Re: Unsafe psyntax label generation

[Mark H Weaver]

Andy Wingo <address@hidden> writes:

> On Tue 24 Jan 2012 15:01, Mark H Weaver <address@hidden> writes:
>
>> `local-eval' combines syntax objects from two different sessions into a
>> single syntax object (in the wrapper procedure), and thus there may be
>> label name collisions.  Now, if this combined syntax object is
>> serialized as a compiled procedure, these labels with the same name will
>> be optimized together into the same string object!
>
> A very good point!
>
> Cf. Aziz's psyntax/expander.ss from r6rs-libraries.dev:
>
>   ;;; (two marks must be eq?-comparable, so we use a string
>   ;;; of one char (this assumes that strings are mutable)).
>   
>   ;;; gen-mark generates a new unique mark
>   (define (gen-mark) ;;; faster
>     (string #\m))
>
>   ;;; every identifier in the program would have a label associated
>   ;;; with it in its substitution.  gen-label generates such labels.
>   ;;; the labels have to have read/write eq? invariance to support 
>   ;;; separate compilation.
>   (define gen-label
>     (lambda (_) (gensym)))
>
> His gensyms are globally unique; he made the exact opposite choice that
> I did in the fd5985271fee3bcb6a290b6ad10525980a97ef8d; interesting.  I
> wonder who is right here.

John Cowan's recent message on scheme-reports "Re: fresh empty strings"
suggests that Ikarus does not merge equal string literals into a single
object, so I guess in that case his `gen-mark' would work properly.

>> Maybe there's something I'm missing here, but if the change you made
>> above was safe, I think the burden of proof is on you to explain why.
>
> Indeed, and I think you found a good indication that it's not right.

How would you like to fix this?  Would you like me to make a new
procedure that creates a universally-unique string?  Most of `gensym's
current code would be moved to that new procedure, and then `gensym'
would use it.

   Thanks,
     Mark