perm filename COMLIS.REP[COM,LSP] blob sn#632909 filedate 1981-12-21 generic text, type T, neo UTF8
Subject: Report on Common Lisp to the Interlisp Community

Introduction

What is Common Lisp?

Common Lisp is a proposal for a new dialect of Lisp, intended be
implemented on a broad class of machines. Common Lisp is an attempt to
bring together the various implementors of the descendents of the
MacLisp dialect of Lisp. The Common Lisp effort is apparently an
outgrowth of the concern that there was a lack of coordination and
concerted effort among the implementors of the descendents of MacLisp.
Common Lisp's principle designers come from the implementors of Lisp
Machine Lisp (at MIT and Symbolics), Spice Lisp (at CMU), NIL (both a
VAX implementation at MIT and the S1 implementation at Lawrence
Livermore), and MacLisp (at MIT, of course.) Also represented, but not
as strongly, are Standard Lisp (at Utah) and Interlisp (at PARC, BBN and
ISI.)

Programs written entirely in Common Lisp are supposed to be readily run
on any machine supporting Common Lisp.  The language is being designed
with the intent first of creating a good language, and only secondarily
to be as compatible as practical with existing dialects (principally
Lisp Machine Lisp.)

Common Lisp attempts to draw on the many years of experience with
existing Lisp dialects; it tries to avoid constructs that are obviously
machine-dependent, yet include constructs that have the possibility of
efficient implementation on specific machines.

Thus, there are several different goals of Common Lisp: (1) to profit
from previous experience in Lisp systems, (2) to make efficient use of
new and existing hardware, and (3) to be compatible with existing
dialects. These various interests pull in different directions on the
design of the language.  The desire to have a core language within which
programs are portable urges that the language be functionally complete,
so that you can write a wide range of programs in it.  The desire to
implement the language on a wide range of machines urges that the
language be compact, and not have features that are tricky to implement.
The goal of making the language good or "right" is at odds with keeping
compatibility with existing languages that suffer various blemishes.
Allowing implementations freedom to perform certain operations more
efficiently encourages vagueness in the specification, but that
vagueness makes the language weaker and can be a rich source of bugs.

These conflicting goals make consensus in the Common Lisp design
difficult. The design of the language is not yet nearly complete. The
amount of convergence and the number of resolved issued is impressive,
but there are many difficult issues ahead.

No machine yet implements Common Lisp. The specification of Common Lisp
is complex enough (and different enough from any existing dialect) that
we believe that a full implementation will involve a significant amount
of work once the design is stable. There is often a large amount of
optimism in the time alloted for future implementations of Lisp dialects
that has yet to be confirmed by any experience we know of.

The November Common Lisp Meeting

On Nov 23-24, 1981, Bill van Melle, as Interlisp representitive and
proxy holder for Larry Masinter attended a meeting of the various
implementors of Common Lisp. The meeting was held at Symbolics in
Cambridge. Also in attendance were Guy Steele (CMU), Dick Gabriel
(Stanford), Jon L White (MIT), George Carrette (MIT), David Moon
(Symbolics), Howard Cannon (Symbolics), Daniel Weinreb (Symbolics), Alan
Bawden (MIT), Rod Brooks (MIT), and Richard Bryan (MIT).

The meeting was called to discuss some of the issues that had arisen in
putting together the Common Lisp specification.  The intention was to
resolve as many as possible of those issues to the satisfaction of the
people concerned. This report concerns both the outcome of that meeting
and the Common Lisp effort in general.

There was no formal voting procedure or official mechanism to determine
the relative "voting strengths" of those present.  As much as possible,
the group tried to resolve issues by acclaim.  Lone dissenters were
generally ignored, unless they were especially vehement.  If there
remained substantial disagreement over an issue even after discussion,
the issue was deferred (though usually by that point the choices on the
issue had at least been narrowed).

Informally, the Lisp Machine representatives tended to have the greatest
clout, and one of their principal concerns was that Common Lisp be as
compatible as possible with Lisp Machine Lisp, even if that meant
retaining blemishes in the language (e.g. bad order of args to putprop,
push, etc).

The interest of the Interlisp community.

The Interlisp community's interest in this discussion is not immediately
obvious. None of the various possible relations with Common Lisp will be
simple: it will be difficult to make Interlisp programs mechanically
transportable to Common Lisp, to make Common Lisp programs mechanically
transportable to Interlisp, or to implement a Common Lisp given an
existing Interlisp.

