[HARLEQUIN][Common Lisp HyperSpec (TM)] [Previous][Up][Next]


Status: Passed v4 (8-1), as amended, Dec-91; v5 reflects the amendment


Reference: Draft 10.156 Initialization Arguments (p.7-6)

(which corresponds to CLtL-II, p.802) Declaring the Validity of Initialization Arguments

(p.7-7) (which corresponds to CLtL-II, p.803)

7.3.5 Keyword Arguments in Generic Functions and Methods

(p.7-20) (which corresponds to CLtL-II, p.792)

Category: CHANGE

Edit History: Version 1, 10/2/91, Kim Barrett

Version 2, 10/8/91, Kim Barrett (Moon & JonL comments)

Version 3, 10/11/91, Kim Barrett (more Moon comments)

Version 4, 11/20/91, Kim Barrett

(clarify :allow-other-keys, make references to 10.156 draft)

Version 5, 11-Feb-92, Kent Pitman

("has no effect on" => "suppresses" per X3J13 meeting amendment)

Problem Description:

A careful reading of 88-002R indicates that users must specify

&ALLOW-OTHER-KEYS in the lambda-lists of methods for initialization functions

if the method also specifies &KEY, else risk having the generic function

keyword argument checking signal an error due to the presence of "slot

filling" initargs. (An error might not occur due to the presence of other

methods which already specify &KEY + &ALLOW-OTHER-KEYS).

Several implementors (and presumably users too) have misread the specification

as saying that &ALLOW-OTHER-KEYS in an applicable initialization method means

that initarg validation is suppressed for those protocols in which the method

takes part.

The result is that the need for and meaning of &ALLOW-OTHER-KEYS in

initialization methods differs significantly from one implementation to



1. Change the syntax of the following generic functions to specify both &KEY


ALLOCATE-INSTANCE class &key &allow-other-keys

INITIALIZE-INSTANCE object &key &allow-other-keys

MAKE-INSTANCE class &key &allow-other-keys

REINITIALIZE-INSTANCE object &key &allow-other-keys

SHARED-INITIALIZE object slot-names &key &allow-other-keys


&key &allow-other-keys


added-slots discarded-slots


&key &allow-other-keys

It is of course possible to retain "&rest initargs" in the syntax description

when needed in the corresponding function description.

2. Clarify that the presence of &ALLOW-OTHER-KEYS in the lambda-list of an

applicable method relevant to the specified initarg validity checking for an

initialization protocol suppresses the validity checking.

3. Clarify that a true value for the initialization argument :ALLOW-OTHER-KEYS

disables initialization argument validation. Make the following changes Draft

10.156 in order make this clarification.

a. Remove the last sentence of the second paragraph of section

(p.7-6), which says

"Error checking of initialization argument names is disabled if the keyword

argument pair whose keyword is :ALLOW-OTHER-KEYS and whose value is non-NIL

appears in the initialization argument list."

b. Replace the last sentence of section (p.7-7), which says

"The meaning of:ALLOW-OTHER-KEYS is the same here as when it is passed to an

ordinary function."


"Validity checking of initialization arguments is disabled if the value of

the initialization argument :ALLOW-OTHER-KEYS is true."

Editorial impact:

The syntax entries for seven functions need to be modified.

A small change to "Declaring the Validity of Initialization Arguments" is

needed in order to clarify the handling of &ALLOW-OTHER-KEYS in initialization



Since &ALLOW-OTHER-KEYS needs to be specified somewhere if &KEY is specified

by any of the applicable methods, it is better to have the generic function

specify it, rather than requiring programmers to remember to include it in

every initialization method that also specifies &KEY.


(defclass test ()

((slot :initarg :slot-filler))


(defmethod initialize-instance :after ((x test) &key) nil)

(make-instance 'test :slot-filler 5)

-> currently might signal an error

-> returns instance of test under proposed change.

Current Practice:

Both Lucid 4.0 and IIM 3.4 specify only &REST in the lambda-lists of the

listed generic functions and the specified methods for those functions, with

the implication that user defined methods which specify &KEY must also specify

&ALLOW-OTHER-KEYS in order to inhibit keyword argument checking by the generic

function from signaling an error due to the presence of "slot filling"

initargs. However, because generic function keyword checking has not yet been

implemented in either Lucid 4.0 or IIM 3.4, users may not be aware of this

requirement. In fact, in IIM 3.4 there are system-supplied initialization

methods that fail to meet this requirement, so there are some implementors who

aren't aware of this problem either.

Franz Allegro 4.0 appears to have the same behavior as described above for

Lucid 4.0 and IIM 3.4. In particular, generic function keyword checking has

not yet been implemented.

Symbolics CLOS and Chestnut's Lisp-to-C translator both specify &KEY and

&ALLOW-OTHER-KEYS in the lambda-lists of the listed generic functions. In

addition, if an applicable method for one of the protocols specifies

&ALLOW-OTHER-KEYS then the specified initarg checking for the protocol is not


Macintosh Common Lisp 2.0b3c4 has "&KEY &ALLOW-OTHER-KEYS" in the lambda list

for MAKE-INSTANCE, but all the others just use "&REST". This asymmetry is an

oversight due to the fact that the method combination code special cases them,

and elides normal generic function keyword checking for all of them (hence it

has never appeared as a "bug"). MCL (like Symbolics and Chestnut) also

provides the additional behavior that if one of the applicable methods

specifies &ALLOW-OTHER-KEYS then the initarg checking for the protocol is not


Cost to Implementors:

Some possibly tricky code in the implementation may have to be reexamined. In

some implementations it looks like this may involve simply ripping out some

special cases.

Cost to Users:

Probably none in most cases. Since none of the implementations surveyed ever

signal an error for the described problem case, it is unlikely that any user

code exists which depends on an error being signaled.

Users can eliminate unnecessary &ALLOW-OTHER-KEYS from initialization methods

at their leisure, since its presence or absence has no effect.

However, those users who have made use of the fairly common but non-portable

meaning of &ALLOW-OTHER-KEYS in applicable methods as a means of suppressing

initarg validation would have to find some other mechanism for this purpose.

One possibility would be to define appropriate :AROUND or primary methods on

MAKE-INSTANCE that add :ALLOW-OTHER-KEYS T to the initialization arguments.

Performance Impact:



Users and implementors will know what &ALLOW-OTHER-KEYS in an initialization

method's lambda-list means. Programmers writing initialization methods that

specify keywords won't have to remember to include &ALLOW-OTHER-KEYS



Requiring all initialization methods that specify &KEY to also specify

&ALLOW-OTHER-KEYS is clearly unaesthetic, so this change is an improvement.


Interestingly, for one reason or another, none of the implementations surveyed

perform generic function keyword checking (as opposed to initarg checking) for

the functions affected by this proposal.

The possibility of having &ALLOW-OTHER-KEYS in the lambda-list of an

applicable initialization method mean that initarg validation should be

inhibited was discussed, especially in light of the fact that several of the

implementations surveyed already provide this behavior. However, some people

questioned whether this was really such a good idea, being unmodular and

perhaps *too* convenient. Also, making such a change would add a real cost to

users who weren't expecting this extension, since they would have to examine

occurances of &ALLOW-OTHER-KEYS in initialization methods to decide whether

they were left over from the time when it was (at least theoretically) needed

to inhibit generic function keyword checking or actually there to do the new


[Starting Points][Contents][Index][Symbols][Glossary][Issues]
Copyright 1996, The Harlequin Group Limited. All Rights Reserved.