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.