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?
The plain function from_str
has been removed in recent versions of Rust. This function is now only available as a method of the FromStr
trait.
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")
}
}
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.
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);
}
Your original code works almost as-is:
use num::bigint::BigInt; // 0.2.0
use std::str::FromStr;
fn main() {
println!("{:?}", BigInt::from_str("1"));
}
You need to switch to std::str::FromStr
and from_str
returns a Result
which requires the {:?}
( Debug
) formatter.
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