Issue DEFVAR-INITIALIZATION Writeup

Issue:        DEFVAR-INITIALIZATION

References:   DEFVAR (p68)

Category:     CLARIFICATION/CHANGE

Edit history: Version 1 by Pitman 02/26/87

              Version 2 by cleanup committee 15-Mar-87 14:45:18

              Version 3 by Masinter (format) 15-Mar-87 18:34:28

              Version 4 by Masinter  5-Jun-87


Problem description:


The description of DEFVAR on p.68 is not adequately clear on what

happens if an initialization value is not provided, as in (DEFVAR FOO).

Does this initialize the variable?


Proposal (DEFVAR-INITIALIZATION:CONSERVATIVE):


If the one-argument form of DEFVAR is used, the value (or lack of value)

of the variable is not changed.


Rationale:


In parent languages to CL, such behavior was documented. The omission of

clear documentation in CL is presumably an accident.


Current Practice:


Most implementations already do not initialize the variable. Some

implementations, however, assume that the missing initial value defaults

to NIL and assume that the variable is always to be initialized if

unbound.


Adoption Cost:


Some implementations suffer a minor incompatible change.  The

modification to systems is presumably trivial.


Benefits:


It's sometimes useful to have the ability to declare a variable without

initializing it. More importantly, though, DEFVAR is used by lots of

users in every implementation and it deserves uniform treatment.


Conversion Cost:


It's stylistically poor to be relying on the variable to be initialized

without writing the initial value explicitly anyway. Except for very

rare situations where a conditional action is taken based on a BOUNDP

test of the variable, user programs are unlikely to be affected in

practice.


Very few user programs are likely to be affected. The incidence rate is

probably sufficiently low that the issue of automatic tools for

conversion is irrelevant.


Aesthetics:


No significant issues are obvious.


Discussion:


The cleanup committee generally supports this clarification.


[Version 3 was distributed at the last X3J13 meeting. This version has

changed only to bring it in line with the current proposal format.]