43 lines
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43 lines
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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: Section 1.4.1.5</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="01_dadd.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="01_da.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="01_daf.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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1.4.1.5 Designators</H2> <P>
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A <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> is an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> that denotes another <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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Where a <A REL=DEFINITION HREF="26_glo_p.htm#parameter"><I>parameter</I></A> of an <A REL=DEFINITION HREF="26_glo_o.htm#operator"><I>operator</I></A> is described as a <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A>, the description of the <A REL=DEFINITION HREF="26_glo_o.htm#operator"><I>operator</I></A> is written in a way that assumes that the value of the <A REL=DEFINITION HREF="26_glo_p.htm#parameter"><I>parameter</I></A> is the denoted <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>; that is, that the <A REL=DEFINITION HREF="26_glo_p.htm#parameter"><I>parameter</I></A> is already of the denoted <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A>. (The specific nature of the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> denoted by a ``<<<I>type</I>>> <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A>'' or a ``<A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> for a <<<I>type</I>>>'' can be found in the Glossary entry for ``<<<I>type</I>>> <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A>.'') <P>
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For example, ``<A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>'' and ``the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of <A REL=DEFINITION HREF="v_debug_.htm#STstandard-outputST"><B>*standard-output*</B></A>'' are operationally indistinguishable as <A REL=DEFINITION HREF="26_glo_s.htm#stream_designator"><I>stream designators</I></A>. Similarly, the <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> <TT>foo</TT> and the <A REL=DEFINITION HREF="26_glo_s.htm#string"><I>string</I></A> <TT>"FOO"</TT> are operationally indistinguishable as <A REL=DEFINITION HREF="26_glo_s.htm#string_designator"><I>string designators</I></A>. <P>
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Except as otherwise noted, in a situation where the denoted <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> might be used multiple times, it is <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A> whether the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> is coerced only once or whether the coercion occurs each time the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> must be used. <P>
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For example, <A REL=DEFINITION HREF="f_mapc_.htm#mapcar"><B>mapcar</B></A> receives a <A REL=DEFINITION HREF="26_glo_f.htm#function_designator"><I>function designator</I></A> as an argument, and its description is written as if this were simply a function. In fact, it is <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A> whether the <A REL=DEFINITION HREF="26_glo_f.htm#function_designator"><I>function designator</I></A> is coerced right away or whether it is carried around internally in the form that it was given as an <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A> and re-coerced each time it is needed. In most cases, <A REL=DEFINITION HREF="26_glo_c.htm#conforming_program"><I>conforming programs</I></A> cannot detect the distinction, but there are some pathological situations (particularly those involving self-redefining or mutually-redefining functions) which do conform and which can detect this difference. The following program is a <A REL=DEFINITION HREF="26_glo_c.htm#conforming_program"><I>conforming program</I></A>, but might or might not have portably correct results, depending on whether its correctness depends on one or the other of the results: <P>
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<PRE>
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(defun add-some (x)
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(defun add-some (x) (+ x 2))
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(+ x 1)) => ADD-SOME
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(mapcar 'add-some '(1 2 3 4))
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=> (2 3 4 5)
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OR=> (2 4 5 6)
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</PRE>
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</TT> <P>
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In a few rare situations, there may be a need in a dictionary entry to refer to the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> that was the original <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> for a <A REL=DEFINITION HREF="26_glo_p.htm#parameter"><I>parameter</I></A>. Since naming the <A REL=DEFINITION HREF="26_glo_p.htm#parameter"><I>parameter</I></A> would refer to the denoted <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>, the phrase ``the <<<I>parameter-name</I>>> <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A>'' can be used to refer to the <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> which was the <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>argument</I></A> from which the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of <<<I>parameter-name</I>>> was computed. <P>
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