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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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