Typed errors (error, panic, and T | ErrorType...)

Typed errors make “this function can fail” explicit in the type system:

• define error types,
• declare error contracts in function return types (T | MyError),
• trigger errors with panic MyError { ... };,
• handle them with match (expr) { ... } (statement form + Terminal Arm Rule),
• and propagate them with the postfix ? operator (call()?).

Full reference: typed errors.

Notes#

Defined in the reference compiler (front-end + native backend subset).

• Canonical spec + rules: typed errors
• Error model overview: errors

Syntax#

error OutOfBounds { index: int, len: int }

fn get_at (xs: &u8[], index: int) -> u8 | OutOfBounds {
  if index < 0 { panic OutOfBounds{ index: index, len: 0 }; }
  return 0;
}

fn caller (xs: &u8[]) -> u8 | OutOfBounds {
  let x: u8 = get_at(xs, 0)?;
  return x;
}

Examples#

Example: ? propagation#

error Boom { code: int }

fn may_boom () -> int | Boom {
  panic Boom{ code: 7 };
}

fn main () -> int | Boom {
  let x: int = may_boom()?;
  return x;
}

Example: handling with match statement#

error Boom { code: int }

fn may_boom (x: int) -> int | Boom {
  if x == 0 {
    panic Boom{ code: 123 };
  }
  return 7;
}

fn main () -> int {
  match (may_boom(0)) {
    value => {
      return value;
    },
    err: Boom => {
      std::abort();
    },
  }
}

See also#

• Full reference: typed errors