106 lines
5.6 KiB
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106 lines
5.6 KiB
HTML
<!-- 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: Special Operator RETURN-FROM</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="s_go.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_return.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="return-from"><I>Special Operator</I> <B>RETURN-FROM</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>return-from</B> <I><I>name</I> [<I>result</I>]</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>name</I>---a <A REL=DEFINITION HREF="26_glo_b.htm#block_tag"><I>block tag</I></A>; not evaluated. <P>
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<I>result</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>; evaluated. The default is <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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Returns control and <A REL=DEFINITION HREF="26_glo_m.htm#multiple_values"><I>multiple values</I></A>[2] from a lexically enclosing <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A>. <P>
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A <A REL=DEFINITION HREF="s_block.htm#block"><B>block</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> named <I>name</I> must lexically enclose the occurrence of <A REL=DEFINITION HREF="#return-from"><B>return-from</B></A>; any <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> <I>yielded</I> by the <A REL=DEFINITION HREF="26_glo_e.htm#evaluation"><I>evaluation</I></A> of <I>result</I> are immediately returned from the innermost such lexically enclosing <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A>. <P>
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The transfer of control initiated by <A REL=DEFINITION HREF="#return-from"><B>return-from</B></A> is performed as described in <A REL=CHILD HREF="05_b.htm">Section 5.2 (Transfer of Control to an Exit Point)</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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(block alpha (return-from alpha) 1) => NIL
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(block alpha (return-from alpha 1) 2) => 1
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(block alpha (return-from alpha (values 1 2)) 3) => 1, 2
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(let ((a 0))
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(dotimes (i 10) (incf a) (when (oddp i) (return)))
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a) => 2
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(defun temp (x)
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(if x (return-from temp 'dummy))
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44) => TEMP
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(temp nil) => 44
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(temp t) => DUMMY
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(block out
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(flet ((exit (n) (return-from out n)))
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(block out (exit 1)))
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2) => 1
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(block nil
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(unwind-protect (return-from nil 1)
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(return-from nil 2)))
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=> 2
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(dolist (flag '(nil t))
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(block nil
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(let ((x 5))
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(declare (special x))
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(unwind-protect (return-from nil)
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(print x))))
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(print 'here))
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>> 5
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>> HERE
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>> 5
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>> HERE
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=> NIL
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(dolist (flag '(nil t))
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(block nil
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(let ((x 5))
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(declare (special x))
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(unwind-protect
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(if flag (return-from nil))
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(print x))))
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(print 'here))
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>> 5
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>> HERE
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>> 5
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>> HERE
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=> NIL
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</PRE>
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</TT> <P>
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The following has undefined consequences because the <A REL=DEFINITION HREF="s_block.htm#block"><B>block</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> exits normally before the <A REL=DEFINITION HREF="#return-from"><B>return-from</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> is attempted. <P>
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<PRE>
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(funcall (block nil #'(lambda () (return-from nil)))) is an error.
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</PRE>
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</TT> <P>
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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="s_block.htm#block"><B>block</B></A>, <A REL=DEFINITION HREF="m_return.htm#return"><B>return</B></A>, <A REL=CHILD HREF="03_a.htm">Section 3.1 (Evaluation)</A> <P>
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<P><B>Notes:</B> None.
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<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/iss152.htm">EXIT-EXTENT:MINIMAL</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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