[R6RS] Re: Zapping SET-CAR! & SET-CDR!

[Richard Kelsey]

   From: Michael Sperber <sperber at informatik.uni-tuebingen.de>
   Date: Wed, 20 Oct 2004 16:51:35 +0200

   RK> I am sympathetic
   RK> to wanting immutable pairs, but I think it is too big change.  Too many
   RK> programs would break.

   [...] I would be much less favorably inclined towards this change if
   a majority at the Scheme workshop hadn't expressed they approved it.

I am surprised by this.  How big a majority?  11 out of 20 is a
majority, 19 out of 20 is a mandate.

   So, would an alternative be to make mutability more explicit in the
   standard?  Either by having something like MAKE-IMMUTABLE!, which
   works on any kind of non-closure heap object, or by having
   IMMUTABLE-CONS and so on.  Then we could have argument lists be
   immutable and at least reap some of the benefits.

I would be a lot happier with having both mutable and immutable
pairs, although it leaves open the question of which kind of pair
MAP, APPEND, and other procedures create.  Having both might be
a problem for some implementations.
                                            -Richard