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4 points by herdrick 4119 days ago | link | parent

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.

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

3 points by CatDancer 4117 days ago | link

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:

  (import (rnrs) (rnrs mutable-pairs (6)))
  (define x ((lambda a a) 'a 'b 'c))
  (set-car! (cdr x) 'd)
  (write x)

  $ 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.


2 points by CatDancer 4115 days ago | link

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/, 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.


4 points by pg 4119 days ago | link

Incidentally, does the (or some) Scheme standard allow an implementation to make cons cells immutable? I.e. is supporting set-car! optional?


5 points by CatDancer 4118 days ago | link

Strongly deprecated:

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?


  (import (rnrs) (rnrs mutable-pairs (6)))

  (define x (cons 'a 'b))
  (set-car! x 'c)
  (write x)

  $ ./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.


1 point by herdrick 4118 days ago | link

Interesting. Have you tried r6rs compat mode?

It looks like SRFI 1, at least, isn't working with r6rs:


2 points by CatDancer 4118 days ago | link

Have you tried r6rs compat mode?

Not any more than I've shared here.