Add Weakguard to decoders library

[sfarthin]

When adding decoders to an already functioning production environment, many times it is helpful to use a weakGuard temporarily. Rather than throwing an exception it would console.error the message (or maybe an option for a custom action?). This way the code can continue to function like it has, but the type can be refined as more variances come in. Eventually the weakGuard can be replaced with guard once one is confident the type is correct.

[nvie]

Yeah, great idea! It will need some good documentation, and configurability, so it may take a bit to do it well. This is an area where you can shoot yourself in the foot.

---

I'm thinking of extending the current Guard API so you can provide an optional callback (called whenever a guard would throw an error), which would allow you to both log/inspect, and handle the error in your preferred way, and opt-in to continuing or keep-failing.

const verify: Guard<Something> = guard(myDecoder, { onError: myCustomCallback });

Since all projects have different preferences, I don't think I can make it more specific than this generic callback structure.

The callback would receive:

• error: Error - The Decoding Error instance (whatever error the guard throws today, literally)
• originalBlob: mixed - The original blob value that failed to decode, for further inspection, or logging, or modification

It would have to return:

• An explicit fallback value (of type T)

---

You could then use it to build your own project-specific "weak guard":

function weakGuard<T>(decoder: Decoder<T>): Guard<T> {
  return guard(decoder, {
    onError: (error: Error, originalBlob: mixed): T => {
      if (process.env.NODE_ENV === 'production') {
        // Report the error to Sentry
        sentry.withScope((scope) => {
          scope.setExtra('blob', originalBlob);
          sentry.captureException(error);
        });

        // But let's allow unexpected inputs through, sacrificing correctness
        // $FlowFixMe - deliberate force-cast here
        return ((originalBlob: any): T);
      } else {
        // Keep failing in development mode
        throw error;
      }
    },
  });
}

---

All of the logic inside of the onError handler is inherently project-specific though. But I could provide this snippet as a good example for how to build something like this.

[sfarthin]

That looks great 💯

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