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
