Issue DOTTED-MACRO-FORMS Writeup

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.