Thus, for the most part, we cast votes in favor of making the language a
good language, seeking completeness and perspicuity.  It ought to be the
case that two programmers speaking different Lisp dialects should be
able to look at each other's programs and have a good chance of
following them without resorting to careful reading of a thick manual.
This argues strongly in favor of keeping the set of standard functions
small, yet allowing them to express the most common programming needs.
There were places where we voted against blatant incompatibilities with
Interlisp, but not always successfully (we agreed that at best, porting
programs between Interlisp and Common Lisp could be done with mechanical
assistance).  There were other occasions where we suggested constructs
taken from Interlisp to help resolve some difficulty, or fill in a gap
in the language definition.

The rest of this report explains some of the major issues, especially
those where compatibility with Interlisp seems problematic.

1. Symbols NIL and T.

These symbols ("litatoms" in Interlisp parlance) have since the
beginning been "special" in most Lisp implementations; e.g. you can't
bind or set them (or you can, producing strange and wondrous effects),
you can't put properties on NIL.  The implementors of the language NIL
decided to remove this blemish by making NIL and T be ordinary symbols.
The value "false" is still the empty list, but it is spelled (), and is
not a symbol; the value "true" is a special "truth" constant, not T,
spelled #t.  [# is a readmacro used to read many sorts of special
constructs.]

Most other implementors who expressed any opinion view this change as
anywhere from silly to absolutely unacceptable.  The original Common
Lisp specification attempted to compromise on this issue by allowing but
not requiring that NIL and () be different entities, and that (symbolp
()) be allowed to be true.  That introduces an undesirable vagueness,
something we had been trying to fight elsewhere.  And it is not a
particularly grand compromise: in order for programs to be transportable
between implementations, and in particular for a program written using
NIL as false to be readable by an implementation where NIL and () are
different, it is necessary at least that the empty list always PRINT as
(); furthermore, programmers must not pass around the symbol T unquoted.

An impressive amount of time was spent arguing about this issue at the
November meeting.  Many people find () and 'T an aesthetic abomination,
and were aggravated that we were wasting so much time on this
comparatively petty issue.  There is little hope that Common Lisp will
eradicate all warts from the face of the language, and this does not
seem to be a wart that is worth removing (David Moon summed it up nicely
by saying that NIL and T are not warts, but rather birthmarks).  It was
thought that this change would cause much distress to the MacLisp, Lisp
Machine Lisp and Interlisp communities for little gain; the NIL
representatives claim a user community as well (not independently
substantiated), but admitted that users have not been flocking to use
NIL and T as variables.

The issue was left unresolved.

2. Case preservation in pnames.

In MacLisp and friends, READ canonicalizes atom names to all uppercase
(except where explicitly quoted).  Interlisp preserves case (although
for terminal input you can be in "raise" mode, where lower-case is read
as upper-case).  The effect in MacLisp is that case is largely
ignored--you type your program in whatever case you want, but ultimately
all symbols are uppercase.  Since MacLisp users typically edit their
programs on files using text editors, the case of their input is in some
sense automatically preserved.  In Interlisp, user programs are edited
inside Lisp and prettyprinted back out to files, so it is important that
case is preserved (assuming you ever want to be able to use lowercase).

It is difficult, if not outright impossible, to have it both ways (have
symbols read in correctly independent of the case in which they are
typed, yet have the case of symbols you care about preserved), but this
may not really be a significant issue.  Except for use of symbols
(rather than strings) in printed text, and for those users who define
distinct functions and/or variables that differ only in case, it appears
that a program in a case-preserving language will still run fine in a
non-case preserving dialect, and that a program in a non-case preserving
dialect will run in a case-preserving one if the program is first
prettyprinted out to a file (so that all the symbols are in the same
case).

3. Sequence Functions.

Common Lisp has the notion of a "sequence", something that has elements
that can be enumerated in an obvious linear fashion.  Standard sequences
are lists, strings and vectors (one-dimensional arrays).  There are
operations that are useful on any sequence, independent of its
representation.  For example, determining the length of the sequence,
extracting/replacing the nth element, sorting the sequence.  Common Lisp
proposes introducing functions that operate on sequences.  Many are old
favorites from the list world, e.g. LENGTH, REMOVE, REVERSE, extended
now to other kinds of sequences.  There are two mjor issues here.

 3a. Type-specific sequence functions.

