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Status: Proposal LARGE, with sections 7, 8, 9 removed, passed Mar 89 X3J13
Issue: FUNCTION-NAME
References: SETF rules for what -place- can be (pp.94-7)
FBOUNDP function (p.90)
FMAKUNBOUND function (p.92)
FUNCTION special form (p.87)
SYMBOL-FUNCTION and setf of symbol-function (p.90)
88-002R pages 1-21, 2-21, 2-26, 2-39, 2-44, 2-46, 2-51, and 2-55
(There are additional references for the MEDIUM and LARGE
proposals, but they are not listed here. They're obvious.)
Related issues: SETF-FUNCTION-VS-MACRO, SETF-PLACES (both subsumed by this)
Category: ADDITION
Edit history: Version 1, 23-Jan-89, by Moon
(based on discussion at Jan X3J13 meeting)
Problem description:
The Common Lisp Object System needs a well-defined way to relate the name
and arguments of a writer function to those of a reader function, because
both functions can be generic and can have user-defined methods. The way
that was adopted into Common Lisp when X3J13 voted to accept document
88-002R was to use a list (SETF reader) as the name of the writer function.
Some changes to the non-object-oriented portion of Common Lisp are required
in order to support this.
This issue has three proposals.
Proposal (FUNCTION-NAME:SMALL):
Add a new concept "function-name" (called "function-specifier" in
88-002R). A function-name is either a symbol or a 2-element list whose
first element is the symbol SETF and whose second element is a symbol.
Implementations are free to extend the syntax of function-names to
include lists beginning with additional symbols other than SETF.
Add a new function (FDEFINITION function-name), which returns the
current global function definition named by function-name, or signals
an error if there is no global function definition. This follows all
the same rules listed for SYMBOL-FUNCTION in CLtL p.90.
Add SETF of FDEFINITION to change the current global function definition
named by a function-name. This follows all the same rules listed for
SETF of SYMBOL-FUNCTION in CLtL p.90.
Change the FBOUNDP and FMAKUNBOUND functions, and the FUNCTION special
form, to accept function-names in place of symbols. Implementation
defined extensions to the syntax of function-names cannot use the
symbol LAMBDA, since FUNCTION already uses that symbol.
Change the rules for SETF places (CLtL pp.94-7) by adding the following
clause after all the existing clauses:
- Any other list whose first element is a symbol, call it reader.
In this case, SETF expands into a call to the function named by the
list (SETF reader). The first argument is the new value and the
remaining arguments are the values of the remaining elements of
-place-. This expansion occurs regardless of whether reader or
(SETF reader) is defined as a function locally, globally, or not at
all. For example,
(SETF (reader arg1 arg2...) new-value)
expands into a form with the same effect and value as
(LET ((#:temp-1 arg1) ;force correct order of evaluation
(#:temp-2 arg2)
...
(#:temp-0 new-value))
(FUNCALL (FUNCTION (SETF reader)) #:temp-0 #:temp-1 #:temp-2...)).
Change the functions GET-SETF-METHOD and GET-SETF-METHOD-MULTIPLE-VALUE
to implement the above change to the rules.
Document that a function named (SETF reader) should return its first
argument as its only value, in order to preserve the semantics of SETF.
Change the macro DEFGENERIC and the function ENSURE-GENERIC-FUNCTION to
refer to the function FDEFINITION where they now refer to the function
Change the macros DEFCLASS, DEFGENERIC, and DEFMETHOD, the special forms
GENERIC-FLET and GENERIC-LABELS, and the functions DOCUMENTATION and
ENSURE-GENERIC-FUNCTION to use the term "function-name" where they now
use the term "function-specifier" or "function specifier".
Rationale for FUNCTION-NAME:SMALL:
This is the minimum change to Common Lisp needed to do what 88-002R says
about (SETF reader). Giving implementations freedom to extend the syntax
of function-names allows for current practice. Changing the name from
"function-specifier" to "function-name" avoids confusion and improves
consistency with the rest of the language, at the cost of a few small
changes to 88-002R.
