Status: passed, as amended, Mar 89 X3J13
References: 11.6 Built-in Packages (pp181-182)
Edit history: 22-Dec-88, Version 1 by Pitman
9-Apr-89, version 2 by Masinter, incorporate
changes per Mar 89 amendments.
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
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.
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
Similarly, rename the package USER to be COMMON-LISP-USER with
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.
=> not specified, due to this proposal. Perhaps #<Package CL>,
perhaps #<Package LISP>, or perhaps something implementation-specific.
=> not specified, not due to this proposal, but because CLtL didn't
specify this explicitly.
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.
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:
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:
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
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
ANSI's position in ISO that we are not trying to legislate the meaning of
-the- LISP dialect would be weakened.
Needless incompatibility would be avoided in a variety of situations.
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.
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.
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
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.