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Portability of ` usage
Date: Wed, 28 Mar 90 10:58 CST
Our application uses ` to construct keyword argument lists
(i.e., property lists) which are then passed around, stored in
data structures, etc. I am concerned about the statement in CLtL
that "...no guarantees are made as to whether the constructed copy
of the template will or will not share list structure with the
template itself." Does this mean that an implementation could
conceivably rig a ` expression to return the same list structure
on each execution, destructively modifying the structure for each
use of , ? For example, consider a function
(DEFUN IN-LIST (ARG) `(,ARG))
Is it really permissible for a Common Lisp implementation to
return the same (according to EQL) value for these two calls?
Of course not, in this example the list would have to be consed afresh each
time. However, consider this example:
(defun list-with-common-tail (head)
`(,head foo bar baz))
There is nothing preventing the Lisp implementation from returning a list
whose CDR is the actual constant in the compiled code -- in fact, most
implementations will do this as an optimization. This means that if you
modify the returned value, you might actually modify the function, depending
on whether the implementation keeps constants in a "read-only" region of
memory or not.
Symbolics' implementation doesn't do this, of course, but if the
standard considers it permissible, we may have to modify our usage
to ensure portability.
Lawrence G. Mayka
AT&T Bell Laboratories
Hope this clarification helps.
-- Chuck Fry
Symbolics Software Support