Issue DEFSTRUCT-SLOTS-CONSTRAINTS-NAME Writeup

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.