On Thu, Apr 11, 2013 at 9:18 AM, Anthony Carrico <acarr...@memebeam.org> wrote: > On 04/11/2013 08:51 AM, Sam Tobin-Hochstadt wrote: >> Yes, an explicit struct involves an explicit indirection, and thus >> produces a regular tree. > > Thanks Sam. I'm glad I asked the question. I guess I did manage to bump > up against a limitation of the type system on my first time out. > > And thanks to Eric for showing me a way around. Bummer that this adds a > spurious struct at runtime. Sam, I think the reason people are pressing > the question is that intuitively it appears as though the type system > would be appeased by a "phantom" struct for the indirection (which > didn't allocate anything at runtime), which would be just as good as a > non-regular datatype. Maybe the catch is in the subtype cases you mentioned?
This is indeed something we've run into before. See section 4.2 of this paper: http://www.ccs.neu.edu/racket/pubs/sfp10-kth.pdf We could probably also introduce something like Haskell's `newtype` to address this, as you suggest, but it's not obvious to me how that would work in interoperation with untyped code, for example. There are a lot of open questions here, and I think we can eventually give better answers to them than we currently have, but we don't have them yet. Sam ____________________ Racket Users list: http://lists.racket-lang.org/users
