Issue: LISP-PACKAGE-NAME (Version 1)

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

Issue:        LISP-PACKAGE-NAME
References:   11.6 Built-in Packages (pp181-182)
Category:     CHANGE
Edit history: 22-Dec-88, Version 1 by Pitman
Status:	      For Internal Discussion

Problem Description:

  Since ANSI Common Lisp will differ from the Common Lisp described by CLtL,
  it will not be possible to have support for both in the same Lisp image
  if ANSI Common Lisp insists on placing its functionality in the package
  named LISP.

  Further, use of the name unqualified name LISP by the ANSI Common Lisp
  community is inconsistent with ANSI's expressed position to ISO that 
  the term "LISP" names a language family rather than a specific dialect
  within that family.

Proposal (LISP-PACKAGE-NAME:COMMON-LISP):

  Define that ANSI Common Lisp uses the package name COMMON-LISP, not LISP.
  Define that the COMMON-LISP package has nickname CL.

  Since some symbols (e.g., T, NIL, and LAMBDA) might have to be shared
  between COMMON-LISP and LISP in implementations simultaneously supporting
  both, clarify that the initial symbols specified by ANSI Common Lisp as
  belonging in the COMMON-LISP package need not have a home package of 
  Common-Lisp.

Test Case:

  In an implementation supporting CLtL's LISP package and 
  the ANSI Common Lisp CL package proposed here:

  (EQ 'LISP:T 'CL:T)
  => not specified, due to this proposal, but probably T

  (EQ 'LISP:CAR 'CL:CAR)
  => not specified, due to this proposal, but probably T

  (EQ 'LISP:FUNCTIONP 'CL:FUNCTIONP)
  => not specified, due to this proposal, but since FUNCTIONP is
     changed incompatibly between CLtL (LISP) and CL (ANSI), there
     are good reasons why this might return NIL.

  (SYMBOL-PACKAGE 'CL:T)
  => not specified, due to this proposal. Perhaps #<Package CL>, 
     perhaps #<Package LISP>, or perhaps something implementation-specific.

  (SYMBOL-PACKAGE 'LISP:T)
  => not specified, not due to this proposal, but because CLtL didn't
     specify this explicitly.

Rationale:

  In practice, some implementations will have very legitimate reasons for 
  wanting to Lisp dialects to be coresident. As it stands, they will have
  little other choice than to make the two use different packages, and so
  will be forced to be incompatible with one or the other dialect unless
  we choose a different package name for the one dialect for which there
  is currently no existing code.

  Not only is this important the CLtL and ANSI Common Lisp communities, but
  also, if we continue to use the name LISP, it sends a signal to the ISO
  Lisp community that the "latest and greatest" Lisp should use the generic
  name LISP, and they may try to use it as well. If ISO Lisp turns out to
  be very different than ANSI Common Lisp, there may be motivation down the
  line for having ISO Lisp and ANSI Common Lisp co-resident, and conflicts
  will inevitably arise if both want to use the name LISP. This will almost
  certainly lead to a confrontation where one Lisp dialect tries to force
  the other out by the artificial means of asserting its right to this
  generic name. Choosing a name which compatibly admits the option of
  introducing other dialects into the environment at a later date without
  conflict is a good way to avoid a class of potential problems.

  Although there are a few problems which could come up due to the symbol
  package of initial symbols being unspecified, experience with 
  implementations that do this suggests that they are very few.
  Problems occur only in the rare circumstance that all of the following
  conditions are met:

   - A symbol S on the LISP package but with home package H (that is not "LISP")
     is shadowed in some package P of implementation A.

   - A program F in package P uses the shadowed symbol H:S by an explicit
     LISP: or H: package qualification. (Only the case of using "LISP:" is
     interesting, of course, since if H were named explicitly, we would be
     outside the bounds of portable code).

   - The program F, referring to H:S, is printed out in implementation A 
     while using package P (or some other package that shadows S, so that
     the H package qualifier appears explicitly) and an attempt is made to
     re-read it in implementation B.

   - Implementation B has no package named H, has a package named H but no
     external symbol named S, or has a package named H with external symbol
     S but the symbol H:S has different semantics in implementation B than
     it did in implementation A.

  In practice, this hardly ever happens. It would happen even less if 
  programmers were explicitly alerted that it was a potential problem they
  needed to guard against.

Current Practice:

  Symbolics Genera already has a package named COMMON-LISP with nicknames
  CL and LISP. As such, this would be an incompatible change for Genera.

Cost to Implementors:

  Small.

  In some cases, this may even have `negative cost' because it will provide
  implementors a way of avoiding incompatible changes to released operators.

Cost to Users:

  Small.

  In some cases, this may even have `negative cost' because existing code
  would be able to continue to run in implementations which chose to support
  both CLtL's LISP and ANSI Common Lisp's CL packages, thereby allowing
  developers to put off a massover changeover, perhaps doing the transition
  more incrementally.

Cost of Non-Adoption:

  Implementations trying to support multiple dialects in the same environment
  would be forced to violate one or the other spec.

  Worse, different implementations faced with the same set of hard choices
  about which spec to violate in order to concurrently support two dialects
  might not make the same choices, leading to even more gratuitous 
  incompatibility.

  ANSI's position in ISO that we are not trying to legislate the meaning of
  -the- LISP dialect would be weakened.

Benefits:

  Needless incompatibility would be avoided in a variety of situations.

Aesthetics:

  Failing to specify the home package of symbols in the LISP and CL packages
  seems unaesthetic because it appears to diminish print/read invertability,
  but as observed above, that case is rare.

  Failiing to specify a way in which lisp dialects can be co-resident is also
  unaesthetic because in practice implementors with a need to do this will do
  so whether the standard allows them or not, and it will be a source of 
  severe divergence among implementations.

Discussion:

  Symbolics Genera offers two co-resident dialects of Lisp: Zetalisp and
  Symbolics Common Lisp. The Symbolics Cloe development environment adds
  a third co-resident dialect, making an environment in which two differing
  Common Lisp dialects (Symbolics Common Lisp and Cloe) must cooperate.
  Already in Cloe it is not possible for the home package to contain 
  package "LISP" since Cloe's concept of what the "LISP" package is differs
  from Genera's concept of what the "LISP" package is, yet they are forced
  by efficiency constraints to share the same symbol. It is Pitman's belief,
  based on extensive experience with Cloe, that failure to pass this proposal
  (or something very like it) will lead to all sorts of trouble for Common
  Lisp users and implementors down the road.

  Pitman strongly supports this proposal.