Incorrect value for the exponential function of (-1)

[jonahpelfrey]

I'm using rust_decimal to implement a Monte Carlo simulation that requires the use of the exponential function (e^x).

While debugging values in the Monte Carlo, I ran into a case where the input to Decimal::exp() was -1, and the output was zero.

Simplified code:

use rust_decimal::prelude::*;

fn main() {
  let decimal = Decimal::from_str("-1").unwrap();
  let value = decimal.exp();
  println!("Value: {:?}", value);
  // Value: 0
}

Wolfram alpha: 0.36787944117
https://www.wolframalpha.com/input/?i=e%5E-1

Any ideas on what might be causing the approximation to be incorrect?