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Return Types & Exceptions

Suppose your function wants to return a String, then all these return types are supported:

  1. Direct return type (fn f() -> String)
  2. Anyhow error (fn f() -> anyhow::Result<String>): The anyhow error will be automatically converted to a Dart exception.
  3. Arbitrary custom error type (fn f() -> Result<String, YourErrorType>): The YourErrorType will be automatically converted to a Dart exception.

In addition, Rust has panic in addition to Result error, thus:

  • When Rust panic occurs, a PanicException will be thrown in Dart.

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

Example​

Example 1: Direct Result​

pub fn f(a: i32, b: i32) -> i32 { a + b }

Example 2: 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 3: 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 4: Custom Error Without backtrace​

pub enum CustomError {
    Error0(String),
    Error1(u32),
}

pub fn return_err_custom_error() -> Result<u32, CustomError> {
    Err(CustomError::Error1(3))
}

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 5: 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.