On 10/27/05, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > > Tomasz Zielonka wrote: > > > Speaking about casts, I was playing with using GADTs to create a > > non-extensible version of Data.Typeable and Data.Dynamic. > > I wonder if it's possible to write such a thing without GADTs (and > > unsafeCoerce, which is used in Data.Dynamic, IIRC). > > Absolutely. Stephanie Weirich did that back in 2000 (actually, earlier: > her paper, TypeSafe cast was presented at ICFP'00). The code does not > use existentials -- only universals. BTW, if you don't plan to store > your dynamics in a data structure (or plan to cast them to the same > thing), then no explicit quantification is needed and the code becomes > Haskell98. Here's her code for the cast, with slight > embellishments.
This is cool! I thought that even if it was possible, the code would be longer and less readable. It isn't. I'm not sure I would be able to do everything I need to do with this implementation of Dynamic (because I have some other operations on Type), but I guess the answer is True. I've known about this paper, but I haven't read it yet. That was a mistake :-) > It should be pointed out that we can cast polymorphic > lists and polymorphic functions. I'm not sure I understand correctly, but I don't think you can (toDyn const) and get a polymorhic function back from fromDyn. I checked and I got an ambiguity error. Can you show an example? > There is a different way to perform casting within a closed universe, > see > http://www.mail-archive.com/haskell@haskell.org/msg13089.html I'll check it out. Best regards Tomasz _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
