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The idea behind Multiple Values is that they are more efficient and
cleaner than the basic alternatives of returning a list, or stashing
values in global variables.
If using Multiple Values is going to be more costly than
these alternatives, then their use will not be widespread.
I think they have suceeded in this regard, and believe they will become more
common at time goes on. However, I think MULTIPLE-VALUE-OR is a case
where it fails to live up to its efficiency goals.
I have written code that needed OR which returned multiple values, (and after
spending a while tracking down a strange bug that ending up being OR not
returning multiple values except if its the LAST form), I wrote it as a macro.
I have since changed the code to store the results in global variables
because it made the function much faster, and speed was important for this
I would like to think Common Lisp implementors will provide non-consing
&Rest args, since they are very useful for many things, as their use
on the MIT-based Lisp Machines shows, but it is not a trivial task.
MULTIPLE-VALUE-OR can be implemented without them (I'll give details
if anyone really wants to know), but perhaps MULTIPLE-VALUE-OR just isn't that
widely applicable to warrant special treatment to make it as efficient
as possible. If people are happy with that, then I can live with
using uglier mechanisms when speed and efficiency are important.