Unwrap Error Message from Guard about decoders

[arb]

Hey @nvie thanks for the prompt and thorough reply! Upgrading doesn't seem like it's in the cards at the moment regrettably. I ended up doing something like this

function tryDecoderGuard<D extends d.Decoder<any>>(
  decoder: D,
  value: unknown,
): [Error, null] | [null, d.DecoderType<D>] {
  let decodedValue = null;
  let error = null;
  try {
    decodedValue = d.guard(decoder)(value);
  } catch (e: any) {
    error = e;
  }
  return [error, decodedValue];
}

So that way, we return a tuple with a decoded and typed value OR a guard error. The caller then can refine on that tuple and execute additional logic. I'm using this code in an Express server and going down the .map route would have been difficult to prevent multiple res.send from happening on a given request.

[nvie]

Note upfront: This is more complicated than it had to be, which is why I simplified it a lot in 2.0. To learn how to adopt from the old to the new APIs, see this migration guide. I would recommend it, because it also gets rid of the complexity incurred by the lemons and debrief dependencies. 🎉

---

To answer your question: Before 2.0, you can use use the debrief library to serialize it directly. This is what the guard internally uses too (it's already a dependency).

Here's the relevant code you could copy:

decoders/src/guard.js

Lines 29 to 35 in a396623

	decoder(blob)
	.mapError((annotation) => {
	const err = new Error('\n' + serializer(annotation));
	err.name = 'Decoding error';
	return err;
	})
	.unwrap();

Where blob is your input.

Note that there are two ways to turn an Annotation into a textual error message:

1. Using import { serialize } from 'debrief, then calling serialize(annotation). This will echo back the original input, and "inline" the error message in a human-readable form. Example of a possible returned multi-line string:

   {
     "foo": "bar",
            ^^^^^ Must be integer
   }
   

2. Using import { summarize } from 'debrief, then calling summarize(annotation). This will return a list-of-strings, each containing the "path" into the object, and the error reason, without echoing back. Less human-friendly, but won't echo back any sensitive data. Example of returned array of strings:

   ['Value at key "foo": Must be integer']
   

Hope that helps!