# `Result`

We've seen that the `Option` enum can be used as a return value from functions that may fail, where `None` can be returned to indicate failure. However, sometimes it is important to express why an operation failed. To do this we have the `Result` enum.

The `Result<T, E>` enum has two variants:

• `Ok(value)` which indicates that the operation succeeded, and wraps the `value` returned by the operation. (`value` has type `T`)
• `Err(why)`, which indicates that the operation failed, and wraps `why`, which (hopefully) explains the cause of the failure. (`why` has type `E`)
``````mod checked {
// Mathematical "errors" we want to catch
#[derive(Debug)]
pub enum MathError {
DivisionByZero,
NonPositiveLogarithm,
NegativeSquareRoot,
}

pub type MathResult = Result<f64, MathError>;

pub fn div(x: f64, y: f64) -> MathResult {
if y == 0.0 {
// This operation would `fail`, instead let's return the reason of
// the failure wrapped in `Err`
Err(MathError::DivisionByZero)
} else {
// This operation is valid, return the result wrapped in `Ok`
Ok(x / y)
}
}

pub fn sqrt(x: f64) -> MathResult {
if x < 0.0 {
Err(MathError::NegativeSquareRoot)
} else {
Ok(x.sqrt())
}
}

pub fn ln(x: f64) -> MathResult {
if x <= 0.0 {
Err(MathError::NonPositiveLogarithm)
} else {
Ok(x.ln())
}
}
}

// `op(x, y)` === `sqrt(ln(x / y))`
fn op(x: f64, y: f64) -> f64 {
// This is a three level match pyramid!
match checked::div(x, y) {
Err(why) => panic!("{:?}", why),
Ok(ratio) => match checked::ln(ratio) {
Err(why) => panic!("{:?}", why),
Ok(ln) => match checked::sqrt(ln) {
Err(why) => panic!("{:?}", why),
Ok(sqrt) => sqrt,
},
},
}
}

fn main() {
// Will this fail?
println!("{}", op(1.0, 10.0));
}
``````