Issue LISP-PACKAGE-NAME Writeup

Status: passed, as amended, Mar 89 X3J13


Issue:        LISP-PACKAGE-NAME

References:   11.6 Built-in Packages (pp181-182)

Category:     CHANGE

Edit history: 22-Dec-88, Version 1 by Pitman

		9-Apr-89, version 2 by Masinter, incorporate

		changes per Mar 89 amendments.


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.


  Similarly, rename the package USER to be COMMON-LISP-USER with

  nickname CL-USER.


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.


Additional comments:


Is it permissible for implementations to define

"LISP" as a nickname for this package, for the

sake of backward compatibility?


Anyone wanting to make LISP a nickname could just as well create a LISP

package which simply imported the appropriate symbols from the CL package.


With only modest additional effort, they could try to make new symbols where

feasable (especially for most functions) and put borrowed functions plopped

in their function cells. The amount of additional storage is small (compared

to implementing a whole new lisp), but it would leave open the possibility for

users upgrading the level of compatibility without hurting the core

system. eg, if I wanted APPEND to signal an error on dotted lists, I

would not consider redefining the system's APPEND for fear of breaking

the world, but if they told me that nothing depended on LISP other than

compatibility code, I might feel ok about redefining (or doing

SHADOWING-IMPORT of LISP:APPEND on a per-implementation basis (with

appropriate sharp conditionals)) in order to up the level of

compatibility.


In fact, though, my guess is that implementations which are not going to

do a serious compatibility effort are better off leaving the package

missing. My experience has been that customers are often happier growing

their own compatibility [or getting it from a public library] than being

stuck with something which really doesn't do what they want but which

seals off the place in the namespace which they needed in order to do

their own thing.