anyhow

anyhow offers versatile and idiomatic error handling capabilities to make your code safer, more maintainable, and errors easier to debug.

This is accomplished through the use of the Result monad type and an implementation of the popular Rust crate with the same name - anyhow. anyhow will allow you to never throw another exception again and have a predictable control flow. When errors do arise, you can add context to better understand the situation that led to the errors. See here to jump right into an example.

What Is a Result Monad Type And Why Use it?

If you are not familiar with the Result type, why it is needed, or it's usages, you can read up on all that here: article

The Better Way To Handle Errors With Anyhow

Before anyhow, with a regular Result type, we had no way to know the context around Errs. anyhow fixes this and more! With the anyhow Result type, we can now add any Object as context around errors. To do so, we can use context or withContext (lazily). Either will only have an effect if a Result is the Err subclass. In the following example we will use Strings as the context, but using Exceptions, especially for the root cause is common practice as well.

import 'package:anyhow/anyhow.dart';

void main() {
  print(order("Bob", 1));
}

Result<String> order(String user, int orderNumber) {
  final result = makeFood(orderNumber).context("Could not order for $user.");
  if(result case Ok(:final ok)) {
    return Ok("Order of $ok is complete for $user");
  }
  return result;
}

Result<String> makeFood(int orderNumber) {
  if (orderNumber == 1) {
    return makePizza().context("Order was number $orderNumber.");
  }
  return Ok("pasta");
}

Result<String> makePizza() {
  return bail("Pizza was missing a topping.");
}

Output

Could not order for Bob.

Caused By:
	0: Order was number 1.
	1: Pizza was missing a topping..
	
StackTrace:
#0      AnyhowResultExtensions.context (package:anyhow/src/anyhow/anyhow_extensions.dart:12:29)
#1      order (package:anyhow/test/src/temp.dart:9:40)
#2      main (package:anyhow/example/main.dart:5:9)
... <OMITTED FOR EXAMPLE>

Now we know keep a record of exactly what was happening at each level in the call stack!

What Would This Look Like Without Anyhow

Before anyhow, if we wanted to accomplish something similar with Result, we had to do:

void main() {
  print(order("Bob", 1));
}

Result<String, String> order(String user, int orderNumber) {
  final result = makeFood(orderNumber);
  if(result case Ok(:final ok)) {
    return Ok("Order of $ok is complete for $user");
  }
  Logging.w("Could not order for $user.");
  return result;
}

Result<String, String> makeFood(int orderNumber) {
  if (orderNumber == 1) {
    final result = makePizza();
    if (result.isErr()) {
      Logging.w("Order was number $orderNumber.");
    }
    return result;
  }
  return Ok("pasta");
}

Result<String, String> makePizza() {
  // What is the context around this error??
  return Err("Pizza was missing a topping.");
}

Which is more verbose/error-prone and may not be what we actually want. Since:

We may not want to log anything if the error state is known and can be recovered from
Related logs should be kept together (in the example, other functions could log before this Result had been handled)
We have no way to get the correct stack traces related to the original issue
We have no way to inspect "context", while with anyhow we can iterate through with chain()

Now with anyhow, we are able to better understand and handle errors in an idiomatic way!

Configuration Options

anyhow functionality can be changed by changing:

Error.hasStackTrace;
Error.displayOrder;
Error.stackTraceDisplayFormat;
Error.stackTraceDisplayModifier;

Which is usually done at startup.

hasStackTrace: With Error.hasStackTrace = false, we can exclude capturing a stack trace:

Could not order for Bob.

Caused By:
	0: Order was number 1.
	1: Pizza was missing a topping.

displayFormat: We can view the root cause first with Error.displayOrder = ErrorDisplayOrder.rootFirst

Pizza was missing a topping.

Additional Context:
	0: Order was number 1.
	1: Could not order for Bob.

StackTrace:
#0      bail (package:anyhow/src/anyhow/functions.dart:6:14)
#1      makePizza (package:anyhow/test/src/temp.dart:31:10)
... <OMITTED FOR EXAMPLE>

stackTraceDisplayFormat: if we want to include none, the main, or all stacktraces in the output.
stackTraceDisplayModifier: Modifies the stacktrace during display. Useful for adjusting number of frames to include during display/logging.

Anyhow Result Type vs Rust Result Type

The Result type for this package is just a typedef of the Result type in the rust package. Thus inheriting all capabilities and the two types/packages can be used seamlessly together -

import 'package:anyhow/anyhow.dart' as anyhow;
import 'package:rust/rust.dart';

void main(){
  Result<int,anyhow.Error> x = Ok(1);
  x.context(2);
  anyhow.Result<int> y = Ok(1);
  y.context(2);
  assert(x == y);
  Cell(1); // e.g. `rust` package type
}

import 'package:anyhow/rust.dart'; // overlays anyhow on top of the `rust` package

void main(){
  Result<int> x = Ok(1);
  x.context(1);
  Cell(1); // e.g. `rust` package type
}

Downcasting

Downcasting is the process of getting the inner error from an an anyhow Error.

import 'package:anyhow/anyhow.dart';

void main(){
  Result<int> x = bail("this is an error message");
  Error error = x.unwrapErr();
  assert(error.downcast<String>().unwrap() == "this is an error message");
  assert(error.downcastUnchecked() == "this is an error message"); // or
}

This may be useful when you want to inspect the root error type.

import 'package:anyhow/anyhow.dart';

void main(){
  Result<int> x = bail("this is an error message").context(1);
  final rootInner = x.unwrapErr().rootCause().downcastUnchecked();
  switch(rootInner) {
    case String():
      print("String found");
    default:
      print("Default reached");
  }
}

Since anyhow makes the trade off that you do not care about the underlying causes inner type, which allows your api's to be more composable and concise, downcasting is expected to be used sparingly.