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<TITLE>CLHS: Issue RECURSIVE-DEFTYPE 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/iss290_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss291.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss292_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue RECURSIVE-DEFTYPE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss291.htm">RECURSIVE-DEFTYPE</A><P>
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<B>Forum:</B> Editorial<P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> (p50)<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> 05-Mar-91, Version 1 by Pitman<P>
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15-Mar-91, Version 2 by Pitman, Loosemore<P>
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(expand scope of proposal to treat both program and<P>
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structural circularities, and to treat both macros<P>
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and type specifiers equivalently)<P>
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<B>Status:</B> For X3J13 consideration<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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In discussion of issue <A HREF="iss078.htm">CONS-TYPE-SPECIFIER</A>, it became apparent that some<P>
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users thought it might be ok to have non-terminating recursive <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A><P>
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definitions. Some vendors balked at this. The case in question, which<P>
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is not [yet] valid Common Lisp at the time the proposal is written, but <P>
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which nevertheless illustrates the point, is Barry Margolin's:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>deftype</B></A> proper-list (<A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&optional</B></A> (element-type t))<P>
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`(<A REL=DEFINITION HREF="../Body/a_or.htm#or"><B>or</B></A> <A REL=DEFINITION HREF="../Body/a_null.htm#null"><B>null</B></A> (<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>cons</B></A> ,element-type proper-list)))<P>
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<P>
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Another case, which is also not valid Common Lisp but which illustrates<P>
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a case that might be more tolerable is Allan Wechsler's:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>deftype</B></A> list-of-length (<A REL=DEFINITION HREF="../Body/f_length.htm#length"><B>length</B></A>)<P>
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(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>if</B></A> (= 0 <A REL=DEFINITION HREF="../Body/f_length.htm#length"><B>length</B></A>)<P>
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'<A REL=DEFINITION HREF="../Body/a_null.htm#null"><B>null</B></A><P>
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`(<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>cons</B></A> t (list-of-length ,(- <A REL=DEFINITION HREF="../Body/f_length.htm#length"><B>length</B></A> 1)))))<P>
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<P>
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<B>Proposal (RECURSIVE-DEFTYPE:EXPLICITLY-VAGUE):<P>
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</B><P>
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Clarify that recursive expansion of the type specifier returned as<P>
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the expansion of a <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> must terminate, including the expansion<P>
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of type specifiers which are nested within the expansion.<P>
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<P>
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Clarify that the consequences are undefined if the result of<P>
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fully expanding a type specifier contains any circular <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A>,<P>
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except within the objects referred to by <A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A> and <A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> type<P>
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specifiers.<P>
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<P>
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Clarify that recursive expansion of the form returned as the<P>
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expansion of a <A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A> must terminate, including the expansion <P>
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of other macros which are subforms of other forms returned by<P>
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<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A>.<P>
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<P>
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Clarify that the consequences are undefined if the result of fully<P>
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macroexpanding a form contains any non-constant circular list<P>
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<A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A>.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This probably protects the status quo in most implementations,<P>
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while giving users a clear sense of what's reasonable and what's not.<P>
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<P>
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<B>Test Cases:<P>
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</B><P>
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These test cases are more contrived in nature, but are chosen to be <P>
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actually plausible to test in existing implementations (something not<P>
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true of the cases mentioned in the problem description):<P>
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<P>
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#1: (<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>DEFUN</B></A> RETURNS-T (X) X)<P>
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(<A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> FOO () '(<A REL=DEFINITION HREF="../Body/a_or.htm#or"><B>OR</B></A> (<A REL=DEFINITION HREF="../Body/t_satisf.htm#satisfies"><B>SATISFIES</B></A> RETURNS-T) FOO))<P>
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<P>
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This is an error because in (<A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> X 'FOO), FOO cannot, in general,<P>
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be fully expanded because the expansion does not terminate.<P>
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<P>
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#2: (<A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> FOO (<A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> (FAST <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>))<P>
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(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>IF</B></A> FAST <P>
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`(<A REL=DEFINITION HREF="../Body/t_satisf.htm#satisfies"><B>SATISFIES</B></A> FOO-FAST)<P>
