On Fri, Jul 6, 2012 at 3:04 PM, Lindsey Kuper <lind...@rockstargirl.org> wrote: > Two of the changes mentioned in the development roadmap that Graydon > sent out earlier this week were "Extend interfaces to full traits" and > "Enforce implementation coherence". Those are addressed by this > proposal for unifying traits and interfaces: > > https://github.com/mozilla/rust/wiki/Proposal-for-unifying-traits-and-interfaces
That last hash table example makes me uneasy because the proposed solution doesn't really solve the problem very nicely at all. If I have a 'hash' trait in a module Hash, and then two different hash function providers FastHash (TM) and QuickHash (TM) want to provide hash functions for use with a hash table, they can not actually be used with any standard types, (without requiring first wrapping every key inside some per-hash-function wrapper type. This is ugly and kinda defeats the point of overloading - if we want to pick the implementation explicitly at each use site, why not just pass in the function dictionary explicitly instead of wrapping the arguments in a special "marker" type whose sole purpose is to make the compiler pick the right instance? This seems.. quite painful. I would be extremely unlikely to use a 'hash' trait in a hash table implementation for this reason. I'd store the hash function as a closure in the table itself, and set it up at construction (this way you can pass in any hash function provider at construction). It almost seems like the guidance for library writers would be to avoid traits because they reduce the ability for users to compose different behaviors (or at the very least each library would provide two versions of everything - one which uses a trait, and one which takes the method dictionary at construction time and stores it with the data structure). Not sure what the right solution is. I think Haskell's system is too inflexible, but this is even less flexible. Maybe there are better options in the literature somewhere? Maybe parameterizing modules is the answer - the Hash module can define instances for pre-existing types using functions that are passed in by the importer of the module (the same module imported twice but parameterized differently would be treated as two independent modules)? -- Sebastian Sylvan _______________________________________________ Rust-dev mailing list Rust-dev@mozilla.org https://mail.mozilla.org/listinfo/rust-dev
