70 lines
5.6 KiB
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70 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 BLOCK</TITLE>
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<A NAME="block"><I>Special Operator</I> <B>BLOCK</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>block</B> <I><I>name</I> <I>form</I>*</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>name</I>---a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>. <P>
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<I>form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</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> if a <A REL=DEFINITION HREF="26_glo_n.htm#normal_return"><I>normal return</I></A> occurs, or else, if an <A REL=DEFINITION HREF="26_glo_e.htm#explicit_return"><I>explicit return</I></A> occurs, the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> that were transferred. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#block"><B>block</B></A> <A REL=DEFINITION HREF="26_glo_e.htm#establish"><I>establishes</I></A> a <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A> named <I>name</I> and then evaluates <I>forms</I> as an <A REL=DEFINITION HREF="26_glo_i.htm#implicit_progn"><I>implicit progn</I></A>. <P>
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The <A REL=DEFINITION HREF="26_glo_s.htm#special_operator"><I>special operators</I></A> <A REL=DEFINITION HREF="#block"><B>block</B></A> and <A REL=DEFINITION HREF="s_ret_fr.htm#return-from"><B>return-from</B></A> work together to provide a structured, lexical, non-local exit facility. At any point lexically contained within <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>forms</I></A>, <A REL=DEFINITION HREF="s_ret_fr.htm#return-from"><B>return-from</B></A> can be used with the given <I>name</I> to return control and values from the <A REL=DEFINITION HREF="#block"><B>block</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>, except when an intervening <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A> with the same name has been <I>established</I>, in which case the outer <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A> is shadowed by the inner one. <P>
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The <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A> named <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> has <A REL=DEFINITION HREF="26_glo_l.htm#lexical_scope"><I>lexical scope</I></A> and <A REL=DEFINITION HREF="26_glo_d.htm#dynamic_extent"><I>dynamic extent</I></A>. <P>
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Once established, a <A REL=DEFINITION HREF="26_glo_b.htm#block"><I>block</I></A> may only be exited once, whether by <A REL=DEFINITION HREF="26_glo_n.htm#normal_return"><I>normal return</I></A> or <A REL=DEFINITION HREF="26_glo_e.htm#explicit_return"><I>explicit return</I></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 empty) => NIL
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(block whocares (values 1 2) (values 3 4)) => 3, 4
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(let ((x 1))
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(block stop (setq x 2) (return-from stop) (setq x 3))
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x) => 2
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(block early (return-from early (values 1 2)) (values 3 4)) => 1, 2
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(block outer (block inner (return-from outer 1)) 2) => 1
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(block twin (block twin (return-from twin 1)) 2) => 2
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;; Contrast behavior of this example with corresponding example of CATCH.
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(block b
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(flet ((b1 () (return-from b 1)))
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(block b (b1) (print 'unreachable))
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2)) => 1
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</PRE>
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</TT> <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_return.htm#return"><B>return</B></A>, <A REL=DEFINITION HREF="s_ret_fr.htm#return-from"><B>return-from</B></A>, <A REL=CHILD HREF="03_a.htm">Section 3.1 (Evaluation)</A> <P>
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<P><B>Notes:</B><P>
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<P>
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