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Comment by DangerousPie | Hacker News Reader

On a related note, how does the `parse()` in `let addr = "127.0.0.1:12345".parse().unwrap();` work? I am assuming that this parses a IP:port string into its components, but it looks like this is just calling a method of the basic String class. So how does it infer what `parse()` is supposed to do?

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parse() is generic over it's return type (which is Result<F, F::Err> where F: FromStr) [1]. The value of which can be inferred.

The result of parse().unwrap() is of type F, which means addr is of type 'F'. afterwards when we call `let sock = TcpListener::bind(&addr, &handle).unwrap();` the compiler can infer from the signature of `TcpListener::bind`[2] what F has to be, namely `std::net::SocketAddr`

[1] https://doc.rust-lang.org/std/primitive.str.html [2] https://tokio-rs.github.io/tokio-core/tokio_core/net/struct....

Where is string to address parse implementation defined? I took a peek at SocketAddr and see a from_str function. Does that name have special significance?

It's at the very bottom of https://doc.rust-lang.org/std/net/enum.SocketAddr.html, the part that says

  impl FromStr for SocketAddr

If you look at parse https://doc.rust-lang.org/std/primitive.str.html#method.pars... it says

    fn parse<F>(&self) -> Result<F, F::Err> where F: FromStr

So any type that implements FromStr can be parse'd from a &str.

which has a link to the actual src which is here-ish and is what you might expect https://doc.rust-lang.org/src/std/up/src/libstd/net/parser.r...

If you look closely, you'll see that the from_str function is part of the implementation of the FromStr trait for the SocketAddr struct. Its this FromStr trait that the parse() method depends on.

The method `from_str` is from the FromStr trait. [1] The `parse` function only works on types that implement FromStr. It's "special" in that it's the conventional way to do things, but the compiler does not treat FromStr specially.

[1]: https://doc.rust-lang.org/std/str/trait.FromStr.html

Rust uses type inference to figure that out. Because `addr` gets passed to `TcpListener::bind` which requires a `SocketAddr`, the type inferencer knows that `addr` should be a SocketAddr and so it all works :)

You might also add that it only works because SocketAddr implements FromStr. Type inference will only determine which version of from_str to call, it won't actually do the parsing.

That type inference goes well beyond most others though.

Wow, thank you for pointing that out!

Due to the resulting thread, I feel legitimately excited to sit down and learn a new-to-me language for the first time in ages :)

I noticed that too, because it get's passed after as an argument, Rust is able to infer it's type.

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