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- To: common-lisp@SAIL.STANFORD.EDU
- Subject: STATELESS/SIDE-EFFECT-FREE Functions
- From: ELIOT@cs.umass.edu
- Date: Fri, 20 May 88 13:13 EDT
From: Jon L White <firstname.lastname@example.org.EDU>
re: . . . Should "ERROR" and ...
re: . . . if a compiler can check these properties it could
also compute them. But currently it would not be allowed
to use those computed properties.
I agree with you that a compiler (and maybe even interpreter?) should do as
much consistency checking as possible. As moon implied in previous mail,
an SSC(*) should be able to determine "simpleness" mechanically. But the
danger of using that information is that it may not have been the user's
_intent_ that this function be marked as "simple" (e.g., he may want to
alter it incompatibly at a later time). An explicit proclamation (or
whatever) is the right vehicle for conveying that intent.
The previous paragraph describes the situation accurately. There seems
to be general agreement that an SSC(*) could mechanically determine
a number of properties of functions that would help a compiler
optimize code. MOON lists a veritable conspiracy of such properties
that are used by the Symbolics compiler. Most people agree that by
default a compiler should not use these properties without some
explicit proclamation (or whatever) indicating that it is OK.
(GZ seems to disagree with this.) There does seem to be a reasonable
amount of interest in making these properties known to compilers.
Using a set of ad-hoc declarations to specify these properties has
several problems. It is inevitable that these declarations will be
defined for the purpose of enabling specific optimizations.
MOON's description of the Symbolics declarations
refered to specific optimization techniques several times. The
STATELESS/SIDE-EFFECT-FREE proposal is intended to enable constant
More importantly there are subtle semantic difficulties with these
declarations. The "curious property" of functions which allocate
modifyable structures seems like a dangerous trap to me. I have
difficulty understanding the meaning and distinctions of the
Symbolics declarations. This means they have to be used slowing and
carefully, and therefore infrequently, and therefore they are not
Even worse is the kind of bugs that can be caused. An incorrect
STATELESS declaration might be ignored in one programing environment
and eventually cause bugs only when the code is ported to another
environment, perhaps years and generations of programmers later.
I consider language features which can cause hidden
bugs of this nature dangerous, and generaly think they should
be avoided. This is why (integer 0 100) is a better type specifyer
The only way to safely use these declarations is with a SSC(*) that
checks to verify that the function definition agrees with the
declaration. But an SSC can compute any of these properties that
it can check, so why make the programmer use the congitive
overhead required to figure out where, when and which declarations
are needed if the compiler could do it automatically? Simply
because the compiler cannot assume that that is the intended
definition of the functions involved.