Function std::fs::read_to_string example code fails to run on command line and needs improving.

[haydonryan]

Location

https://doc.rust-lang.org/std/fs/fn.read_to_string.html

cargo 1.74.0 (ecb9851af 2023-10-18)
rustc 1.74.0 (79e9716 2023-11-13)

Summary

I'm still very new to rust, but as part of advent of code this year found that the example for std::fs::read_to_string fails. I was also surprised to see the return value being a socket address not a string. I created a simple test program and the sample code returns an error. It would be much better having this return a string and print it out.

2124 $ cat main.rs 
use std::fs;
use std::net::SocketAddr;
use std::error::Error;

fn main() -> Result<(), Box<dyn Error>> {
    let foo: SocketAddr = fs::read_to_string("address.txt")?.parse()?;
    Ok(())
}

2125 $ cat address.txt
123 rusty lane
san francisco 94999

2126 $ cargo run
warning: unused variable: `foo`
 --> src/main.rs:6:9
  |
6 |     let foo: SocketAddr = fs::read_to_string("address.txt")?.parse()?;
  |         ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: `tester` (bin "tester") generated 1 warning (run `cargo fix --bin "tester"` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `/home/haydon/workspace/rust-test-pr/tester/target/debug/tester`
Error: AddrParseError(Socket)

I'm submitting a PR shortly and would love some feedback on it.

[jdahlstrom]

The example uses the parse method to parse into a SocketAddr -- note that it's read_to_string()?.parse()?, not just read_to_string()?! The read_to_string function itself returns a String as expected. A SocketAddr is an IP address and port number, so if you put something like 192.168.0.123:4567 into address.txt, the example should work fine.

Now, I do think that the example makes matters needlessly confusing by introducing the SocketAddr type and requiring the reader to have familiarity with how parse works, both of which are just red herrings. Particularly given that parse uses return type inference, plus delegation to the FromStr trait, to be able to parse a string into a number of different types.

[haydonryan]

Thankyou!

I've updated this in the PR and also added a second instance of using socketaddr for fs::read.