Error Handling

Though QangLang does not have the throw/try/catch/finally seen in other languages, it does have a couple of ways to surface errors.

Using Assertions

assert

assert can be used to test if a given condition is truthy. It takes in an optional message. If the condition is falsy, the program exits with the message.

assert(true == false, "This will result in an error.");

assert_eq

assert_eq is a more specific case of assert that checks for equality, and exits if not equal.

assert_eq(true, false, "This will also result in an error");

assert_not_nil

assert_not_nil is another specific case of assert where the program exits if the provided value is equal to nil.

assert_not_nil(nil, "This is also an error because nil is not not nil.");

Using Result

Result is a base abstract class with two concrete implementations

• Ok - Used to wrap the value
• Err - Used to instead return an error if the result value is no longer valid.

fn validate_value_is_greater_than_zero(value) {
  return when (value) {
    is NUMBER => value > 0
      ? Ok(value)
      : Err("Value must be greater than 0.");
    else => Err("Value must be a number.")
  };
}

Once a value is wrapped in the Result, there are a number of methods that can be used to manipulate the value.

var maybe_double = validate_value_is_greater_than_zero(10)
  .map((num) -> num * 2)
  .ok(); // 20

var maybe_squared = validate_value_is_greater_than_zero(-5)
  .map((num) -> num * num)
  .ok(); // nil

For information other methods on Result, see the Result reference.

Custom Errors

Sometimes it might be required to store more than an error message inside an Err, but also require a clear error message. A new class ineheriting from Err can be used to handle that, with the to_string method overriden for the custom message.

class MyErr : Err {
  init(message, kind) {
    super.init({{
      message = message or "Unknown.",
      kind = kind or "UNKNOWN"
    }});
  }

  to_string() {
    return this._error.kind + " Error: " + this._error.message;
  }
}

var error = MyErr("Oops.", "REALLY_BAD");

println(error.to_string()); // "REALLY_BAD Error: Oops."