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96 lines
8.4 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: Accessor GET</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="f_symb_5.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_symbol.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_rempro.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<HR>
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<A NAME="get"><I>Accessor</I> <B>GET</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>get</B> <I>symbol indicator <TT>&optional</TT> default</I> => <I>value</I><P>
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<TT>(setf (</TT><B>get</B> <I>symbol indicator <TT>&optional</TT> default</I><TT>) </TT><I>new-value</I><TT>)</TT><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>symbol</I>---a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>. <P>
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<I>indicator</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>default</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. The default is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<I>value</I>---if the indicated property exists, the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> that is its <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A>; otherwise, the specified <I>default</I>. <P>
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<I>new-value</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#get"><B>get</B></A> finds a <A REL=DEFINITION HREF="26_glo_p.htm#property"><I>property</I></A> on the <A REL=DEFINITION HREF="26_glo_p.htm#property_list"><I>property list</I></A>[2] of <I>symbol</I> whose <A REL=DEFINITION HREF="26_glo_p.htm#property_indicator"><I>property indicator</I></A> is <A REL=DEFINITION HREF="26_glo_i.htm#identical"><I>identical</I></A> to <I>indicator</I>, and returns its corresponding <A REL=DEFINITION HREF="26_glo_p.htm#property_value"><I>property value</I></A>. If there are multiple <A REL=DEFINITION HREF="26_glo_p.htm#property"><I>properties</I></A>[1] with that <A REL=DEFINITION HREF="26_glo_p.htm#property_indicator"><I>property indicator</I></A>, <A REL=DEFINITION HREF="#get"><B>get</B></A> uses the first such <A REL=DEFINITION HREF="26_glo_p.htm#property"><I>property</I></A>. If there is no <A REL=DEFINITION HREF="26_glo_p.htm#property"><I>property</I></A> with that <A REL=DEFINITION HREF="26_glo_p.htm#property_indicator"><I>property indicator</I></A>, <I>default</I> is returned. <P>
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<A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="#get"><B>get</B></A> may be used to associate a new <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> with an existing indicator already on the <I>symbol</I>'s <A REL=DEFINITION HREF="26_glo_p.htm#property_list"><I>property list</I></A>, or to create a new assocation if none exists. If there are multiple <A REL=DEFINITION HREF="26_glo_p.htm#property"><I>properties</I></A>[1] with that <A REL=DEFINITION HREF="26_glo_p.htm#property_indicator"><I>property indicator</I></A>, <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="#get"><B>get</B></A> associates the <I>new-value</I> with the first such <A REL=DEFINITION HREF="26_glo_p.htm#property"><I>property</I></A>. When a <A REL=DEFINITION HREF="#get"><B>get</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> is used as a <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> <I>place</I>, any <I>default</I> which is supplied is evaluated according to normal left-to-right evaluation rules, but its <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> is ignored. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(defun make-person (first-name last-name)
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(let ((person (gensym "PERSON")))
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(setf (get person 'first-name) first-name)
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(setf (get person 'last-name) last-name)
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person)) => MAKE-PERSON
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(defvar *john* (make-person "John" "Dow")) => *JOHN*
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*john* => #:PERSON4603
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(defvar *sally* (make-person "Sally" "Jones")) => *SALLY*
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(get *john* 'first-name) => "John"
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(get *sally* 'last-name) => "Jones"
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(defun marry (man woman married-name)
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(setf (get man 'wife) woman)
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(setf (get woman 'husband) man)
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(setf (get man 'last-name) married-name)
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(setf (get woman 'last-name) married-name)
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married-name) => MARRY
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(marry *john* *sally* "Dow-Jones") => "Dow-Jones"
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(get *john* 'last-name) => "Dow-Jones"
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(get (get *john* 'wife) 'first-name) => "Sally"
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(symbol-plist *john*)
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=> (WIFE #:PERSON4604 LAST-NAME "Dow-Jones" FIRST-NAME "John")
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(defmacro age (person &optional (default ''thirty-something))
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`(get ,person 'age ,default)) => AGE
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(age *john*) => THIRTY-SOMETHING
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(age *john* 20) => 20
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(setf (age *john*) 25) => 25
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(age *john*) => 25
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(age *john* 20) => 25
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</PRE>
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</TT> <P>
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<P><B>Side Effects:</B> None.
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<P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B><P>
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<P>
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Should signal an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_tp_err.htm#type-error"><B>type-error</B></A> if <I>symbol</I> is not a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>. <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_getf.htm#getf"><B>getf</B></A>, <A REL=DEFINITION HREF="f_symb_4.htm#symbol-plist"><B>symbol-plist</B></A>, <A REL=DEFINITION HREF="f_rempro.htm#remprop"><B>remprop</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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<PRE>
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(get x y) == (getf (symbol-plist x) y)
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</PRE>
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</TT> <P>
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<A REL=DEFINITION HREF="26_glo_n.htm#number"><I>Numbers</I></A> and <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>characters</I></A> are not recommended for use as <I>indicators</I> in portable code since <A REL=DEFINITION HREF="#get"><B>get</B></A> tests with <A REL=DEFINITION HREF="f_eq.htm#eq"><B>eq</B></A> rather than <A REL=DEFINITION HREF="f_eql.htm#eql"><B>eql</B></A>, and consequently the effect of using such <I>indicators</I> is <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A>. <P>
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There is no way using <A REL=DEFINITION HREF="#get"><B>get</B></A> to distinguish an absent property from one whose value is <I>default</I>. However, see <A REL=DEFINITION HREF="f_get_pr.htm#get-properties"><B>get-properties</B></A>. <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/iss306.htm">SETF-GET-DEFAULT:EVALUATED-BUT-IGNORED</A><LI> <A REL=CHILD HREF="../Issues/iss269.htm">PLIST-DUPLICATES:ALLOW</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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