Infinite not being used

Example:

extern crate num;

use num::{BigInt, BigRational, ToPrimitive};

fn main() {
    let n = BigInt::from(-2);
    let n = n.pow(4001);
    println!("{:?}", n.to_f64());
    let r = BigRational::from(n);
    println!("{:?}", r.to_f64());
}

Result:

None
Some(-inf)

I was expecting some minus infinite and got none. A workaround is to convert to BigRational but it is expensive, a clone is required.

[cuviper]

You can also see ToPrimitive sticking to finite values when narrowing, e.g. std::f64::MAX.to_f32() returns None. It's been that way ever since it was first created in rust-lang/rust#9250 (specifically here), and it doesn't appear that this point was debated.

At that time, bigints only directly supported converting to 64-bit integers, and floats would convert from there by the trait default, so larger values would just be None. Then rust-num/num#145 added explicit conversions from bigints to floats, noting that None was used instead of Some(INFINITY), but we didn't debate that point.

So this is actually inconsistent on BigRational's part, which is relatively recent: rust-num/num-rational#52.

That's not to insist that we shouldn't change, but we should be careful and consistent about it.

[cuviper]

Another inconsistency: u128::MAX.to_f32() returns Some(INFINITY).

Understood, thank you for clarifying the situation, closing.

[cuviper]

I was starting to think we should change -- do you think not?

To make it consistent? Yes. Would it be for finite value or would infinite value be included?

[cuviper]

I think we should include infinities -- if nothing else, the fact that they can be positive or negative is better than None.

Agree. Reopening to track the issue.