"Generic" sequence functions may incur significant overhead over
type-specific sequence functions; and generic functions "catch fewer
errors", e.g., where you think you're handing around a list but really
it's a string.  The former is really only significant for a few
primitive operations (elt, length) that can be cleverly open-coded; and
it's not clear the latter is much worse than the situation that exists
already with numbers. (All of the Common Lisp numeric functions are
"generic" in that they accept numbers of any type (integers, floating
point numbers or ratios, sometimes complexes) and behave like
type-specific functions of the type suggested by their arguments.)

An earlier proposal of Common Lisp had the notion of type-specific
functions in addition to each generic function, e.g., list-length,
string-length and vector-length in addition to length. This was rejected
at the November meeting; we opted instead for a means of declaring the
types of arguments in those cases where there is efficiency or
documentation concern.  The preferred choice for declaration was a type
declaration wrapper around designated arguments, transparent to the
interpreter (except for optional error checking) but useable by the
compiler.  The THE construct from the Interlisp DECL package was
tentatively considered for this.

  3b. Variations of common sequence functions.

The issue here is how to specify in a convenient way the various forms
of the same basic operation.  For example, in traditional Lisp, there
are some functions that differ only in whether they test elements of a
list using EQ or EQUAL. (E.g., MEMB and MEMBER in Interlisp.)  Common
Lisp wants you to be able to write something that means "remove from y
all elements that satisfy condition p", where the condition might mean
"EQ to z", "EQUAL to z", or some arbitrary condition, without having to
make the common cases ugly or awkward (the first two cases are MacLisp's
REMOVE and REMQ functions, I believe).

An earlier Common Lisp draft had a profusion of separate functions for
each of these. This was rejected at the November meeting, but no
alternative was agreed upon. The major contenders were

 (1) some keyword based scheme, e.g., (member x lst :test (function
superequal)), or (remove lst :if-not (function baz)); and somewhat less
enthusiastically,

 (2) a functional programming style, where each sequence operator has a
functional operator that produces a function to do the desired thing,
e.g. the two previous examples would be ((fmember (function superequal)
x) lst) and ((fremove (function (lambda (x) (not (baz x)))) lst).

As an adjunct to (1), it was also recommended that LOOP (see below) be
used to handle the less common cases.

4. Iterative constructs

Some of the MacLisp dialects have a construct called LOOP, which is
similar to Interlisp's CLISP iterative constructs.  There are some
objections to it (it is too complicated; code-walking programs have to
parse it, and the translation into a PROG with GO's is ugly to analyze),
but most people seem to think it is a win, or at least is more desirable
than having lots of different functions to do special kinds of
iterations.  It's not clear that the more violent objections are based
on anything more than lack of familiarity.

It was proposed that we wait for the Lisp Machine people to submit a
revised spec for LOOP that counters some of the objections; if the
result is satisfactory, several functions may fall by the wayside.

4. Multiple values.

Lisp Machine Lisp allows functions to return multiple values.  The idea
is that this allows a function to return more than one result to its
caller without having to create a list containing the results, or
resorting to the unaesthetic method of setting freely some variables
that are bound in the caller.  In order to use the multiple results, the
caller encloses the call in a construct that in some way spreads the
results; the principle methods are a "multiple-value setq" form and a
"multiple-value bind" form.  If the caller just uses the result of a
function as a single value, it gets the first of the multiple values and
the rest are discarded.  Thus, most of the time, you need never know
that a function returns multiple values; this means that system
functions can return possibly useful extra values that the caller can
use or not as it chooses. For example, QUOTIENT returns the remainder as
the second value, since the remainder is frequently computed anyway as a
side effect of the division.

Common Lisp currently requires the support of the multiple values
feature.  Unfortunately, the use of multiple values severely corrupts
the semantics of Lisp, and nobody has yet figured out how to "do
multiple values right".  The major corruption is that, as currently
designed, you can't bind a variable to multiple values.  This means it
is difficult to transmit multiple values in a transparent fashion,
making it a tricky, if not impossible business to "break" or "advise" a
function that returns multiple values.  It also inhibits some compiler
optimizations; e.g. it is no longer true that you can transform the
sequence (SETQ X (FOO)), (RETURN X) into (RETURN (FOO)), because the
latter may return multiple values while the former never does.  It
complicates an implementation's call/return mechanism to accomodate an
infrequent case, resulting in somewhat arbitrary restrictions on how
multiple values are handled by certain forms.  If FOO returns multiple
values, then should (PROG1 (FOO) --)?  What about (OR (FOO) (FIE))?
(AND (FOO) (FIE))?  Requiring that these forms transmit multiple values
somewhat cramps the implementation in order to satisfy a very infrequent
case, yet not requiring this leaves gaping inconsistencies in the
handling of multiple values.

