Issue DEFSTRUCT-SLOTS-CONSTRAINTS-NUMBER Writeup

Issue:          DEFSTRUCT-SLOTS-CONSTRAINTS-NUMBER

References:     CLtL p.307 & 86-003 p.4

Category:       CHANGE

Edit history:   Revision 1 by Skona Brittain 05/13/88


Problem Description:


Structures defined by defstruct currently are required to have at least

one slot.  This seems to have been a mistake in the design of the language.


Proposal (DEFSTRUCT-SLOTS-CONSTRAINTS-NUMBER:ALLOW-ZERO):


Allow a call to defstruct to have zero slot-descriptions.

i.e. change the + to a * in the syntax of calls to defstruct 

given at the bottom of page 307 of CLtL.


Test Case:


(defstruct s), which is not allowed according to CLtL, would be allowed.


Rationale:


The current restriction is in marked contrast to the generality allowed 

elsewhere in the language.  And removing it slightly increases the 

usefulness of defstruct - by allowing the zero slot case when it may be 

deemed useful and by not requiring a check for it when it doesn't matter.


Current Practice:


KCL allows zero slots.  


Cost to Implementors:


None for implementations that currently allow zero slots.

Very slight for others.


Cost to Users:


None.


Benefits:


Slightly increases the usefulness of defstruct and is aesthetic.


Aesthetics:


In general, it is more aesthetic to allow for generality rather than to

specifically prohibit a particular case.  And the generality in this case 

is consistent with that of many other features of the language, such as 

that arrays can be empty, functions like + and list can take zero arguments, 

etc.


Discussion: 


Although this issue was mentioned in Guy's original issues file, it has

not been officially discussed since.