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128 lines
8.5 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: Macro PROG, PROG*</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_nth_va.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_prog1c.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="prog"><A NAME="progST"><I>Macro</I> <B>PROG, PROG*</B></A></A> <P>
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<P>
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<P><B>Syntax:</B><P>
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<P>
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<B>prog</B> <I>({<I>var</I> | (<I>var</I> [<I>init-form</I>])}*) <I>declaration</I><B>*</B> {<I>tag</I> | <I>statement</I>}*</I><P> => <I><I>result</I><B>*</B></I><P>
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<P>
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<B>prog*</B> <I>({<I>var</I> | (<I>var</I> [<I>init-form</I>])}*) <I>declaration</I><B>*</B> {<I>tag</I> | <I>statement</I>}*</I><P> => <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>var</I>---variable name. <P>
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<I>init-form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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<I>declaration</I>---a <A REL=DEFINITION HREF="s_declar.htm#declare"><B>declare</B></A> <A REL=DEFINITION HREF="26_glo_e.htm#expression"><I>expression</I></A>; not evaluated. <P>
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<I>tag</I>---a <A REL=DEFINITION HREF="26_glo_g.htm#go_tag"><I>go tag</I></A>; not evaluated. <P>
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<I>statement</I>---a <A REL=DEFINITION HREF="26_glo_c.htm#compound_form"><I>compound form</I></A>; evaluated as described below. <P>
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<I>results</I>---<A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></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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Three distinct operations are performed by <A REL=DEFINITION HREF="#prog"><B>prog</B></A> and <A REL=DEFINITION HREF="#progST"><B>prog*</B></A>: they bind local variables, they permit use of the <A REL=DEFINITION HREF="m_return.htm#return"><B>return</B></A> statement, and they permit use of the <A REL=DEFINITION HREF="s_go.htm#go"><B>go</B></A> statement. A typical <A REL=DEFINITION HREF="#prog"><B>prog</B></A> looks like this: <P>
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<PRE>
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(prog (var1 var2 (var3 init-form-3) var4 (var5 init-form-5))
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declaration*
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statement1
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tag1
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statement2
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statement3
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statement4
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tag2
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statement5
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...
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)
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</PRE>
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</TT> <P>
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For <A REL=DEFINITION HREF="#prog"><B>prog</B></A>, <I>init-forms</I> are evaluated first, in the order in which they are supplied. The <I>vars</I> are then bound to the corresponding values in parallel. If no <I>init-form</I> is supplied for a given <I>var</I>, that <I>var</I> is bound to <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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The body of <A REL=DEFINITION HREF="#prog"><B>prog</B></A> is executed as if it were a <A REL=DEFINITION HREF="s_tagbod.htm#tagbody"><B>tagbody</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>; the <A REL=DEFINITION HREF="s_go.htm#go"><B>go</B></A> statement can be used to transfer control to a <I>tag</I>. <I>Tags</I> label <I>statements</I>. <P>
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<A REL=DEFINITION HREF="#prog"><B>prog</B></A> implicitly establishes a <A REL=DEFINITION HREF="s_block.htm#block"><B>block</B></A> named <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> around the entire <A REL=DEFINITION HREF="#prog"><B>prog</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>, so that <A REL=DEFINITION HREF="m_return.htm#return"><B>return</B></A> can be used at any time to exit from the <A REL=DEFINITION HREF="#prog"><B>prog</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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The difference between <A REL=DEFINITION HREF="#progST"><B>prog*</B></A> and <A REL=DEFINITION HREF="#prog"><B>prog</B></A> is that in <A REL=DEFINITION HREF="#progST"><B>prog*</B></A> the <A REL=DEFINITION HREF="26_glo_b.htm#binding"><I>binding</I></A> and initialization of the <I>vars</I> is done <A REL=DEFINITION HREF="26_glo_s.htm#sequentially"><I>sequentially</I></A>, so that the <I>init-form</I> for each one can use the values of previous ones. <P>
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<P><B>Examples:</B><P>
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<PRE>
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(prog* ((y z) (x (car y)))
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(return x))
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</PRE>
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</TT> returns the <A REL=DEFINITION HREF="26_glo_c.htm#car"><I>car</I></A> of the value of <TT>z</TT>. <P>
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<PRE>
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(setq a 1) => 1
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(prog ((a 2) (b a)) (return (if (= a b) '= '/=))) => /=
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(prog* ((a 2) (b a)) (return (if (= a b) '= '/=))) => =
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(prog () 'no-return-value) => NIL
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</PRE>
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</TT>
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<PRE>
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(defun king-of-confusion (w)
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"Take a cons of two lists and make a list of conses.
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Think of this function as being like a zipper."
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(prog (x y z) ;Initialize x, y, z to NIL
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(setq y (car w) z (cdr w))
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loop
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(cond ((null y) (return x))
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((null z) (go err)))
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rejoin
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(setq x (cons (cons (car y) (car z)) x))
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(setq y (cdr y) z (cdr z))
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(go loop)
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err
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(cerror "Will self-pair extraneous items"
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"Mismatch - gleep! ~S" y)
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(setq z y)
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(go rejoin))) => KING-OF-CONFUSION
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</PRE>
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</TT> This can be accomplished more perspicuously as follows: <P>
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<PRE>
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(defun prince-of-clarity (w)
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"Take a cons of two lists and make a list of conses.
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Think of this function as being like a zipper."
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(do ((y (car w) (cdr y))
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(z (cdr w) (cdr z))
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(x '<TT>()</TT> (cons (cons (car y) (car z)) x)))
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((null y) x)
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(when (null z)
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(cerror "Will self-pair extraneous items"
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"Mismatch - gleep! ~S" y)
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(setq z y)))) => PRINCE-OF-CLARITY
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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="s_block.htm#block"><B>block</B></A>, <A REL=DEFINITION HREF="s_let_l.htm#let"><B>let</B></A>, <A REL=DEFINITION HREF="s_tagbod.htm#tagbody"><B>tagbody</B></A>, <A REL=DEFINITION HREF="s_go.htm#go"><B>go</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><P>
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<A REL=DEFINITION HREF="#prog"><B>prog</B></A> can be explained in terms of <A REL=DEFINITION HREF="s_block.htm#block"><B>block</B></A>, <A REL=DEFINITION HREF="s_let_l.htm#let"><B>let</B></A>, and <A REL=DEFINITION HREF="s_tagbod.htm#tagbody"><B>tagbody</B></A> as follows: <P>
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<PRE>
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(prog variable-list declaration . body)
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== (block nil (let variable-list declaration (tagbody . body)))
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</PRE>
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</TT> <P>
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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/iss097.htm">DECLS-AND-DOC</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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