On Mon, Apr 29, 2013 at 9:30 PM, Erik S <sw...@earthling.net> wrote: > On 4/29/2013 2:00 PM, Graydon Hoare wrote: >> Yes. Formalizing and completing the definition of the constant >> expression subgrammar would be extremely valuable. It's one of the >> major high-risk areas remaining the the design space. -Graydon > VHDL is a very different language from Rust (it's a hardware design > language, not a normal programming language), but it has the nicest > constant system I've worked with. > > The rule is basically that constant expressions can contain: > > Literals (i.e. 5, 0x20) > Expressions that depend on constants (i.e. 0x5 + 10 * const_k) > Any "pure" function whose arguments are constant. (i.e. 5 + f_pure(5, > const_k) ) > > It's this last rule that is truly beautiful. You can use the same > functions in both constant initialization and run-time code, with only > the requirement that they are pure. Pure functions are ones whose output > depends only on their arguments (and constants). Allowing functions to > initialize constants avoids the whole annoyance in C/C++ where you have > to use macros to make things compile-time const. It also allows a > surprising amount of compile-time optimization. > > I don't know how realistic this is for constants in a language like rust > - but it would be very elegant if it could work. >
We used to have a notion of pure functions in Rust, but that was removed recently. It originally existed for typestate (a feature we removed a while ago) and more recently to assist the borrow checker. We removed it because in Rust, it was impractical to define an easily-checkable pure subset of the language. Bringing it back in is somewhat less likely than adding significant whitespace. Cheers, Tim -- Tim Chevalier * http://catamorphism.org/ * Often in error, never in doubt "Too much to carry, too much to let go Time goes fast, learning goes slow." -- Bruce Cockburn _______________________________________________ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
