Nope. All you can do is use mcons ('mutable cons'), mcar, mcdr, etc. But apparently there are only a few functions that work with these things. It's like using lists in a language that doesn't support lists well. Useless.
Hmmmmmm... I had thought that r6rs compatibility mode wasn't very useful, if it just made car a synonym for mcar etc. But it does more, for example unlike in regular plt-4 where in (lambda args ...) args is an immutable list, in r6rs mode:
#!r6rs
(import (rnrs) (rnrs mutable-pairs (6)))
(define x ((lambda a a) 'a 'b 'c))
(set-car! (cdr x) 'd)
(write x)
(newline)
$ plt-4.1.5/bin/mzscheme -t a.scm
(a d c)
This looks like it could solve a lot of problems with a port to plt-4, since otherwise we'd need to be rewriting the Arc compiler to change the expansion of (fn args ...) etc.
Played around with it a bit more, r6rs appears problematic as apparently ++ is not a legal symbol in r6rs (!)
I took a look at PLT's implementation of lambda for r6rs/r5rs (it's in collects/r5rs/main.ss), and they just simply convert to a mutable list if the lambda has a rest parameter:
(define-syntax (r5rs:lambda stx)
;; Convert rest-arg list to mlist, and use r5rs:body:
(syntax-case stx ()
[(_ (id ...) . body)
(syntax/loc stx (#%plain-lambda (id ...) (r5rs:body . body)))]
[(_ (id ... . rest) . body)
(syntax/loc stx
(#%plain-lambda (id ... . rest)
(let ([rest (list->mlist rest)])
(r5rs:body . body))))]))
(the list->mlist is the part I'm looking at)
So having for example (fn args ...) compile to an (arc-lambda args ...) which does the same thing might be simpler than trying to get Arc to compile and run in the whole complicated r6rs environment.
I participated a bit in the discussion while r6rs was being created, but it quickly became apparent to me that the goals that the editors were striving for weren't things that I personally cared about.
So, anyway, due to the "compromise", can we get mutable pairs if we run MzScheme in r6rs compatibility mode?
a.scm:
#!r6rs
(import (rnrs) (rnrs mutable-pairs (6)))
(define x (cons 'a 'b))
(set-car! x 'c)
(write x)
(newline)
$ ./mzscheme a.scm
(c . b)
So yes, though it turns out that in r6rs compatibility mode cons is really just mcons, so we don't actually gain anything.
