Re: constant folding/smashing

[NGALL@G.BBN.COM]

    Date: Tue, 7 Jun 88 10:24:20 PDT
    From: cperdue@Sun.COM (Cris Perdue)
    ...
    > It is this aspect of `constant'ly returning the same (EQ) object that
    > is meant in the sentence on pg. 86: "[QUOTE] allows any Lisp object to
    > be written as a constant value in a program."  It is the value of the
    > (QUOTE ...) form that is held constant, not the object itself.  The
    > same (EQ) object will always be referenced, but that object may be
    > modified over time (if it is a composite object).
    
    The problem with this conclusion is that if the storage for constants
    is "collapsed" together, it is impossible to know how many constants
    are changing if you change a constant.  A constant in someone
    else's code, even system code, may be changed by your side effect.
    This is an intolerable situation, so side-effects on constants
    are an error.
				    -Cris
		
I don't believe the following form is in error
   (SETF (SYMBOL-VALUE (QUOTE X)) 1)
even though X is wrapped by (QUOTE ...) and is therefore being
"written as a constant value in a program." (See above) The reason
this ISN'T an "intolerable situation" is that in the case of a symbol,
EQUAL is the same as EQ, so you don't get UNEXPECTED side-effects.

What WOULD be intolerable would be to follow your rule ("side-effects
on constants are in error") and say that the above form is in error.
That would force people to write such things as
   (SETF (SYMBOL-VALUE (INTERN "X")) 1)

My point is that in the form
   (SETF (SYMBOL-VALUE (QUOTE X)) 1)
the quoted object is no more or less "constant" than in the form
   (SETF (FIRST (QUOTE (A B C))) 1)
So why does QUOTE make the components of the list (1 2 3)
unmodifiable but not the components of X?

What causes the UNEXPECTED side effects in the latter form is the
folding of EQ but not EQUAL objects into one object.  I am currently
thinking of a way to use DEFCONSTANT to limit such CONSTANT folding in
a CLtL compatible way (i.e., don't constant fold the values of
DEFCONSTANT).

-- Nick