Proposal (FUNCTION-NAME:MEDIUM):
Everything in FUNCTION-NAME:SMALL, and in addition:
Change the DEFUN macro to accept a function-name for its name argument,
instead of only accepting a symbol. If function-name is (SETF sym),
the body is surrounded by an implicit block named sym.
Rationale for FUNCTION-NAME:MEDIUM:
Keeping DEFUN consistent with DEFMETHOD is a good idea. Also 88-002R
says "The name of a generic function, like the name of an ordinary
function, can be either a symbol or a two-element list whose...", which
implies this change to DEFUN.
Proposal (FUNCTION-NAME:LARGE):
Everything in FUNCTION-NAME:MEDIUM, and in addition the following
numbered points, each of which could be adopted independently,
except where explicitly noted:
1. Change the function COMPILE to accept a function-name as its name
argument.
2. Change the function DISASSEMBLE to accept a function-name as its name
argument.
3. Change the FTYPE, INLINE, and NOTINLINE declarations and proclamations
to accept function-names, not just symbols, as function names.
4. Change the FLET and LABELS special forms to accept a function-name in
the name position, not just a symbol.
5. Change the TRACE and UNTRACE macros to accept function-names, not just
symbols, in the function name positions.
6. Change the ED function to accept (ED function-name) in place of
(ED symbol).
7. Change the syntax of a function call to allow a function-name as the
first element of the list, rather than allowing only a symbol.
8. Change the DEFMACRO macro and the MACROLET special form to accept a
function-name in the name position, not just a symbol. Change the
MACRO-FUNCTION function to accept function-names, not just symbols.
Change the last rule for SETF places to use
((SETF reader) #:temp-0 #:temp-1 #:temp-2...)
in place of
(FUNCALL (FUNCTION (SETF reader)) #:temp-0 #:temp-1 #:temp-2...)
so that (SETF reader) can be defined as a macro. This depends on item
7. If item 4 is rejected, MACROLET should be stricken from this item.
9. Add an optional environment argument to FDEFINITION, SETF of
FDEFINITION, FBOUNDP, and FMAKUNBOUND. This is the same as the
&environment argument to a macroexpander. This argument can be used to
access local function definitions, to access function definitions in the
compile-time remote environment, and to modify function definitions in
the compile-time remote environment.
10. Change the second, third, fourth, fifth, seventh, and ninth rules for
SETF places so that they only apply when the function-name refers to the
global function definition, rather than a locally defined function or
macro. (The ninth rule is the one that refers to DEFSETF and
DEFINE-SETF-METHOD; the other rules listed are the ones that list
specific built-in functions). The effect of this change is that SETF
methods defined for global functions are ignored when there is a local
function binding; instead, the function named (SETF reader), which may
have a local function binding, is called. This change is most useful
in connection with item 4, but does not actually depend on it.
11. Clarify that the eighth rule for SETF places (the one for macros)
uses MACROEXPAND-1, not MACROEXPAND.
Rationale for FUNCTION-NAME:LARGE:
This extends the new feature throughout the language, in order to make
things generally more consistent and powerful. Point by point:
1,2,3 - one should be able to compile, examine, and make declarations
about functions regardless of whether they are named with symbols or
with lists.
4 - locally defined non-generic SETF functions are a logical companion
to locally defined generic SETF functions, which can be defined with
GENERIC-FLET or GENERIC-LABELS. They make sense on their own, since one
might define a local reader function and want a local writer function
to go with it.
5,6 - one should be able to apply development tools to functions
regardless of how they are named. The function DOCUMENTATION was already
updated to work for function-names by 88-002R. There might be some
difficulty with implementation-dependent syntax extensions to TRACE and
UNTRACE conflicting with this new syntax.
7 - this restores consistency between the FUNCTION special form and the
first element of a function call form.
8 - it seems more consistent to allow macros to be named the same way
that ordinary functions are named. However, this might be considered
redundant with DEFSETF.
9 - this is not needed by the "chapter 1 and 2" level of CLOS, but might
be used by the metaobject based implementation of ENSURE-GENERIC-FUNCTION.
