Expand description
eval takes a &str and a &mut VecDeque<f64>, evaluates the expression,
and prints the result, pushing the results onto the stack of type
VecDeque
It does not return an error, but will instead silently fail if the expression is invalid. Additionally, it mutates the stack parameter, rather than returning a new one. For this reason, eval is less safe, and safe_eval is recommended over it.