Hi Felix, Thanks a lot for the detailed explanation.
On Tue, Jul 22, 2014 at 7:08 PM, Felix S. Klock II <pnkfe...@mozilla.com> wrote: > Aravinda (cc’ing rust-dev)- > > It seems like you are trying to program in Rust as if it were a > dynamically-typed language, or one with runtime-type reflection (i.e. like > Java). At least, that is my best guess at where your misunderstanding lies. > > All functions in Rust, even generic ones, need to have their types > resolved at compile-time. A generic function can have different concrete > types substituted in for its type parameters at different call-sites, but > in the end, a particular call-site needs to resolve to a single type at > compile-time; the type cannot be left for later resolution at program > runtime. > > In a signature like your: > > fn get_value(settings:HashMap<String, MyTypes>, key: &'static str) -> > T; > > the particular instance of `MyTypes` that is returned will depend on which > `key` is passed in; therefore, the `T` above could only be dynamically > determined based on the runtime computation. It inherently cannot be > resolved at compile-time, and therefore it is not statically typed. > > ---- > > Rust is not alone in offering this kind of generic types; many programming > languages use a similar logic for determining types at compile time. It > just gets fuzzy if one is used to languages that maintain types at runtime > and do not enforce restrictions like the one I outlined above. > > These type systems are often said to offer “parametric polymorphism”; I > mention that solely to give you some guidance for a term to search for when > goggling this subject. (Though I will say up front that a lot of the > results you get on this topic can be very academic and language > research-oriented.) > > Here is a tutorial that may help you get a handle on the concepts here: > > http://lucacardelli.name/Papers/BasicTypechecking.pdf > > (Yes, it is from 1987. I think that is why it probably one of the better > descriptions I was able to find quickly: At that time, these ideas were not > as widely popularized as they were today, so Cardelli took his time > explaining the notions and assumed little about the audience.) > > rust-dev members: If others know of freely available introductions to this > topic, I’m all ears; I just didn’t see any obvious winners in my searches. > > Cheers, > -Felix > > > On 22 Jul 2014, at 14:24, Aravinda VK <hallimanearav...@gmail.com> wrote: > > Sorry for the incomplete mail. > > What I wanted is, > get_value(MyStr("Rust".to_str())) returns String, > get_value(MyBool(true)) returns bool and, > get_value(MyInt(100)) returns int > > I was trying to store generic value in hashmap, as in the example below, > > use std::collections::hashmap::HashMap; > > #[deriving(Show)] > enum MyTypes{ > MyBool(bool), > MyStr(String), > MyInt(int) > } > > fn main(){ > let mut settings:HashMap<String, MyTypes> = HashMap::new(); > > settings.insert("port".to_str(), MyInt(8000)); > settings.insert("name".to_str(), MyStr("Rust".to_str())); > settings.insert("enabled".to_str(), MyBool(true)); > > println!("{}", settings); > } > > So to get the value out of hashmap, I need a generic function which checks > the respective type and returns value. Some thing like > > fn get_value(settings:HashMap<String, MyTypes>, key: &'static str) -> T{ > match settings.get(&key) { > MyBool(x) => x, > MyStr(x) => x, > MyInt(x) => x > } > } > > But I don't know how to make this work. > > Thanks. > > > > On Tue, Jul 22, 2014 at 4:55 PM, Felix S. Klock II <pnkfe...@mozilla.com> > wrote: > >> Aravinda (cc’ing rust-dev)- >> >> You didn’t show us exactly what you had tried to do to get your code to >> work, nor did you really describe what it is you want here. >> >> E.g. you seem to want `get_value(MyBool(true))` to return a boolean, but >> since `MyBool` belongs to the `MyTypes` enum, that implies that `get_value` >> when applied to any variant of `MyTypes` (including `MyInt` or `MyStr`) >> should also return a boolean … does that seem right to you? >> >> In any case, I suspect the missing piece of the puzzle for you is that >> you need to write an `impl` for the type in question. I.e. something along >> the lines of: >> >> impl MyTypes { >> fn render(&self) -> String { >> match *self { >> MyBool(x) => format!("{:b}", x), >> MyStr(ref x) => x.clone(), >> MyInt(x) => format!("{:d}", x), >> } >> } >> } >> >> (except revised from an Impl for the type to being an impl of some trait >> for the type). >> >> Here is a link to a playpen with your code, and with a couple of example >> `impl`s for enums (like the one above) tossed in, including an impl of one >> instance of your `Value<S>` trait. >> >> http://is.gd/RofN9R >> >> There is more discussion of writing implementations that also provides an >> example with a simpler enum) in the Rust tutorial, see: >> >> http://doc.rust-lang.org/tutorial.html#methods >> >> Cheers, >> -Felix >> >> On 22 Jul 2014, at 11:45, Aravinda VK <hallimanearav...@gmail.com> wrote: >> >> > Hi, >> > >> > I am trying to create a generic function to return value depending on >> the enum passed. But I don't know to create a generic trait for enum. >> > >> > In following example, print_value works but I don't know how I can >> write a generic get_value function to get value from enum. >> > >> > #[deriving(Show)] >> > enum MyTypes{ >> > MyBool(bool), >> > MyStr(String), >> > MyInt(int) >> > } >> > >> > fn print_value(arg: MyTypes){ >> > match arg{ >> > MyBool(x) => println!("Bool: {}", x), >> > MyStr(x) => println!("String: {}", x), >> > MyInt(x) => println!("Int: {}", x), >> > } >> > } >> > >> > >> > fn main(){ >> > print_value(MyBool(true)); >> > >> > // Following lines not working, how to write get_value func? >> > // let a: bool = get_value(MyBool(true)); >> > // println!("{}", a); >> > } >> > >> > >> > In case of struct it is simple, >> > >> > struct MyInt { >> > value: int >> > } >> > >> > struct MyBool{ >> > value: bool >> > } >> > >> > trait Value<S>{ >> > fn get(&self) -> S; >> > } >> > >> > impl Value<int> for MyInt{ >> > fn get(&self) -> int{ >> > self.value >> > } >> > } >> > >> > impl Value<bool> for MyBool{ >> > fn get(&self) -> bool{ >> > self.value >> > } >> > } >> > >> > fn get_value<S, T: Value<S>>(arg: T) -> S{ >> > arg.get() >> > } >> > >> > fn main(){ >> > let a: bool = get_value(MyBool{value: true}); >> > println!("{}", a); >> > >> > let b: int = get_value(MyInt{value: 100}); >> > println!("{}", b); >> > } >> > >> > Please help in writing generic function for enum. >> > >> > >> > >> > -- >> > Regards >> > Aravinda | ಅರವಿಂದ >> > http://aravindavk.in >> > _______________________________________________ >> > Rust-dev mailing list >> > Rust-dev@mozilla.org >> > https://mail.mozilla.org/listinfo/rust-dev >> >> > > > -- > Regards > Aravinda | ಅರವಿಂದ > http://aravindavk.in > > > -- Regards Aravinda | ಅರವಿಂದ http://aravindavk.in
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