What is the correct way to convert an i128 to an f64

Question

I would like to convert an i128 to an f64.

I know that I can silently cast with as.

let nb: i128 = 1337;
let converted = nb as f64;

But it doesn't give me the opportunity to handle any error.

Is there a way to do something similar to

let nb: i128 = 1337;
let converted: f64 = nb.try_into()?;

Where I can handle the error, if the converted value doesn't fit into an f64 ?

Answer 1

There is no standard f64::try_from(i128).
You can make your own function relying on a check after the reciprocal as coercion.

fn try_to_f64(v: i128) -> Result<f64, &'static str> {
    let attempt = v as f64;
    (attempt as i128 == v)
        .then_some(attempt)
        .ok_or("precision loss in i128 to f64 conversion")
}

fn main() {
    for v in [
        123_456_789,
        123_456_789_123,
        123_456_789_123_456,
        123_456_789_123_456_789,
    ] {
        println!("{} ~~> {:?}", v, try_to_f64(v));
    }
}
/*
123456789 ~~> Ok(123456789.0)
123456789123 ~~> Ok(123456789123.0)
123456789123456 ~~> Ok(123456789123456.0)
123456789123456789 ~~> Err("precision loss in i128 to f64 conversion")
*/