CLHS: Issue DEFMACRO-LAMBDA-LIST Writeup

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## Issue DEFMACRO-LAMBDA-LIST Writeup

    Status: Passed, as amended, Mar 89 X3J13
    Issue:        DEFMACRO-LAMBDA-LIST
    Forum:	      Cleanup
    References:   8.1 Macro Definition (CLtL pp144-151),
    	      Issue DESTRUCTURING-BIND
    Category:     CLARIFICATION/CHANGE
    Edit history: 30-Jan-89, Version 1 by Pitman
    			17-Mar-89, Version 2 by Masinter
    			 9-Apr-89, Version 3 by Masinter
    				Incorporate amendments as per Mar 89 X3J13
    			10-Apr-89, V.4 Masinter (forgot an amendment)

    Problem Description:

      The description of the DEFMACRO lambda list currently contains some
      mis-statements and leaves some ambiguities:

      1. Can &BODY, &WHOLE, and &ENVIRONMENT appear at recursive levels of the
         DEFMACRO lambda list?

         The description of &WHOLE (p145) specifies that &WHOLE must occur ``first
         in the lambda list,'' but the description of a lambda list says that
         ``a lambda may [recursively] appear in place of the parameter name.''
         Consequently, the question arises whether &WHOLE should be permitted to
         be a synonym for &REST at inner levels of a DEFMACRO lambda list.

         The descriptions of &BODY and &ENVIRONMENT do not contain syntactic
         restrictions on where they may appear.

      2. Does using &WHOLE affect the pattern of arguments permitted by DEFMACRO.

    Proposal (DEFMACRO-LAMBDA-LIST:TIGHTEN-DESCRIPTION):

      1. a. Specify that &BODY may appear at any level of a DEFMACRO lambda list.

         b. Specify that &WHOLE may appear at any level of a DEFMACRO
            lambda list. At inner levels, the &WHOLE variable is bound to
    		the corresponding part of the argument, as with &REST, but
    		unlike &REST, other arguments are also allowed.

         c. Specify that &ENVIRONMENT may only appear at the top level of a
    	DEFMACRO lambda list.

      2. Clarify that using &WHOLE does not affect the pattern of arguments
         specified by DEFMACRO.

      3. Clarify that  &ENVIRONMENT may appear anywhere in the top level
         of a DEFMACRO lambda list, as in the following examples:

            (&whole W &environment E A B)   ``After &Whole''
            (&environment E &whole W A B)   ``Before &Whole''
            (&whole W A B &environment E)   ``Last''
            (&whole W A &environment E B)   ``Middle''

       4. &ENVIRONMENT can only appear once in a DEFMACRO lambda list.

    Examples:

      1. (DEFMACRO DM1A (&WHOLE FORM A B)  ...) is defined.
         (DEFMACRO DM1B (A (&WHOLE B C &OPTIONAL D) E) ...) is defined.
    		It allows simultaneousaccess to the THIRD of the whole
    		form as B and to the destructuring of that list into
    		C and D.

         (DEFMACRO DM1C (A B &ENVIRONMENT ENV) ...)     is defined.
         (DEFMACRO DM1D (A (B &ENVIRONMENT ENV) C) ...) is undefined.

         (DEFMACRO DM1E (A B &BODY X) ...)     is defined.
         (DEFMACRO DM1F (A (B &BODY X) C) ...) is defined.

      2. (DEFMACRO DM2A (&WHOLE X) `',X)

         (MACROEXPAND '(DM2A))   => (QUOTE (DM2A))
         (MACROEXPAND '(DM2A A)) is an error.

         (DEFMACRO DM2B (&WHOLE X A &OPTIONAL B) `'(,X ,A ,B))

         (MACROEXPAND '(DM2B))       is an error.
         (MACROEXPAND '(DM2B Q))     => (QUOTE ((DM2B Q) Q NIL))
         (MACROEXPAND '(DM2B Q R))   => (QUOTE ((DM2B Q R) Q R))
         (MACROEXPAND '(DM2B Q R S)) is an error.

    Rationale:

      1. a. An example on p151 makes it clear that this was the intent.

    	b. There's no cogent reason to forbid &WHOLE at inner levels.
         The example on p.150 uses &WHOLE in a non-top-level position.

    	This simplifies the implementation of DEFMACRO if we introduce a
            DESTRUCTURING-BIND which does not understand &ENVIRONMENT.

         c. The environment is never different at top level than embedded.
    	Permitting &ENVIRONMENT to occur embedded would only encourage
    	the misconception that there was a potential difference.

    	This simplifies the implementation of DEFMACRO if we introduce a
            DESTRUCTURING-BIND which does not understand &ENVIRONMENT.

      2. This allows useful syntax checking.

         One can always trivially write
    	(DEFMACRO ... (&WHOLE FORM &REST IGNORE) ...)
         to get around the error checking this forces.

      3. <vote at Mar 89 X3J13>

    Current Practice:

      1. a. Symbolics Genera permits &BODY at any level. This is compatible.
         b. Symbolics Genera seems to permit &WHOLE at any level. When embedded,
    	it is treated as a synonym for &REST.
         c. Symbolics Genera does not permit &ENVIRONMENT except at top level.
    	This is compatible.

      2. Symbolics Genera has this behavior when &WHOLE appears at top level,
         but not at recursive levels (where &WHOLE is treated as a synonym for
         &REST).

    Cost to Implementors:

      This seems to be what CLtL intended and what most implementations
      perform.

    Cost to Users:

      We think this is possibly (probably) upward compatible with most
      current practice.

    Cost of Non-Adoption:

      Some fuzzy places in DEFMACRO continue to exist.

      It's harder to introduce DESTRUCTURING-BIND because describing its relation
      to DEFMACRO is difficult.

    Benefits:

      The language is a little tighter.

    Aesthetics:

      Negligible impact.

    Discussion:

      Part 2 of this issue came up during the discussion of DOTTED-MACRO-FORMS
      but was not pursued until now.

      Pitman supports these clarifications.

      A previous version disallowed &WHOLE at inner levels, because
      of the mistaken impression that &WHOLE was equivalent to &REST.
      However, additional arguments are not allowed after &REST,
      while they are for &WHOLE.

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