On Wed, 3 Sep 2014 15:17:22 +0900, Alex Shinn <alexsh...@gmail.com> said:
> On Wed, Sep 3, 2014 at 2:00 PM, Kevin Wortman <kwort...@gmail.com> wrote: >> ... >> Are cycles actually possible in strict immutable structures? When you >> create a head node, there's no way for it to have a circular link to a tail >> node that doesn't exist yet. > Yes, if you provide implementation support for it, > see http://docs.racket-lang.org/reference/shared.html > Also for lazy data structures see > http://www.haskell.org/haskellwiki/Tying_the_Knot, > for which I have a Scheme version lying around somewhere. By the way, some languages offer yet another approach, in principle similar to the latter, but more procedural. It is to make the object construction protocol a little more elaborate so the user (not just the implementor) can see object _allocation_ as a separate step from object _initialization_. Probably needless to add, in this way a reference to the object becomes available (for plugging into other objects) before the construction of the object is finished (with references to those other objects plugged into it). This is just with regards to the possibility of cycles in strictly immutable structures; whether this can be a candidate for adopting by Scheme is another question. There is also the more general approach of "arbitrarily delayed once-only component initialization", where any component of an immutable object can be initialized at any time after object creation (no matter how much later), but only once. If I recall correctly, Oz allows (something like) that, but I forget how exactly it is called there. ---Vassil. -- Vassil Nikolov | Васил Николов | <vniko...@pobox.com> "Be careful how you fix what you don't understand." (Brooks 2010, 185) _______________________________________________ Scheme-reports mailing list Scheme-reports@scheme-reports.org http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports
