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DECLARE-MACROS (Version 1)
- To: CL-Cleanup@SAIL.STANFORD.EDU
- Subject: DECLARE-MACROS (Version 1)
- From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
- Date: Thu, 22 Oct 87 10:41 EDT
- Cc: KMP@STONY-BROOK.SCRC.Symbolics.COM, Moon@STONY-BROOK.SCRC.Symbolics.COM
References: Declaration Syntax (p154)
Edit history: 22-Oct-87, Version 1 by Pitman
Status: For Internal Discussion
It is permissible for a macro call to expand into a declaration and be
recognized as such. This linguistic feature provides some useful
flexibility, but has a number of disadvantages:
* Operations on the executable portion of a body of code inside a
binding form (such as inserting an additional form) is a complicated
operation. This is because one or more trial macro expansions must be
done in order to pass over any declarations or documentation string
and find the beginning of the body.
* In some cases, it may be desirable to ask a question about a
particular piece of code without actually modifying the code. Since
the presence of *MACROEXPAND-HOOK* in the language means that a call
to MACROEXPAND can be a destructive operation, some seemingly harmless
inquiry operations about an expression during program analysis can
risk destructively modifying the expression.
* In order to find the end of the declarations, MACROEXPAND must be
called until a non-macro form is seen or until a macro does not expand
into a macro. In some interpreters which do macroexpansion on the fly,
this may cause inefficiency because macro expansion of the first form
in a body must be done twice. In implementations where this is
optimized, the implementor may resent the fact that an optimization is
needed in the first place.
* Various code analysis tools have been shown to have been significantly
slowed down by the need to expand macros in order to determine whether
a binding is SPECIAL when analyzing a variable binding form. This is
particularly serious when macro invocations are deeply nested; the
number of macro expansions can be expontential in the depth of nesting
unless a macro-expansion caching mechanism is added. For example,
certain efficiency problems in the Symbolics compiler have been traced
to this feature.
* User macros must be very careful about finding declarations in a body
of code by doing proper macro expansion. Often, however, naive users
don't realize this and so unknowingly write buggy code. This problem can
be (and is) defined away as simply a programmer error, but this is a
place where we could fairly straightforwardly redefine the language to
better accomodate what has been shown to be a common expectation of the
Make it illegal for a macro call to expand into a DECLARE form and be
recognized as such.
It should still be possible for a macro call to expand into a PROCLAIM
The advantages provided by the ability to have a macro form expand into
a declaration have been shown in practice to not be worth the price paid
elsewhere in the language.
Most or all implementations support the old behavior even though few
user programs probably need it.
Some user macros are careful about finding declarations in a body of code
by doing proper macro expansion, but some probably cheat and look just
for explicit uses of DECLARE. The cheat probably works most of the time.
The nature of this change is such that implementations which did not
choose to change would simply be supporting an implementation-dependent
extension (except for some `minor' worry about destructive modification
due to macro expanding while *MACROEXPAND-HOOK* is set to something
which implemented displacing macros).
In any case, there might be several places in which the interpreter,
compiler, and system macros had knowledge about doing macro expansion
before declaration processing. The change is not trivial, but most of
its complexity is likely to be in finding the places which need change
and not in making the actual change.
The efficiency of some tools may be improved.
User macros which must do minor surgery on bodies of code will be
easier to write.
Most users probably do not write macros which expand into DECLARE forms
so most users are probably not affected.
Users who do exploit this feature probably know that they do. In any
case, compilers could be made to detect cases where this feature is
being exploited and warn about it.
Rewrites must be devised on a case-by-case basis. A common sort of
rewrite would take the form:
Old code: (DEFMACRO SPEEDY () `(DECLARE (SPEED 3) (SAFETY 0)))
(LET (..bindings..) (SPEEDY) ..body..)
New code: (DEFMACRO SPEEDY-LET (BVL &BODY FORMS)
`(LET ,BVL (DECLARE (SPEED 3) (SAFETY 0)) ,@FORMS))
(SPEEDY-LET (..bindings..) ..body..)
This change simplifies the semantics of the language slightly and
probably tends to better support the assumptions of naive programmers
In some cases involving complicated extensions to declarations, it
may be slightly harder to express such extensions in a modular way.
Experience thus far has shown such cases to be rare, however.
Symbolics took an in-house poll of people who take advantage of the
feature and it was generally agreed that in most cases where this
feature is used at all, that it would be just as easy to work around
using the sample rewrite techniques cited above.
Moon `credits' Pitman for (against some opposition) pushing this
`feature' down everyone's throats in the original CL design process.
Pitman admits this was an expensive mistake. Moon and Pitman support
this change as an important simplification to the language.