10 - this change was in SETF-FUNCTION-VS-MACRO and makes item 4 more useful.
11 - this change was in SETF-FUNCTION-VS-MACRO and is a good idea, but
actually is independent of everything else being proposed here.
Examples:
;This is an example of the sort of syntax 88-002R allows
(defmethod (setf child) (new-value (parent some-class))
(setf (slot-value 'child parent) new-value)
(update-dependencies parent)
new-value)
(setf (child foo) bar)
;If SETF of SUBSEQ was not already built into Common Lisp,
;it could have been defined like this, if the MEDIUM or LARGE
;proposal is adopted.
(defun (setf subseq) (new-value sequence start &optional end)
(unless end (setq end (length sequence)))
(setq end (min end (+ start (length new-value))))
(j 0 (1+ j)))
((= i end) new-value)
(setf (elt sequence i) (elt new-value j))))
;The preceding example would have to be defined like this
;if only the SMALL proposal is adopted. This is a method
;all of whose parameter specializer names are T.
(defmethod (setf subseq) (new-value sequence start &optional end)
(unless end (setq end (length sequence)))
(setq end (min end (+ start (length new-value))))
(j 0 (1+ j)))
((= i end) new-value)
(setf (elt sequence i) (elt new-value j))))
;Another example, showing a locally defined setf function
(defun frobulate (mumble)
(let ((table (mumble-table mumble)))
(flet ((foo (x)
(gethash x table))
((setf foo) (new x)
(setf (gethash x table) new)))
..
(foo a)
..
(setf (foo a) b))))
;get-setf-method could implement setf functions by calling
;this function when the earlier rules do not apply
(defun get-setf-method-for-setf-function (form)
(temp-vars (do ((a (cdr form) (cdr a))
((null a) v))))
(values temp-vars
(cdr form)
(list new-value)
`(funcall #'(setf ,(car form)) ,new-value ,@temp-vars)
`(,(car form) ,@temp-vars))))
Current practice:
No implementation supports exactly what is proposed. Symbolics Genera
and the TI Explorer support something close to the MEDIUM proposal, but
differing in a number of details. Symbolics Genera supports items 1, 2,
3, 6, and 11, and modified forms of items 5 and 8, of the LARGE proposal.
Moon considers this proposal's variations from Symbolics current practice
to be an improvement, although incompatible in some cases.
Many implementations currently support only symbols as function names.
Symbolics Genera and the TI Explorer have some additional function-name
syntaxes.
Cost to Implementors:
The SMALL and MEDIUM proposals are estimated to be no more than 50 lines
of code and require no changes to the "guts" of the interpreter and
compiler. Most of the code for this can be written portably and was
shown on two slides at the X3J13 meeting.
Some of the changes in the LARGE proposal are trivial, some require
the compiler to use EQUAL instead of EQ to compare function names, and
items 4, 7, and 8 might require a more substantial implementation
effort. Even that effort is estimated to be negligible compared to
the effort required to implement CLOS.
Cost to Users:
No cost to users, other than program-understanding programs, since this
is an upward compatible addition.
As with any language extension, some program-understanding programs may
need to be enhanced. A particular issue here is programs that assume
that all function names are symbols. They may use GET to access
properties of a function name or use EQ or EQL (perhaps via MEMBER or
ASSOC) to compare function names for equality. Such programs will need
improvement before they can understand programs that use the new feature,
but otherwise they will still work.
Cost of non-adoption:
We would have to make some other language change since the language
became inconsistent when 88-002R was adopted.
Performance impact:
This has no effect on performance of compiled code. It might slow
down the compiler and interpreter but not by very much.
Benefits:
CLOS will work as designed.
Esthetics:
Some people dislike using anything but symbols to name functions.
Other people would prefer that if the change is to be made at all,
the LARGE proposal be adopted so that the language is uniform in its
treatment of the new extended function names. Other proposals for
how to deal with SETF in CLOS were considerably less esthetic,
especially when package problems are taken into account.
