Modification of https://github.com/fivemoreminix/rsc and https://gist.github.com/NickAger/d58f3120388fe899cad1022a01c162b9 - now works.

main.rs

use rsc::{tokenize, parse, Interpreter, TokenizeError, ParseError, InterpretError};

// error handling based on: 
// * https://doc.rust-lang.org/rust-by-example/error/multiple_error_types/wrap_error.html

#[derive(Debug)]
struct EquationError(pub String);

impl<'input_string> From<TokenizeError<'input_string>> for EquationError {
    fn from(err: TokenizeError<'input_string>) -> EquationError {
        EquationError(format!("Tokenize Error: {:?}", err))
    }
}

impl<'input_string> From<ParseError<'input_string, f64>> for EquationError {
    fn from(err: ParseError<'input_string, f64>) -> EquationError {
        EquationError(format!("Parse Error: {:?}", err))
    }
}

impl<'input_string> From<InterpretError<'input_string>> for EquationError {
    fn from(err: InterpretError<'input_string>) -> EquationError {
        EquationError(format!("Interpret Error: {:?}", err))
    }
}
type EvaluateResult = std::result::Result<f64, EquationError>;

fn evaluate<'input>(input: &str, interpreter: &mut Interpreter<f64>) -> EvaluateResult {
    let tokens = tokenize(&input)?;
    let expr = parse(&tokens)?;
    let result = interpreter.eval(&expr)?;

    return Ok(result);
}

fn pr(x: f64) {
    println!("{}", x);
}


fn main() -> Result<(), EquationError> {
    // Constructs an f64 interpreter with included variables
    println!("Starting");
    let mut interpreter = Interpreter::default();
    
    let result1 = evaluate("5^2", &mut interpreter)?;
    pr(result1); // prints "25"
    let result2 = evaluate("x = 3", &mut interpreter)?; 
    pr(result2); // prints "3"
    let result3 = evaluate("x(3) + 1", &mut interpreter)?; 
    pr(result3); // prints "10"

    return Ok(());   
}