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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: Issue VARIABLE-LIST-ASYMMETRY Writeup</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="../Issues/iss357_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss358.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss359_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue VARIABLE-LIST-ASYMMETRY Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss358.htm">VARIABLE-LIST-ASYMMETRY</A><P>
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<B>References:</B> CLtL pgs. 110, 122, 131<P>
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<B>Category:</B> ADDITION<P>
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<B>Edit history:</B> 26-Jul-88, Version 1 by Skona Brittain<P>
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04-Aug-88, Version 2 by Skona Brittain<P>
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08-Oct-88, Version 3 by Pitman<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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The syntax of items in the variable-list for various control structues<P>
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(do, do*, let, let*, prog, prog*, and compiler-let) varies. This<P>
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variation seems unnecessary.<P>
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<P>
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The allowed variations are indicated in the following chart:<P>
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<P>
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do & do*: (var) (var init) (var init step)<P>
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prog & prog*: var (var) (var init) n.a.<P>
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let & let*: var (var val) n.a.<P>
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compiler-let var (var value)<P>
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<P>
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Note that just plain `` var '' is prohibited in do forms<P>
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and that the case of ``(var)'' is prohibited in let forms<P>
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and compiler-let forms.<P>
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<P>
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<B>Proposal (VARIABLE-LIST-ASYMMETRY:SYMMETRIZE):<P>
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</B><P>
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Allow all the variations in all of the forms;<P>
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i.e. add the prohibited cases mentioned above.<P>
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<P>
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I.e. change the variable-list in the syntax descriptions as follows:<P>
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<P>
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do & do*: ( { var | (var [init [step]] ) }* )<P>
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let & let*: ( { var | (var [value] ) }* )<P>
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compiler-let: ( { var | (var [value] ) }* )<P>
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<P>
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<B>Test Cases:<P>
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</B><P>
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(let (a (b) (c 3)) ... ) would be valid.<P>
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(let* (a (b) (c 3)) ... ) would be valid.<P>
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(do (a (b) (c 3)) ... ) would be valid.<P>
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(do* (a (b) (c 3)) ... ) would be valid.<P>
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(compiler-let (a (b) (c 3)) ... ) would be valid.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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The asymmetry is unnecessary and impedes learning of CL.<P>
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<P>
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Any other way to make these cases consistent, such as either<P>
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omitting just ``var'' from do & do* and prog & prog*, or<P>
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omitting ``(var)'' from let & let* and prog & prog*, <P>
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would be an incompatible change to the language. <P>
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This way just adds the flexibility found in some of the forms to all of them.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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KCL allows ``(var)'' in let & let* but not ``var'' in do & do*.<P>
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<P>
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Symbolics Genera allows all three cases in all the forms; i.e. it conforms<P>
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to this proposal.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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Extremely slight. (May involve subtracting code rather than adding it).<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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None.<P>
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<P>
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<B>Cost of Non-Adoption:<P>
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</B><P>
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The variation in syntax makes them harder to learn.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Ease of learning.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Symmetry is more aesthetic than asymmetry, at least to some of us.<P>
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<P>
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<B>Discussion: <P>
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</B><P>
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Pitman supports this proposal.<P>
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<P>
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The issue about whether the atomic ``var'' should be allowed at all in the <P>
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variable lists for let & let* is a separate issue. (So is whether<P>
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it should mean that the var is initially bound to nil.) Since it is allowed, <P>
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this proposal merely says that the alternative syntax of an atom within a <P>
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list with no initial value, ``(var)'', should also be allowed.<P>
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<P>
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</PRE>
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