SETF would be more esthetic, but less powerful, if it had only the
proposed setf functions and did not have setf macros. Such a major
incompatible change is of course out of the question; however, if setf
functions are stressed over setf macros, SETF will be much easier to
teach.
Discussion:
Moon supports at least FUNCTION-NAME:MEDIUM. He does not necessarily
approve of all parts of FUNCTION-NAME:LARGE.
!
Additional Comments:
On the whole, I like this presentation much better than either of the
other two writeups that were circulated previously. I suspect that it
might be necessary to vote on each of the items in the LARGE proposal
individually, though. I think I would support items 1, 2, and 11, and
don't have any particular objections to 3, 5, and 6. For item 4, if
consistency with GENERIC-FLET and GENERIC-LABELS is an object, another
alternative is to change those two special forms to be like ordinary
FLET and LABELS, instead of vice versa.
- - - - - - -
I support FUNCTION-NAME:MEDIUM and may support LARGE once I think about
it some more.
As I explained in Hawaii, support for either of these is based on the
:conc-name bugs being removed from the condition system. Of course, I
believe the best way to do that is to CLOSify it.
- - - - - - - -
I'm still thinking about this, but while I am I wanted point out that
MEDIUM is unacceptable to me because I don't think FLET and DEFUN should
disagree on what they permit as defined names. If FLET were added to
MEDIUM, I suspect I'd think it was an internally consistent position.
LARGE has an appeal to me in general, but I'm still mulling over
the specifics.
- - - - - - - - - -
I favor the FUNCTION-NAME:LARGE proposal, because it defines a single,
useful notion of what a function name is. The other proposals have
the flaw that there are two kinds of function names: symbols, and
extended names, with only some of the Lisp primitives accepting the
latter. This may be convenient for some implementations, for the
short term, but it fragments the language.
I have two other comments on the proposal.
A. Reducing the Cost to Implementors
One observation you could put in the Cost To Implementors section is
that none of the SMALL, MEDIUM, or LARGE proposals require changes to
the "guts" of the interpreter and compiler. This is because an
implementation is free to use plain symbols internally to name
functions, and use a hack like JonL's SETF:|3.FOO.BAR| mapping to
convert non-symbol names to symbols. This conversion would be done as a
part of parsing the handful of forms which accept function names, and
then all other passes of the interpreter and compiler (the "guts") would
just see symbols. (By "parsing" I mean ensuring the right number and
type of syntactic subforms. You can see that this is a very early and
simple stage of processing.) Or, Lisp compilers with an "alphatization"
phase could perform function name symbolization at that phase.
B. Finishing the Job of Regularization
I'd like to suggest two additions to your smorgasbord of options in the
FUNCTION-NAME:LARGE section of the proposal. One addition would
regularize a major special case of functions--lambda expressions. The
other addition would reaffirm an unstated regularity in the language,
that function names can stand in for functions under FUNCALL and APPLY.
Not only can the treatment of symbolic and setf-list function names be
regularized, but lambda too can be treated in a consistent manner.
If these two points are added to your proposal, the language as a whole
would have a completely uniform treatment of functions and function
names. Here they are:
13. Declare that any function name is a suitable argument to FUNCALL and
APPLY. In such a case, the function name is passed to FDEFINITION,
and the result (which may in turn be a function name) is called.
That is, the following two expressions are equivalent, when fname
is a function name:
(FUNCALL fname x y)
<==>
(FUNCALL (FDEFINITION fname) x y)
Note that the definition is sought in the global environment.
Compare with the rule which applies to a function name occurs,
syntactically, as the car of a list in code:
(fname x y)
<==>
(FUNCALL (FUNCTION fname) x y)
<==> (under proposal item 9)
(FUNCALL (FDEFINITION fname <local-environment>) x y)
12. Declare that any lamba expression (i.e., a list whose car is LAMBDA and
whose cdr is a well-formed lambda argument list and body) is a function
name. The effects of the function name accessors on lambda expressions
are as follows. FDEFINITION returns an implementation-defined value which
is the function specified the lambda expression, closed in the global
environment. This FDEFINITION value cannot be changed by SETF.
