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96 lines
7 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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<HTML>
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<TITLE>CLHS: Macro WHEN, UNLESS</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="m_or.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_data_a.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="m_case_.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="when"><A NAME="unless"><I>Macro</I> <B>WHEN, UNLESS</B></A></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>when</B> <I>test-form <I>form</I><B>*</B></I> => <I><I>result</I><B>*</B></I><P>
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<P>
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<B>unless</B> <I>test-form <I>form</I><B>*</B></I> => <I><I>result</I><B>*</B></I><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>test-form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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<I>forms</I>---an <A REL=DEFINITION HREF="26_glo_i.htm#implicit_progn"><I>implicit progn</I></A>. <P>
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<I>results</I>---the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> of the <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>forms</I></A> in a <A REL=DEFINITION HREF="#when"><B>when</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> if the <I>test-form</I> <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> or in an <A REL=DEFINITION HREF="#unless"><B>unless</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> if the <I>test-form</I> <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>; otherwise <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#when"><B>when</B></A> and <A REL=DEFINITION HREF="#unless"><B>unless</B></A> allow the execution of <I>forms</I> to be dependent on a single <I>test-form</I>. <P>
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In a <A REL=DEFINITION HREF="#when"><B>when</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>, if the <I>test-form</I> <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, the <I>forms</I> are <I>evaluated</I> in order from left to right and the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> returned by the <I>forms</I> are returned from the <A REL=DEFINITION HREF="#when"><B>when</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. Otherwise, if the <I>test-form</I> <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, the <I>forms</I> are not <I>evaluated</I>, and the <A REL=DEFINITION HREF="#when"><B>when</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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In an <A REL=DEFINITION HREF="#unless"><B>unless</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>, if the <I>test-form</I> <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, the <I>forms</I> are <I>evaluated</I> in order from left to right and the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> returned by the <I>forms</I> are returned from the <A REL=DEFINITION HREF="#unless"><B>unless</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. Otherwise, if the <I>test-form</I> <A REL=DEFINITION HREF="26_glo_y.htm#yield"><I>yields</I></A> <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, the <I>forms</I> are not <I>evaluated</I>, and the <A REL=DEFINITION HREF="#unless"><B>unless</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></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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(when t 'hello) => HELLO
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(unless t 'hello) => NIL
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(when nil 'hello) => NIL
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(unless nil 'hello) => HELLO
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(when t) => NIL
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(unless nil) => NIL
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(when t (prin1 1) (prin1 2) (prin1 3))
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>> 123
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=> 3
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(unless t (prin1 1) (prin1 2) (prin1 3)) => NIL
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(when nil (prin1 1) (prin1 2) (prin1 3)) => NIL
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(unless nil (prin1 1) (prin1 2) (prin1 3))
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>> 123
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=> 3
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(let ((x 3))
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(list (when (oddp x) (incf x) (list x))
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(when (oddp x) (incf x) (list x))
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(unless (oddp x) (incf x) (list x))
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(unless (oddp x) (incf x) (list x))
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(if (oddp x) (incf x) (list x))
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(if (oddp x) (incf x) (list x))
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(if (not (oddp x)) (incf x) (list x))
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(if (not (oddp x)) (incf x) (list x))))
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=> ((4) NIL (5) NIL 6 (6) 7 (7))
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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> 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="m_and.htm#and"><B>and</B></A>, <A REL=DEFINITION HREF="m_cond.htm#cond"><B>cond</B></A>, <A REL=DEFINITION HREF="s_if.htm#if"><B>if</B></A>, <A REL=DEFINITION HREF="m_or.htm#or"><B>or</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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(when test {form}+) == (and test (progn {form}+))
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(when test {form}+) == (cond (test {form}+))
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(when test {form}+) == (if test (progn {form}+) nil)
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(when test {form}+) == (unless (not test) {form}+)
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(unless test {form}+) == (cond ((not test) {form}+))
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(unless test {form}+) == (if test nil (progn {form}+))
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(unless test {form}+) == (when (not test) {form}+)
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</PRE>
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</TT> <P>
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<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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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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