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<H2>Issue SUBSETTING-POSITION Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss333.htm">SUBSETTING-POSITION</A><P>
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<B>References:</B> X3J13 committee and sub-committee meetings<P>
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<B>Category:</B> Policy<P>
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<B>Edit history:</B> 12-DEC-88, Version 1 by Chapman<P>
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9-JAN-89, Version 2 by Chapman <P>
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10-JAN-89, Version 3 by Chapman <P>
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20-FEB-89, Version 4 by Chapman <P>
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<P>
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<P>
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<B>Problem Description:<P>
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</B> <P>
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Should the CL <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> be partitioned such that an implementation<P>
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could chose a subset of all the CL facilities to implement and <P>
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still be a conforming implementation?<P>
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What subsets should be specified in the draft <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> we submit to<P>
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ANSI?<P>
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What position should we take if someone should propose a subset?<P>
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<P>
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Subsets might omit syntax, functions, admissable values or arguments<P>
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to functions, or data types. For example, a subset might disallow <A REL=DEFINITION HREF="../Body/d_specia.htm#special"><B>SPECIAL</B></A><P>
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declarations (a syntactic subset), might omit the <A REL=DEFINITION HREF="../Body/f_sin_c.htm#cos"><B>COS</B></A>, <A REL=DEFINITION HREF="../Body/f_sin_c.htm#sin"><B>SIN</B></A>, <A REL=DEFINITION HREF="../Body/f_asin_.htm#atan"><B>ATAN</B></A> functions,<P>
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might disallow the :TEST keyword to <A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A>, might restrict <A REL=DEFINITION HREF="../Body/f_ldiffc.htm#tailp"><B>TAILP</B></A><P>
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to work on proper lists, or might omit complex numbers. Each of these is a<P>
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"subset" in the sense that a subset of correct programs for the "full"<P>
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language would be correct for the "subset" language.<P>
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<P>
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Subsets can have various levels of "determinability" for programs. The<P>
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issue is: how easy is it to tell whether a program written in the "full"<P>
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language would run in a "subset" implementation? Except for the<P>
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(non-trivial) issue of macro expansions, some subsets are "lexically"<P>
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determinable, e.g., if a function is omitted, you can tell if the program<P>
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uses it by scanning the program. Some subsets are "dynamically"<P>
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determinable, e.g, a subset might signal an error if the :TEST argument to<P>
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<A REL=DEFINITION HREF="../Body/f_mk_has.htm#make-hash-table"><B>MAKE-HASH-TABLE</B></A> is <A REL=DEFINITION HREF="../Body/f_equalp.htm#equalp"><B>EQUALP</B></A>. Some subsets are neither lexically nor<P>
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dynamically determinable, e.g., if the subset implements dynamic extent for<P>
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rest lists, it may be impossible to tell even with run-type checks whether<P>
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the a program written in the "full" language would conform.<P>
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<P>
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Some "subsets" might be merely restrictive interpretations, e.g., a<P>
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"run-time" implementation that made <A REL=DEFINITION HREF="../Body/f_ed.htm#ed"><B>ED</B></A>, <A REL=DEFINITION HREF="../Body/m_tracec.htm#trace"><B>TRACE</B></A>, <A REL=DEFINITION HREF="../Body/m_tracec.htm#untrace"><B>UNTRACE</B></A> into no-ops and made<P>
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<A REL=DEFINITION HREF="../Body/f_break.htm#break"><B>BREAK</B></A> halt the program execution rather than "enter the debugger"; since we<P>
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cannot define what "enter the debugger" means, we might want to define<P>
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explicitly this subset as a reasonable one for embedded systems.<P>
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<P>
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Proposal (<A HREF="iss333.htm">SUBSETTING-POSITION:NONE</A>)<P>
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<P>
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The draft <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> we submit to ANSI contains *no* subsets. <P>
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In the section on "subsetting" it should be mentioned<P>
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that Lisp is a "small" language with a "big" library.<P>
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<P>
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We are not opposed in principle to one or more subset definitions.<P>
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<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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We have no well-defined proposals for subset definitions, and didn't have<P>
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the time or energy to pursue their definition, or confidence that we could<P>
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reach convergence on a reasonable definition.<P>
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<P>
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There are several properties that a subset definition must have to be<P>
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considered: it must be well defined in terms of conformance of code, programs, <P>
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and implementations; all valid programs in the subset must be valid programs in<P>
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the full language; the subset definition should address how it can be<P>
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determined if a program in the full language is valid in the subset. <P>
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<P>
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<P>
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<P>
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<B>Current Practice:<P>
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</B> <P>
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Pascal has two levels of conforming implementations -- level 1 contains<P>
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level 0 and conformant arrays. This was a compromise necessary to achieve<P>
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international agreement. The 1981 PL/I was subsetted and the <P>
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results were a range of implementations between the<P>
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subset and the full language; nobody wanted to use the subset so vendors<P>
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were forced to implement the full language eventually anyway.<P>
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Cobol had multiple levels of subsets. However, <P>
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the only two levels that were important were the minimum <P>
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subset and the full language. The middle levels were<P>
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seldom used other than transient points to the full language.<P>
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Fortran was subsetted. It was felt that subsetting encouraged<P>
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vendors to implement Fortran and therefore proliferate its usage,<P>
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but users were quite annoyed that one Fortran was considerably<P>
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different from another. <P>
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The new Fortran language standards committee is<P>
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banning subsetting.<P>
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At one point, there was an ANSI <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> for "Minimal Basic". It was <P>
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too minimal. Later work on ANSI Basic involved a rather different-looking <P>
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language and a number of optional extensions for such things as real-time process control. <P>
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SPARC feels that subsets aren't good for facilitating interchange, but serve the<P>
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purpose of allowing timely implementation of the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>.<P>
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<P>
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<P>
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<B>Adoption Cost:<P>
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</B> <P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B> <P>
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This policy will <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> a basis for making decisions in X3J13.<P>
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<P>
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<B>Conversion Cost:<P>
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</B> <P>
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None.<P>
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<P>
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<B>Aesthetics:<P>
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</B> <P>
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None.<P>
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<P>
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<B>Discussion:<P>
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</B>Jeff Dalton says: <P>
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"I'd be happier if it were fairly easy for someone reading the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A><P>
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to determine which part was the "library" and which the core language.<P>
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For example, where do we find <A REL=DEFINITION HREF="../Body/f_funcal.htm#funcall"><B>FUNCALL</B></A> and APPLY?<P>
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The draft C <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> has an explicit division. Section 3 is<P>
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"Language" and section 4 is "Library". It may not be necessary<P>
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to go that far for Common Lisp."<P>
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<P>
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GLS says: "I am in agreement with <A HREF="iss333.htm">SUBSETTING-POSITION:NONE</A>."<P>
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<P>
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Loosemore says: "... even if the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> doesn't define<P>
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any subsets, that is not going to prevent subsets from happening, and<P>
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perhaps the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> ought to define some terminology to describe such<P>
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implementations (even if it's only to say that they can't call<P>
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themselves Common Lisps at all)."<P>
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<P>
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<P>
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RPG says: "One point worth making might be that a conforming program that is<P>
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delivered may not also be a conforming implementation."<P>
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<B>Paraphrased: <P>
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</B>A conforming implementation does not have to be present in order to deliver<P>
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a conforming program.<P>
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One could produce a linkable version of an application that would <P>
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include almost no part of the Lisp environment.<P>
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Therefore, subsets are not mandated for this case. <P>
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</PRE>
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