FBOUNDP always returns T, and MAKUNBOUND is an error.
Esthetics:
The effect of items 11 and 12 is to complete the regularization of
Common Lisp's treatment of functions and function names. The total
effect of proposal items 1 through 12 is that Lisp has just two notions
for referencing function objects: FUNCTIONS, which are Lisp objects that
directly represent executable code, and FUNCTION NAMES, which can denote
functions. Symbols, SETF function names, and lambda expressions are all
examples of the latter notion. The former notion is highly
implementation dependent. Function names can occur as syntactic
entities in code. FUNCALL and APPLY work uniformly on both functions
and function names, with a consistent semantics.
Lambda expressions are often thought to denote "anonymous" functions, so
it may seem paradoxical to treat them as names. The paradox is only
apparent, since the expression itself has the properties of a Lisp
function name: It is (typically) a cons tree which can be read, printed,
and stored in source files, and it denotes a well-defined Lisp function.
Benefit to Users:
Function names are useful for representing objects in remote
environments, because they need not be bound at all times to the same
function, or to any function, and because they are typically stable in
meaning across reads and prints, where plain functions are not.
Programs which deal simultaneously with remote and local environments,
such as CLOS, can probably be simplified, since function names
can be used uniformly, rather than an ad-hoc mixture of functions
and function names.
The language as a whole become more uniform from these additions and
clarifications, making it easier to learn and use. (See Esthetics.)
Cost to Implementors:
Interpreters which currently have a special case check for application
of lambda expressions would need to modify this check to call
FDEFINITION when a list of any sort is encountered. Note that all
Common Lisps already must perform some such check, since lambda
expressions can be funcalled (and this is currently a very special case,
the only standard case of a list being funcalled). This means that
every Lisp already has a place to insert the required call to
In some implementations, FDEFINITION of a lambda expression could be that
lambda-expression itself. In others featuring a pre-eval codewalk, the
walk would be done by FDEFINITION, which would return an appropriate
closure.
Cost of Non-adoption:
Rather than two notions for function references (functions and function
names), there would be several notions, each corresponding to the valid
inputs for particular group of primitives. APPLY and FUNCALL would
accept functions, symbolic names, and lambda expressions, but not setf
function names. FDEFINITION and its kind would accept symbols and setf
function names but not lambda expressions. If the :LARGE proposal is
not adopted, this fragmentation would also apply to the various syntaxes
involving function names; some names would be acceptable to DEFUN
but not to FLET, etc.
- - - - - - - - - - - - -
> 13. Declare that any function name is a suitable argument to FUNCALL and
> APPLY. In such a case, the function name is passed to FDEFINITION,
> and the result (which may in turn be a function name) is called.
I don't think this is such a good idea. The case of automatically coercing
a symbol to a function is needed because it provides a portable mechanism
for indirect addressing of a function; I haven't seen a reason to need this
for non-symbol function specs. But more important is that coercing a
symbol to a function is a trivial operation that is reasonable to do at
run time on each call without adding a significant amount of overhead.
FDEFINITION, on the other hand, is a much more expensive operation -- at
best it might use GET to do a property list lookup, or it could be using
string-append and INTERN to convert the name to a symbol. In either case,
I think this is more work than you want to do on each call.
> 12. Declare that any lamba expression (i.e., a list whose car is LAMBDA and
> whose cdr is a well-formed lambda argument list and body) is a function
> name. The effects of the function name accessors on lambda expressions
> are as follows. FDEFINITION returns an implementation-defined value which
> is the function specified the lambda expression, closed in the global
> environment. This FDEFINITION value cannot be changed by SETF.
> FBOUNDP always returns T, and MAKUNBOUND is an error.
The exceptions for SETF and MAKUNBOUND show that this is not really as
consistent as you might like. Furthermore, the FUNCTION special form would
have to treat a LAMBDA expression as a function, not a function name, in
order for it to be lexically scoped. It seems like this might just cause
confusion rather than consistency.
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