Issue: DEFSTRUCT-SLOTS-CONSTRAINTS-NAME (Version 5)

[masinter.pa@Xerox.COM]

Changes from Version 4 are: 

Clarify that foo:a and bar:a cannot both be slots in the same structure.
Clarify that this only affects DEFSTRUCT macro, not STRUCTURE-CLASS.
Extend Problem Description & Rationale.


!
Issue:          DEFSTRUCT-SLOTS-CONSTRAINTS-NAME
References:     CLtL p.308 & 86-003 p.4
Category:       CLARIFICATION

Edit history:   Version 1 by Skona Brittain 05/13/88
                Version 2, Masinter, 14-Sep-88
                Version 3, Masinter, 23-Sep-88
                Version 4, Masinter, 31-Oct-88
                Version 5, Masinter, 12-Jan-89

Problem Description:

The case of two slots of a structure having the same name is not 
discussed in CLtL. Is it allowed?  The problem is that the
name of slot accessors and the keyword arguments of the 
constructor function is determined only by the SYMBOL-NAME
of the slot designator; the meaning of slot accessors and
the constructor function is unspecified.

Proposal (DEFSTRUCT-SLOTS-CONSTRAINTS-NAME:DUPLICATES-ERROR):

It is an error for two slots in a structure type to have the same symbol-name;
that is, the SYMBOL-NAME of the slot names should not be STRING=.
This holds when they were both named directly by the same call to defstruct
or when one is present by virtue of being in an included structure.

The situation of expanding a DEFSTRUCT macro with a duplicate name "should
signal an error." (While not yet formally defined, the intent is that 
the error signalling may occur when compiling a file that contains
duplicate names or when evaluating a DEFSTRUCT form with duplicate names
in an interpreter.)

This proposal only affects the operation of the DEFSTRUCT
macro, and not the STRUCTURE-CLASS or structures defined
with DEFCLASS>

Examples:

(defstruct struc slot slot) would be an error.  So would
(defstruct (struc2 (:include struc1)) slot) if preceded by
(defstruct struc1 slot).

(defstruct struct package-1:slot package-2:slot) is also an
error. Slot accessors are interned in the current *PACKAGE*
at the time of the evalution of the DEFSTRUCT. 

Rationale:

Since it would be difficult to prescribe reasonable behavior for
DEFSTRUCT, it should be considered an error.

Current Practice:

In KCL, if two slots have the same name, no warning message is 
given but mysterious behavior ensues.  (Their default values are 
both whatever is given for the second one, neither can be given a
different value via a call to the constructor function, only the 
second one's value can be changed by setf...)

Cost to Implementors:

None.

Cost to Users:

None.

Cost of Non-Adoption:

Possible confusion.

Benefits:

Clarity.

Aethetics:

Something that is not well-defined and leads to erratic behavior 
should be explicitly considered an error.

Discussion: 

Although this issue was mentioned in Guy's original issues file, it has
not been officially discussed since.  

This issue was first circulated to X3J13 June 1988.

This proposal does not address the issue of whether NIL is a legitimate
slot name. There seems to be no current reason why it might be prohibitied.

The compiler committee is proposing to address generally the issue 
of how macro-expansion errors during compile-file might be caught and
turned into compiler warnings.


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