Issue CONSTANT-MODIFICATION Writeup

Forum:		Compiler

Issue:		CONSTANT-MODIFICATION

References:	CLtL p. 78, 87

		Issue CONSTANT-COLLAPSING

Category:	CLARIFICATION

Edit History:   V1, 07 Nov 1988, Sandra Loosemore

		V2, 12 Dec 1988, Sandra Loosemore




Problem Description:


CLtL states that an implementation is permitted to "collapse"

constants appearing in code to be compiled if they are EQUAL.  This

implicit sharing of compiled data structures may result in

unpredictable behavior if destructive operations are performed.

However, CLtL does not explicitly allow or disallow destructive

operations on constants.  



Proposal CONSTANT-MODIFICATION:DISALLOW:


Clarify that it is an error to destructively modify objects which are 

self-evaluating forms or which appear inside of a QUOTE special form.



Rationale:


Disallowing modification of constants consistently in all situations,

rather than just in compiled code, is proposed because in some

compiled-only situations it may be difficult to distinguish between

"compiled" and "interpreted" code.



Current Practice:


Many implementations "collapse" compiled constants.


Many implementations treat compiled constants as read-only.  In

PSL/PCLS, for example, quoted data structures in compiled code are

copied into a part of memory that is not scanned by the garbage

collector.  The TI Explorer's loader also copies constants into a

write-protected memory area.



Cost to implementors:


None.



Cost to users:


User programs which perform destructive operations on constants are

already nonportable.



Benefits:


Many novice programmers do not realize that modifying quoted data

structures is an error in many implementations.  Including an explicit

statement in the standard that doing so is a bad idea will reduce

confusion.



Discussion:


The issue of whether implementations are permitted to copy constants

seen by EVAL or COMPILE is discussed separately as issue QUOTE-MAY-COPY.