Language

Errors are values

A fallible function returns a result that its caller must handle. The notation T ! E means a successful T or an error E.

Return a result

fn positive(value: num): num ! str {
  if value <= 0 { return Err("expected a positive number") }
  return Ok(value)
}

let count = positive(3) catch 1
log `{count}`

A result cannot be silently dropped. Use catch when you have a fallback, match when both paths need work, or try when the caller should handle the error.

Propagate an error

fn doubled(value: num): num ! str {
  let valid = try positive(value)
  return Ok(valid * 2)
}

The propagating bind valid := positive(value) is shorthand for let valid = try positive(value). Both belong in a context that can propagate the error.

Recover or inspect

match positive(-1) {
  Ok(value) => { log `{value}` },
  Err(problem) => { log `{problem}` },
}

Catch takes a fallback expression. Use match when recovery needs statements. Errors returned as values are different from VM traps: a trap is fatal to the instance unless handled at a supported task boundary.