Result / Exceptions

  1. For Result/Error, theanyhow::Result/anyhow::Error is supported. It will be automatically converted to a Dart Exception.
  2. For panics, it will also be automatically captured and converted to Dart exceptions.
  3. For error hierarchy, or arbitrary error types, it is also supported. For example, you can create your own CustomError (such as using thiserror), and it will automatically be converted to a new Dart class.

If you want to see stack traces (backtraces), this doc page discusses how to configure it.


Example 1: Anyhow Result

For example, the following code, when called by Dart code, will throw Dart exceptions.

pub fn f() -> anyhow::Result<i32> { bail!("oops I failed") }

Example 2: Panic

All functions below, when called, will throw Dart exceptions at the Dart side due to the panic.

pub fn g1() -> i32 { panic!("oops I failed") }
pub fn g2() -> anyhow::Result<String> { panic!("oops I failed") }
pub fn g3() -> Result<Vec<u8>, CustomError> { panic!("oops I failed") }

Example 3: Custom Error Without backtrace

pub enum CustomError {

pub fn return_err_custom_error() -> Result<u32, CustomError> {

Becomes something that can be used like this:

try {
    final r = await api.returnErrCustomError();
    print("received $r");
} catch (e) {
    print('dart catch e: $e');
    expect(e, isA<CustomError>());

Example 4: Custom Error With backtrace

Errors with custom fields are also supported, and you can even pass a backtrace:

pub enum CustomStructError {
    Error0 { e: String, backtrace: Backtrace },
    Error1 { e: u32, backtrace: Backtrace },

As for how to fill it in or use it, you can refer to thiserror crate for some hints.