Issue: FUNCTION-TYPE-REST-LIST-ELEMENT (Version 5)

[masinter.pa@Xerox.COM]

I attempted to shorten the discussion section while at the same time be more explicit that there were some serious objections to it. I wound up cutting a lot of it out. Do you think I should put it back in?


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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.