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Date: Mon, 16 May 88 17:55:30 PDT
From: Jon L White <email@example.com>
One more interesting question related to what is STATELESS arises out
of noting the distinction drawn between Symbolic's declarations
(LT:SIDE-EFFECTS LT:REDUCIBLE) and (LT:SIDE-EFFECTS LT:SIMPLE REDUCIBLE).
Is CAR STATELESS? I say yes, even though the Symbolics terminology
seems to imply that it is sensitive to side-effects.
CAR is certainly sensitive to side-effects, in the sense that a side-effect
on its argument can change its result. It's true that CAR doesn't
reference any (obvious) free variables.
So, I would prefer to say that the argument to CAR can be modified by
side-effecting functions; hence a compiler must keep track not only of the
properties of the function it is compiling a call to, but also of the
potential alterations to the arguments to that function. The same analysis
applies to arrays, hash-tables, defstructs and so on, as well as to cons cells.
As I said in my earlier message, the Symbolics declarations do not
attempt to keep track of aliasing. Thus there is no distinction between
side-effects that are known to be connected with a particular object,
and side-effects in general. Aliasing can be fairly difficult to
track in Lisp, and even more so in a language extended with locatives
(which allow non-type-specific operations to access aliases).