Issue: DOTTED-MACRO-FORMS (Version 3)

[Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>]

Issue:        DOTTED-MACRO-FORMS
References:   forms (p54), lists and dotted lists (pp26-27),
	      DEFMACRO (p145), destructuring macro arguments (p146)
Category:     CLARIFICATION/CHANGE
Edit history: 28-Jun-88, Version 1 by Pitman   (explicitly-vague vs allow)
	      01-Oct-88, Version 2 by Masinter (disallow)
	      15-Nov-88, Version 3 by Pitman   (revive allow, flush disallow)

Problem Description:

  CLtL is not explicit about whether macro forms may be dotted lists.

  p54 says that only certain forms are "meaningful": self-evaluating
   forms, symbols, and "lists".

  pp26-27 defines "list" and "dotted list". It goes on to say
   ``Throughout this manual, unless otherwise specified, it is an
   error to pass a dotted list to a function that is specified
   to require a list as an argument.''

  p146 states that in DEFMACRO destructuring, ``the argument
   form that would match the parameter is treated as a
   (possibly dotted) list, to be used as an argument forms list
   for satisfying the parameters in the embedded lambda list.''
   It goes on to say that ". var" is treated like "&rest var"
   at any level of the defmacro lambda-list.

Proposal (DOTTED-MACRO-FORMS:ALLOW):

 Define that it is permissible for a macro form (or subform) to be
 a dotted list when "&REST var" or ". var" is used to match it. It
 is the responsibility of the macro to recognize and deal with such
 situations.

Rationale:
  
 Some implementations permit dotted lists in macro forms at toplevel.
 Most or all implementations permit dotted lists in macro forms at
 embedded levels. This proposal makes the language internally
 consistent without requiring changes to existing code.

 Also, there's no reason to unnecessarily restrict &REST since there
 is no computational overhead and since there's no dispute about how
 to interpret programmer intent in this gray area.

Test Case:

  #1: (DEFMACRO MACW (&WHOLE W &REST R) `(- ,(CDR W)))
      (MACW . 1) => ??

  #2: (DEFMACRO MACR (&REST R) `(- ,R))
      (MACR . 1) => ??

  #3: (DEFMACRO MACX (&WHOLE W) `(- ,(CDR W)))
      (MACX . 1)

    (MACW . 1) => -1 under this proposal.
    (MACR . 1) => -1 under this proposal.

    (MACX . 1) is an error under CLtL semantics and is not
	       changed by this proposal. The reason it is an
	       error is that the argument pattern does not
	       match. The pattern is dictated by the arguments
	       -other than- the &WHOLE argument, so the pattern
	       is () and MACX cannot be called with any arguments.

Current Practice:

  A. Some implementations bind W to (MACW . 1) in #1 and #3
   		      and bind R to 1 in #1 and #2.

  B. Some implementations bind W to (MACW . 1) in #3
		      and signal a syntax error in #1 and #2.

  C. Some implementations signal a syntax error in #1, #2, and #3.
     Symbolics Genera is such an implementation.

Cost to Implementors:
  
 Some implementations would have to eliminate an error check.

 Some implementations which try to use APPLY of a normal lambda
 to accomplish part of the destructuring (in the non-recursive case)
 would have to be slightly more careful.
  
Cost to Users:
  
 None. This change is upward compatible.
  
Benefits:

 People would know what to expect.

Aesthetics:

 Mixed opinion: certainly it is better to specify whether they are
 allowed or an error than to be vague.

 Some feel that disallowing dotted macro forms helps catch syntax errors.
 Some feel that allowing dotted macro forms makes the language more regular.

Discussion:

 Goldman@VAXA.ISI.EDU raised this issue on Common-Lisp.
 This issue came up primarily in the context of program-written programs;
 a macro used in the program generated code might occasionally use
 a dotted tail to a list to explicitly represent special conditions.
 
 Allowing dotted macro forms may blur the data/code distinction too much, 
 particularly for people who are new to Lisp. On the other hand, some people
 argue that the point of macros is to help blur that distinction. Macro
 forms are data which must be translated to program, and only once the
 program claims to be macroexpanded ought syntax restrictions be imposed.

 This proposal was rewritten from `DISALLOW' to `ALLOW' after Steele pointed
 out in a recent meeting that dotted lists are allowed in subforms and 
 that permitting them at toplevel would be the most internally consistent
 interpretation.

 Pitman supports DOTTED-MACRO-FORMS:ALLOW.