32 lines
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32 lines
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<TITLE>CLHS: Section 2.4.8.17</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="02_dhp.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="02_dh.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="02_dhr.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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2.4.8.17 Sharpsign Plus</H2> <P>
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<TT>#+</TT> provides a read-time conditionalization facility; the syntax is <TT>#+</TT><I>test</I><TT> </TT><I>expression</I><TT></TT>. If the <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> <I>test</I> succeeds, then this textual notation represents an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> whose printed representation is <I>expression</I>. If the <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> <I>test</I> fails, then this textual notation is treated as <A REL=DEFINITION HREF="26_glo_w.htm#whitespace"><I>whitespace</I></A>[2]; that is, it is as if the ``<TT>#+</TT> <I>test</I> <I>expression</I>'' did not appear and only a <A REL=DEFINITION HREF="26_glo_s.htm#space"><I>space</I></A> appeared in its place. <P>
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For a detailed description of success and failure in <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expressions</I></A>, see <A REL=CHILD HREF="24_aba.htm">Section 24.1.2.1 (Feature Expressions)</A>. <P>
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<TT>#+</TT> operates by first reading the <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> and then skipping over the <I>form</I> if the <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> fails. While reading the <I>test</I>, the <A REL=DEFINITION HREF="26_glo_c.htm#current_package"><I>current package</I></A> is the <TT>KEYWORD</TT> package. Skipping over the <I>form</I> is accomplished by <A REL=DEFINITION HREF="26_glo_b.htm#binding"><I>binding</I></A> <A REL=DEFINITION HREF="v_rd_sup.htm#STread-suppressST"><B>*read-suppress*</B></A> to <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> and then calling <A REL=DEFINITION HREF="f_rd_rd.htm#read"><B>read</B></A>. <P>
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For examples, see <A REL=CHILD HREF="24_abaa.htm">Section 24.1.2.1.1 (Examples of Feature Expressions)</A>. <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss318.htm">SHARPSIGN-PLUS-MINUS-PACKAGE:KEYWORD</A><P></UL><HR>
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<A REL=NAVIGATOR HREF="../Front/StartPts.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Starting Points]" SRC="../Graphics/StartPts.gif" ALIGN=Bottom></A><A REL=TOC HREF="../Front/Contents.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Contents]" SRC="../Graphics/Contents.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Master.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Index]" SRC="../Graphics/Index.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Symbol.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Symbols]" SRC="../Graphics/Symbols.gif" ALIGN=Bottom></A><A REL=GLOSSARY HREF="../Body/26_a.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Glossary]" SRC="../Graphics/Glossary.gif" ALIGN=Bottom></A><A HREF="../Front/X3J13Iss.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Issues]" SRC="../Graphics/Issues.gif" ALIGN=Bottom></A><BR>
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