Issue FUNCTION-TYPE-REST-LIST-ELEMENT Writeup

Forum:         Cleanup

Issue:         FUNCTION-TYPE-REST-LIST-ELEMENT

References:    CLtL p. 27, 47-48, 61

               "Artifical Intelligence Programming", Charniak et. al.

               X3J13/86-003 (A:>GLS>clarifications.text.4)

Category:      CLARIFICATION, ADDITION

Edit history:  Version 1, 23-Nov-1987 Sandra Loosemore

               Version 2, 15-Jan-1988 Sandra Loosemore

	           (incorporate comments from Scott Fahlman & others)

               Version 3, 13-Feb-88 Masinter

               Version 4,  2-Oct-88 Masinter (update references,

								discussion)

               Version 5, 14-Nov-88 Masinter (add to discussion)

Related issues: FUNCTION-TYPE-KEY-NAME, 

                FUNCTION-TYPE-ARGUMENT-TYPE-SEMANTICS

                REST-LIST-ALLOCATION


Problem description:


The FUNCTION type specifier list is provided to allow declaration of

function argument types and return value types.  This type specifier uses a

syntax similar to the usual lambda list syntax to specify which types go

with which lambda list variables.  However, this is actually of limited

usefulness in the context of a declaration, where one normally wants type

information about the actual arguments which can be passed to the function

rather than the lambda variables to which they are bound.


There is a particular problem with &REST lambda variables, which are always

bound to a value of type LIST.  For the sake of consistency, it would seem

that the corresponding type given in the FUNCTION declaration must also be

LIST, but since this provides no information about the actual arguments,

some users/implementors have instead adopted the convention of supplying

the type of the actual arguments which are gathered into the list.  


CLtL is vague on the issue, mentioning only that &REST may appear in the

type specifier without touching upon its interpretation.


Proposal (FUNCTION-TYPE-REST-LIST-ELEMENT:USE-ACTUAL-ARGUMENT-TYPE):


Clarify that, in the FUNCTION type specifier, the type specifier provided

with &REST is the type of each actual argument, not the type of the

corresponding lambda variable.


Example:


The type of the function + would be specified as:


(FUNCTION (&REST NUMBER) NUMBER)


Rationale:


This is more useful than specifying that the type of a &REST parameter must

be LIST, since it provides information about the actual arguments.


Current practice:


There does not appear to be any concensus on this issue.  Most Common Lisp

implementations currently ignore FUNCTION type declarations. The only

examples found so far are in a text book on Common Lisp, which follows the

proposed syntax.


Cost to Implementors:


Implementations that ignore the FUNCTION type specifier may continue to do

so.  Probably only a small amount of code would have to be written/changed

in implementations that currently think that the  &REST argument should be

LIST.


Cost to Users:


Users who have been using the convention that the &REST type parameter must

be LIST will have to change their code.  However, because this issue is so

unclear, the FUNCTION type specifier is probably not used very much.


Cost of non-adoption:


If nothing is done, the FUNCTION type specifier will continue to be of

limited use for its intended purpose.


Benefits:


Adopting the proposal will clear up an area of confusion in the language

design.


Esthetics:


Debatable.  One the one hand, since the argument type syntax used by the

FUNCTION type specifier mirrors normal lambda-list syntax, it would be

cleaner and less confusing to provide the type of the lambda variable

rather than the type of the actual arguments. However, considering the

types specified in the FUNCTION specifier to be the types of the actual

arguments rather than the types of the parameters as seen on the receiving

end makes the proposed semantics more palatable.


Discussion:


This issue provoked considerable debate in the cleanup committee and at

X3J13. 


Many people objected to this proposal, and would prefer the alternative

that the type given after a &REST in a function declaration apply to the

value of the formal parameter rather than the actual arguments. This would

be even more useful if complex LIST type specifiers were part of Common

Lisp (as the proposal in issue LIST-TYPE-SPECIFIER might add) or if it were

possible to declare, for example, &REST {keyword integer}*.


Some additional arguments against this proposal are the apparent mismatch

between the external declarations of type and the internal ones. It might

be that this proposals presumes that rest lists are always lists, and the

following is illegal:


 (DEFUN FOO (&REST X) X)

 (APPLY #'FOO T)


which is not otherwise explicitly forbidden, but for which there is no

legitimate declaration.