In Rust, how do I use implemented trait FromStr on

问题:

I am trying to get this program to compile:

extern crate num;
use num::bigint::BigInt;
use std::from_str::FromStr;

fn main () {
    println!("{}", BigInt::from_str("1"));
}

But the output from rustc is

testing.rs:6:20: 6:36 error: unresolved name `BigInt::from_str`.
testing.rs:6     println!("{}", BigInt::from_str("1"));
                                ^~~~~~~~~~~~~~~~
note: in expansion of format_args!
<std macros>:2:23: 2:77 note: expansion site
<std macros>:1:1: 3:2 note: in expansion of println!
testing.rs:6:5: 6:43 note: expansion site
error: aborting due to previous error

I suspect I am doing something trivially wrong, but I've tried searching for examples and tried a bunch of different changes and nothing I tried worked.

How do I change my source code so this compiles?

回答1:

The answers that mention from_str are not going to work with recent versions of Rust, this function has been removed.

The modern way to parse values is the .parse method of str:

extern crate num;
use num::bigint::BigInt;

fn main() {
    match "1".parse::<BigInt>() {
        Ok(n)  => println!("{}", n),
        Err(_) => println!("Error")
    }
}

回答2:

extern crate num;
use num::bigint::BigInt;

fn main () {
    println!("{}", from_str::<BigInt>("1"));
}

In function calls, you need to put :: before the angle brackets.

回答3:

This works for calling the trait implementation directly instead of through the utility function. This is not idiomatic.

extern crate num;
use num::bigint::BigInt;
use std::from_str::FromStr;

fn main () {
    let x : Result<BigInt,_> = FromStr::from_str("1");
    println!("{}", x);
}