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3 points by CatDancer 4117 days ago | link | parent

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.