hash-table)) => #<HASH-TABLE EQL 0/120 32005763>
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(setf (gethash table 1) 'one) => ONE
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(setf (gethash table 2) 'two) => TWO
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;; Implementation A
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(let ((*print-readably* t)) (print table))
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Error: Can't print #<HASH-TABLE EQL 0/120 32005763> readably.
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;; Implementation B
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;; No standardized #S notation for hash tables is defined,
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;; but there might be an implementation-defined notation.
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(let ((*print-readably* t)) (print table))
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>> #S(HASH-TABLE :TEST EQL :SIZE 120 :CONTENTS (1 ONE 2 TWO))
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=> #<HASH-TABLE EQL 0/120 32005763>
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;; Implementation C
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;; Note that #. notation can only be used if *READ-EVAL* is true.
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;; If *READ-EVAL* were false, this same implementation might have to
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;; signal an error unless it had yet another printing strategy to fall
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;; back on.
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(let ((*print-readably* t)) (print table))
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>> #.(LET ((HASH-TABLE (MAKE-HASH-TABLE)))
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>> (SETF (GETHASH 1 HASH-TABLE) ONE)
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>> (SETF (GETHASH 2 HASH-TABLE) TWO)
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>> HASH-TABLE)
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=> #<HASH-TABLE EQL 0/120 32005763>
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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>, <A REL=DEFINITION HREF="m_pr_unr.htm#print-unreadable-object"><B>print-unreadable-object</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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The rules for ``<A REL=DEFINITION HREF="26_glo_s.htm#similarity"><I>similarity</I></A>'' imply that <TT>#A</TT> or <TT>#(</TT> syntax cannot be used for <A REL=DEFINITION HREF="26_glo_a.htm#array"><I>arrays</I></A> of <A REL=DEFINITION HREF="26_glo_e.htm#element_type"><I>element type</I></A> other than <A REL=DEFINITION HREF="t_t.htm#t"><B>t</B></A>. An implementation will have to use another syntax or signal an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_pr_not.htm#print-not-readable"><B>print-not-readable</B></A>. <P>
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<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/iss276.htm">PRINT-READABLY-BEHAVIOR:CLARIFY</A><LI> <A REL=CHILD HREF="../Issues/iss089.htm">DATA-IO:ADD-SUPPORT</A><P></UL><HR>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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