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`(<A REL=DEFINITION HREF="../Body/a_and.htm#and"><B>AND</B></A> <A REL=DEFINITION HREF="../Body/t_symbol.htm#symbol"><B>SYMBOL</B></A> (FOO :FAST T))))<P>
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<P>
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This is well-defined because in (<A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> X 'FOO), FOO can, in general,<P>
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be fully expanded without error because the recursion always terminates.<P>
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<P>
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#3: Macro definitions like:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A> MAPCAR1 (FN <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A>)<P>
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`(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>IF</B></A> (<A REL=DEFINITION HREF="../Body/a_null.htm#null"><B>NULL</B></A> ,<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A>) '()<P>
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(<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A> (<A REL=DEFINITION HREF="../Body/f_funcal.htm#funcall"><B>FUNCALL</B></A> ,FN (<A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>CAR</B></A> ,<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A>))<P>
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(MAPCAR1 ,FN (<A REL=DEFINITION HREF="../Body/f_car_c.htm#cdr"><B>CDR</B></A> ,<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A>)))))<P>
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<P>
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would be explicitly undefined.<P>
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<P>
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#4: Forms involving explicit circularities, such as<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>PROGN</B></A> . #1=((<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'FOO) . #1#))<P>
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<P>
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would be explicitly undefined.<P>
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<P>
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#5: Type specifiers involving explicit circularities, such as<P>
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<P>
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a. (<A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> 3 '(<A REL=DEFINITION HREF="../Body/a_and.htm#and"><B>AND</B></A> #1=(T . #1#)))<P>
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<P>
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would be explicitly undefined, except for cases like<P>
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<P>
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b. (<A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> 3 '(<A REL=DEFINITION HREF="../Body/a_eql.htm#eql"><B>EQL</B></A> #1=(T . #1#)))<P>
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<P>
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which would still be well-defined.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Genera implements this proposal.<P>
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<P>
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#1: The proposal makes this an error.<P>
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Genera uses a TYPE-EXPAND function internally at compile time and<P>
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so cannot handle this case.<P>
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<P>
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#2: The proposal requires this to work.<P>
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Genera handles this case.<P>
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<P>
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#3: The proposal makes this an error.<P>
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Genera can manage to do (<A REL=DEFINITION HREF="../Body/f_mapc_.htm#mapcar"><B>MAPCAR</B></A> #'<A REL=DEFINITION HREF="../Body/f_1pl_1_.htm#1PL"><B>1+</B></A> '(1 2 3)) as a standalone form,<P>
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but blows out during lexical analysis both in the interpreter and<P>
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compiler if you put it in a definition.<P>
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<P>
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#4: The proposal makes this case an error.<P>
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Genera can't handle this case at all.<P>
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<P>
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#5: The proposal makes #5a an error but requires #5b to work.<P>
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Genera doesn't handle #5a but does handle #5b.<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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Probably none.<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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Little or none. Code which exploited the erroneous cases were already<P>
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not portable in practice, so probably no users relied on them.<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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Portability pitfalls waiting for unsuspecting users.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Compiler writers who didn't want to handle this won't go to sleep every<P>
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night in fear that they really should have and that some day someone will<P>
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be knocking on their door asking why not.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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This is intended to be equivalent to the status quo for macros, although<P>
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it doesn't have to be spelled out there because the presence of <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A><P>
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pretty much forces it. In any case, it clarifies that <A REL=DEFINITION HREF="../Body/m_deftp.htm#deftype"><B>DEFTYPE</B></A> and <A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A><P>
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must be essentially consistent on this issue.<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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If Sandra Loosemore's TYPE-EXPAND proposal (part of issue TYPE-SPECIFIER-P)<P>
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passed, this view of the world would pretty much be forced anyway.<P>
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<P>
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Of Margolin's example, Rob MacLachlan (at CMU) wrote:<P>
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``I am fairly sure that nobody has addressed this issue. <P>
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Although I have been aware of it, I kept quiet about it, <P>
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because my compiler gags on recursive deftypes, and I was<P>
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afraid someone would decide that was a bug.''<P>
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</PRE>
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