No real decision was reached.  We argued against multiple values until
someone has figured out how to do them right, but this was not a popular
sentiment.  It was proposed that there be a construct that allows you to
bind a variable to the (conceptual) multiple-value vector returned from
a function, so that a program can at least pass values thru itself
without great hassle.  There is still a fair amount of design to go on
here.

6. Arithmetic

There is a great deal of interest that Common Lisp support a full range
of arithmetic, at least as much as you would want for an algebraic
manipulation system such as Macsyma.  (We believe this is the primary
goal of the S1 implementation, for example.)  In addition to
conventional integers and floating-point numbers, Common Lisp provides
arbitrary-precision integers (bignums), ratios (i.e., fractions
consisting of two integers, possibly big themselves) and complex numbers
(pairs of floating-point numbers).

We did not argue against requiring these in the Common Lisp standard,
because their presence seemed necessary if there were in fact going to
be Common Lisp algebraic manipulation systems.

There were some questions on how wide a range of functions to supply
(e.g. should there be trigonometric functions that work in degrees?),
and how much to shield the user from "optional" types, such as complex.
For example, what should (SQRT -1) return--a complex number or an error?

 7. Characters.

Common Lisp includes an elaborate specification for how characters are
handled.  It tries to encompass varying character sets (e.g. to let
Common Lisp work on machines using EBCDIC), keyboards with multiple
shifts (e.g. control, meta, super, hyper,...), and multiple fonts.
Characters are (possibly) distinguished as a separate datatype (not
integer character codes or litatoms), and a large collection of
functions is supplied to avoid ever having to do arithmetic on
characters (e.g., alphanumericp, upper-casep).  Those functions also
serve the purpose of allowing various character sets.  Shifts and fonts
are accommodated as extra numerical attributes of a character, with
functions to test/extract/set those attributes.

Unfortunately, there is a lot of confusion here.  Shift bits such as
Meta- or Greek- are relevant to keyboard input (on some
machines/keyboards), but make no sense in files.  Similarly, keystrokes
don't intrinsically have fonts.  Identifying fonts by a small set of
numbers is a confining notion; our experience in Interlisp-D is that it
is better to have "change font" on a stream with ability for full font
specification (family, size, boldness, etc); font numbers only lead to
confusion between systems with different "font-sets".  The
implementation may still have font numbers at some level, but it is not
so good at the user level. There was little discussion of this issue at
the November meeting.

The Common Lisp specification also tries to define a common "standard"
character set, which is basically all ASCII printing characters, plus
some format characters.  The discussion on format characters contains
some of the same confusion mentioned above.  For example, one person
argued strenuously that <rubout> be a standard character, and then
people argued on about whether <rubout> could appear in files.  (He
wanted a character such that he could write (eq (tyi) #\Rubout) in order
to write a simple input routine that any Common Lisp could run.)

8. Name conflicts.

There are some hopeless incompatibilities with Interlisp of a syntactic
nature, part of why a mechanical pre-processor will be important in
moving programs between Interlisp and Common Lisp.  For example, PUTPROP
and PUSH do not take their "new value" argument last, as is the
convention most everywhere else.  Indexing functions (NTH, ELT) are
zero-based instead of one-based.  SELECTQ specifies the default
differently.  The function LISTP means "list or NIL", and there is a
separate function CONSP that does what Interlisp's LISTP does.

9. Is a "property list" a list in prop value format?

Some implementations want freedom in representing a symbol's "property
list".  E.g. Standard Lisp wants to use an a-list.  Discussion proceeded
on how to properly abstract the property list, and conjectures on to
what extent programmers rely on the format of the property list. This
hasn't been resolved yet.

10. Macros vs Special Forms vs functions.

Common Lisp currently prohibits special forms (NLAMBDA's in Interlisp)
in APPLY and FUNCALL (= APPLY*).  The reasoning is that some of these
may be implemented via macros rather than NLAMBDA functions which
explicitly evaluate their arguments.

11. NIL vs optional.

There are some places in the Common Lisp design where a null argument is
treated differently than an omitted argument.  We have requested that
these be avoided, on the grounds of obvious confusion.