# New Type Idiom

The `newtype` idiom gives compile time guarantees that the right type of value is supplied to a program.

For example, an age verification function that checks age in years, must be given a value of type `Years`.

    struct Years(i64);

    struct Days(i64);

    impl Years {
        pub fn to_days(&self) -> Days {
            Days(self.0 * 365)
        }
    }

    impl Days {
        /// truncates partial years
        pub fn to_years(&self) -> Years {
            Years(self.0 / 365)
        }
    }

    fn old_enough(age: &Years) -> bool {
        age.0 >= 18
    }

    fn main() {
        let age = Years(5);
        let age_days = age.to_days();
        println!("Old enough {}", old_enough(&age));
        println!("Old enough {}", old_enough(&age_days.to_years()));
        // println!("Old enough {}", old_enough(&age_days));
    }

Uncomment the last print statement to observe that the type supplied must be `Years`.

### See also:

`structs`