Implicitly here, you seem to be saying you're fine tossing that kind of reasoning for lists. Are you?
Robby On Tue, Feb 1, 2011 at 2:58 PM, Matthias Felleisen <[email protected]> wrote: > > I am with Matthew. Immutable structs -- other than historically 'blessed' > ones -- give you inductive reasoning. If you allow cycles anyway, that's out > the window. > > > > On Feb 1, 2011, at 3:47 PM, Matthew Flatt wrote: > >> At Tue, 1 Feb 2011 12:38:44 -0800, David Herman wrote: >>> Just thinking out loud, but is there any way to use the unsafe operations to >>> make the shared library work for immutable data? If you can prove it's >>> actually safe, then there'd be no harm to users. (After all, as you say, >>> that's what it's doing with pairs.) >> >> The difference is that pairs are defined a priori as allowing the >> possibility of cycles, so you know that possibility exists when working >> with pairs. >> >> If my module gives yours a structure type, however, I may not want to >> automatically give yours the capability to create cyclic instances of >> the structure type. In that sense, creating a cycle is not always safe. >> >> Furthermore, implementing the cycle by creation plus mutation would >> mean introducing mutation for a supposedly immutable datatype, which >> isn't necessarily safe if the intermediate state is visible. Attempting >> to hide the intermediate state creates all sorts of new problems for >> features like properties and guards. >> >> Along those lines, granting the capability to create cyclic instances >> seems to be just about tantamount to granting mutability, so `shared' >> might as well require mutability. >> >> _________________________________________________ >> For list-related administrative tasks: >> http://lists.racket-lang.org/listinfo/users > > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
