Issue: IGNORE-VARIABLE (Version 1)

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

This was already being discussed under DESTRUCTURING-BIND. I'm
spawning a new issue name to help partition/focus the discussion.

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Issue:        IGNORE-VARIABLE
Forum:        Cleanup
References:   IGNORE declaration (p160)
Category:     CHANGE
Edit history: 06-Feb-89, Version 1 by Pitman
Status:       For Internal Discussion

Problem Description:

  Many users of Symbolics Common Lisp (under Symbolics Genera) have grown
  used to the variable named `IGNORE' receiving special treatment and have
  complained that Common Lisp does not offer the same `feature.'

Proposal (IGNORE-VARIABLE:SPECIAL-TREATMENT):

  1. Define that the variable IGNORE (in the LISP package only) is always
     implicitly ignored. It is an error to use the variable IGNORE.
     A declaration of (IGNORE IGNORE) is permitted, but redundant. 

  2. Permit the variable IGNORE (or any variable declared ignored with the
     IGNORE declaration) to be a duplicated variable in a binding list.

Rationale:

  1. Greater syntactic conciseness.
     This is effectively current practice in some implementations.

  2. If only one variable is going to be dignified in this way, it must
     be possible to repeat it. If it makes sense to repeat IGNORE, it makes
     sense to repeat any name declared IGNORE.

Test Case:
                                                    CLtL        Proposed
  #1: (DEFUN FOO (IGNORE) T)                        may warn    ok
  #2: (DEFUN FOO (IGNORE) IGNORE)                   ok          is error
  #3: (DEFUN FOO (IGNORE IGNORE) T)                 is error    ok
  #4: (DEFUN FOO (IGNORE IGNORE) IGNORE)            is error    is error
  #5: (DEFUN FOO (X X) (DECLARE (IGNORE X)) T)      is error    ok
  #6: (DEFUN FOO (X X) (DECLARE (IGNORE X)) X)      is error    is error

Current Practice:

  Symbolics Genera currently treats all variables with the name "IGNORE"
  (regardless of package) as ignored variables and complains if you try
  to use them. It provides no way to turn off the `feature.'

  Symbolics Cloe does not special-case variables named "IGNORE".

Cost to Implementors:

  Small.

Cost to Users:

  Although the change is technically incompatible, very few programs would
  be likely to require change, since a name like IGNORE is unlikely to have
  been used for anything other than an ignored variable.

Cost of Non-Adoption:

  Some programs would be clumsier to write.

Benefits:

  Less verbose code in some cases.
  Existing code in some dialects would not require translation.

Aesthetics:

  At first brush, some might argue that this makes a bad special case in
  the treatment of symbols as variables. However, on closer inspection,
  it's clear that the language already treats some symbols magically:

   - Symbols on the keyword package are treated specially with respect
     to their values.

   - Symbols like *PRINT-LEVEL*, etc. have pre-defined special meanings
     and cannot be bound without knowing their conventions.

   - Symbols like NIL and MOST-POSITIVE-FIXNUM have pre-defined values
     and cannot be bound at all.

  The thing which makes these changes palatable is that none of them is
  based on the symbol's name. As such, anyone unhappy with this treatment
  can simply make a new package that shadows the symbol and the symbol will
  be treated normally if that is what is desired.

Discussion:

  When translating Macsyma from Zetalisp to Common Lisp a few years ago,
  the use of Maclisp/LispM-style IGNORE was so pervasive that Pitman
  ultimately just did (PROCLAIM '(SPECIAL IGNORE)) to muffle all the
  would-be warnings. Except for the situation of duplicate variable names,
  this is a totally portable solution, but it is -not- efficient.

  Some people have suggested that (PROCLAIM '(IGNORE IGNORE)) should
  work, but others argue that PROCLAIM should not, in general, be assumed
  to affect lexical references.

  Moon and Pitman mildly support option SPECIAL-TREATMENT.

  Symbolics Common Lisp users have often expressed a desire to see this
  feature in portable Common Lisp.

  Some Symbolics users have complained specifically about the 
  incompatibility between Symbolics Genera and Symbolics Cloe in their
  treatment of the variable IGNORE. Although Cloe has CLtL to fall back
  on for justification, Cloe is invariably seen as the "bad guy" in their
  reports because it stubbornly keeps them from getting the functionality
  they want on the basis of what to them seems an irrelevant religious
  or philosophical concern.

  Some people might want to see a symmetric treatment of an ignored
  variable in SETQ and MULTIPLE-VALUE-SETQ.

  Maclisp used to permit NIL to denote an ignored variable. In general
  this worked out well. The disadvantages to using NIL over using
  IGNORE are:
   - The common kind of macroexpansion error where NIL is substituted
     for a useful value is harder to detect. 
   - There would be a potential conflict between the any meaning for NIL
     in its `proper role' at the end of a dotted list and any opposing
     meaning that a destructuring operation (eg, DEFMACRO, LOOP, 
     DESTRUCTURING-BIND) might want to assign to a dotted variable in
     that position.

  Note well: Experience with Symbolics Genera shows that special case
  treatment based only on the name -- or some substring thereof -- and
  not also on the package is a bad idea because there is no way to get
  away from the feature if you don't like it. It is therefore an important
  aspect of this proposal that only LISP:IGNORE is affected, and that an
  EQ-test (and not a substring comparison) is the criterion for 
  identifying this magic variable.