Issue DEFVAR-DOCUMENTATION Writeup

Issue:        DEFVAR-DOCUMENTATION

References:   DEFVAR, DEFPARAMETER, DEFCONSTANT (pp68-9)

Category:     CLARIFICATION

Edit history: 30-Jun-87, Version 1 by Pitman

              23-Nov-87, Version 2 by Masinter


Problem Description:


CLtL is not explicit about whether the documentation part of DEFVAR,

DEFPARAMETER, and DEFCONSTANT special forms is evaluated.


Proposal (DEFVAR-DOCUMENTATION:UNEVALUATED):


Clarify that the documentation part of DEFVAR, DEFPARAMETER, and DEFCONSTANT

special forms is not evaluated. That is, it must be a literal string, not a form

which evaluates to a string.


Examples:


(DEFVAR *MY-VARIABLE* (CONSTRUCT-INITIAL-VALUE) "A documentation string")  ; OK

(DEFVAR *MY-VARIABLE* (CONSTRUCT-INITIAL-VALUE) GENERIC-DOCUMENTATION-STRING)  ;

would be an error


Rationale:


To ensure portability, implementations must agree on whether or not this

position is evaluated. Specifying that the position is unevaluated is the

conservative thing to suggest, and consistent with the (unevaluated)

documentation strings in DEFUN, DEFSTRUCT.


Current Practice:


Some implementations evaluate this position. Others do not. 


Cost to implementors:


Implementations that did not already check might usefully add a check in the

macro expansion for DEFVAR, DEFPARAMETER and DEFCONSTANT to assert that the

DOCUMENTATION, if supplied, was a string. The change is likely trivial.


Cost to users:


Code which uses other than a literal string is not portable, so no portable

programs will be broken. Some non-portable programs which rely on a particular

vendor's interpretation would have to be rewritten. Automatic tools to detect

most offending cases could trivially be constructed. (We know of no current

uses.)


Benefits:


Code portability would be improved. Some programming environment tools might

assume that documentation strings were determinable without evaluation.


Aesthetics:


Slight improvement; this implies consistent treatment for documentation strings

in all defining forms.


Discussion:


We think this is a good idea.