Forum: Public Review
References: Dalton's public review comment #5 (X3J13/92-2005)
Section 188.8.131.52.3 (Self-Evaluating Objects), p 3-9
QUOTE, p 3-65
Edit history: 30 Dec 1992, Version 1 by Loosemore
Status: Proposal SPECIFY passed (7+4)-1 on letter ballot 93-306
Section 184.108.40.206.3 (Self-evaluating objects) states that the
"consequences are undefined if literal objects, including
self-evaluating objects, are destructively modified. A
similar rule appears on p. 3-65 in the description of QUOTE.
What counts as part of such an object for the purposes of this
rule and consequently cannot be modified?
It is important to resolve this, because users do such things
as put hash table literals in code and then add new key-value
associations. It is not clear whether or not this is permitted.
It is not even clear whether a symbol's property list counts as
part of the symbol for the purposes of this rule.
Other places in the standard that refer to limitations on destructive
operations (such as sections 3.6 and 3.7, or the discussion of
compiler-macros), are similarly vague about exactly which operations
are considered "destructive".
Specify that the following operations are considered destructive:
Using it as an argument to the function RANDOM.
Altering or destructively modifying the CAR or CDR of the cons.
Altering or destructively modifying any array element;
changing the fill pointer, dimensions, or displacement of
the array (regardless of whether the array is actually adjustable);
or altering the contents of any array that is displaced to this array
or that otherwise shares its contents with this array.
Altering or destructively modifying any key or its corresponding
value, or adding or removing entries from the hash table.
Altering or destructively modifying the contents of any slot.
Altering or destructively modifying the contents of any slot, or
changing the class of the object.
Altering the readtable-case; altering the syntax type of any
character in this readtable; altering the reader macro function
associated with any character in this readtable; or altering the
reader macro functions associated with characters defined as
dispatching macro characters in this readtable.
Performing I/O operations on the stream, or closing the stream.
all other types
(including number, character, symbol, package, pathname, function,
method, method-combination, condition, restart)
There are no destructive operations defined on these data types.
It solves the problem in a way that is generally consistent with
what is considered to be a component of an object for the purposes
of determining similarity of constants.
Cost to implementors:
Cost to users:
Specifying this behavior is more aesthetic than leaving it vague.
Probably the easiest way to handle this is to add a new subsection
to section 3.7 containing the list above, and add appropriate references
(e.g., in the glossary entry for "destructive").