Originally I was trying grab the last number of items in a table (something similar to your code in prompt.arc where you are grabbing/printing the last n items passed into the repl).

When using a list, as you had, we can easily grab the last 'n' items in the list since lists maintain order.

    (= records* (list "record 1 text" "record 2 text" "record 3 text" "record 4 text" "record 5 text" "record 6 text"))

    (def lastn (n nlist)
      (firstn n (rev nlist)))

    arc>(each record (lastn 3 records*)
      (prn record)) 

    record 6 text
    record 5 text
    record 4 text

    (= records* (obj 1 "record 1 text" 2 "record 2 text" 3 "record 3 text" 4 "record 4 text" 5 "record 5 text" 6 "record 6 text"))

    arc> (each record (firstn 3 (sort > (keys records*)))
         (prn (records* record)))

    record 6 text
    record 5 text
    record 4 text

    arc> (for i (len records*)(- (len records*) 3)
          (prn (records* i)))

    record 6 text
    record 5 text
    record 4 text
    record 3 text

    arc> (for i (len records*)(- (len records*) 2) 0
          (prn (records* i)))

So.... instead I wrote 'grab':

    (def lastn (n nlist)
      (firstn n (rev nlist)))

    (def grab (which n them (o sop nil))
	 (case (type them)
	        table (case which first (if (no sop) (firstn n (sort < (keys them)))(firstn n (sort sop (keys them))))
	                          last  (if (no sop) (firstn n (sort > (keys them)))(firstn n (sort sop (keys them)))))
	        cons  (case which first (if (no sop) (firstn n them)(firstn n (sort sop them))) 
	                          last  (if (no sop) (lastn n them) (lastn n (sort sop them))))))
	

    arc>(each record (grab 'last 3 records*)
        (prn (records* record)))

    record 6 text
    record 5 text
    record 4 text

    (def memo (f)
      (with (cache (table) nilcache (table))
        (fn args 
          (or (cache args)
              (and (no (nilcache args))
                   (aif (apply f args)
                        (= (cache args) it)
                        (do (assert (nilcache args))
                            nil)))))))

    (def intersperse (x ys)
      (and ys (cons (car ys)
                    (mappend [list x _] (cdr ys)))))

    (define (expand-sexpr sym)
      (build-sexpr (reverse (tokens (lambda (c) (or (eqv? c #\.) (eqv? c #\!)))
                                    (symbol->chars sym)
                                    '()
                                    '()
                                    #t))
                   sym))

    (define (build-sexpr toks orig)
      (cond ((null? toks)
             'get)
            ((null? (cdr toks))
             (chars->value (car toks)))
            (#t
             (list (build-sexpr (cddr toks) orig)
                   (if (eqv? (cadr toks) #\!)
                       (list 'quote (chars->value (car toks)))
                       (if (or (eqv? (car toks) #\.) (eqv? (car toks) #\!))
                           (err "Bad ssyntax" orig)
                           (chars->value (car toks))))))))

    (define (tokens test source token acc keepsep?)
      (cond ((null? source)
             (reverse (cons (reverse token) acc)))
            ((test (car source))
             (tokens test
                     (cdr source)
                     '()
                     (let ((rec (if (null? token)
                                acc
                                (cons (reverse token) acc))))
                       (if keepsep?
                           (cons (car source) rec)
                           rec))
                     keepsep?))
            (#t
             (tokens test
                     (cdr source)
                     (cons (car source) token)
                     acc
                     keepsep?))))

    (def get (index) [_ index])

    arc> (map !a (list (obj a 1) (obj a 2)))
    (1 2)

    (def expand-metafn-call (f args)
      (if (is (car f) 'compose)
           ((afn (fs)
              (if (caris (car fs) 'compose)            ; nested compose
                   (self (join (cdr (car fs)) (cdr fs)))
                  (cdr fs)
                   (list (car fs) (self (cdr fs)))
                  (cons (car fs) args)))
            (cdr f))
          (is (car f) 'no)
           (err "Can't invert " (cons f args))
           (cons f args)))

Arc needs to do some list marshaling in any case to deal with nil -- and as such it reimplements nearly all of the list operations, deferring to mzscheme functions only for a few things. Because of this, using `mcons' shouldn't be too problematic (not even for speed, since arc functions in general are not careful about cyclic lists, a plain implementation will be fast enough -- for example, mzscheme's `map' is now implemented in scheme). I have done some tests in the past to measure

Probably the only major problem is how to deal with a rest argument. One option is to make it illegal to mutate rest arguments. Another is to hook on the fact that rest arguments are not modified often -- so instead of copying them to a mutable list on every call, they can be converted only when needed, by the functions that change the contents of these lists. Given the above (need to implement list functionality anyway), this wouldn't be difficult -- basically some wrapper around a mzscheme list that can do the necessary work. As a side benefit it can be used to deal with nil too, which will make that run faster as well.

Currently, in arc.arc, list is defined as

  (def list args args)

Another is to hook on the fact that rest arguments are not modified often -- ... they can be converted only when needed

How could this possibly work??

  ((fn args
     (= (cadr args) 'X)
     args)
   'a 'b 'c)

As a side benefit it can be used to deal with nil too, which will make that run faster as well.

Can you explain? (I need smaller steps to be able to follow you :-)

(BTW, this will be much faster than using r6rs or r5rs modes.)

Yes, I was getting that impression browsing through the r6rs and r5rs code.

  > (require scheme/mpair)
  > (require (lib "1.ss" "srfi"))
  > (lset-difference eq? (list 1 2) (list 2 3))
  (1)
  > (lset-difference eq? (mlist 1 2) (mlist 2 3))
  {1 2}

  #!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)

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))))]))

http://www.r6rs.org/final/html/r6rs-rationale/r6rs-rationale...

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)

  Indeed, our experience is that making the result of `cons' immutable
  does not create many porting problems. Nevertheless, if your code does
  use `set-car!' or `set-cdr!', and if converting to a more functional
  style is difficult, then consider using `mcons' and the `scheme/mpair'
  library.

The usual techniques for enabling change involve exhaustive tests as well as not implementing stuff that might be unrequired. And DSLs. Hence Ruby gets a lot of attention because it's so easy to build a quick DSL in it. But as you've mentioned elsewhere, a layer of lisp macros in a program is effectively the DSL for that system.