Rust by Example

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));

}

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

To obtain the newtype's value as the base type, you may use tuple syntax like so:

struct Years(i64);

fn main() {

let years = Years(42);

let years_as_primitive: i64 = years.0;

}

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