ex.rs

    if let Some(value_or_func) = scope.find(&expression.function) {
        if let &ValueOrFunc::Func(ref func) = value_or_func {
            match *func {
                Func::Builtin(f) => {
                    return f(scope, expression.parameters);
                }
                Func::UserDefined(ref block) => Ok(Value::Nil),
            }
        } else {
            return Err(format!("'{}' is not callable", expression.function));
        }
    } else {
        return Err(format!("Undeclared identifier '{}'", expression.function));
    }