Multiple errors for records?

[f-f]

This PR is not for merging - I am working on purescript/spago#1090, and noticed that since we have a list of errors when decoding, it feels like records should parse "in parallel", i.e. the code should try to parse all the keys and accumulate errors.

It currently doesn't, and bails out as soon as something errors (as demonstrated by the test, which should complain about n and m), and I guess this is a consequence of defining record sequentially with recordProp - @garyb do you see any way we could get multiple errors when decoding records?

[garyb]

I gave this some thought but nothing easy is springing to mind. Assuming the recordProp codec was reworked to not just bind on decoding codecR and instead the errors were captured and combined, I don't think the error type combines things in a helpful way there, so yeah the sequential nature is a problem.

I did try figuring out a more "parallel" formulation for object/prop codecs when putting this version of the library together, but I couldn't come up with anything that worked out. I even tried dropping the Codec underpinning but that made other things worse. I feel like it should be possible though.

[f-f]

Something that I think is related is how the paths in a nested record error are appended. I printed one of the failures from Spago:

 {
  "causes": [
    {
      "causes": [
        {
          "causes": [],
          "message": "No value found",
          "path": "$.package.publish.license"
        }
      ],
      "message": "Could not decode PublishConfig",
      "path": "$.package.publish.license"
    }
  ],
  "message": "Could not decode PackageConfig",
  "path": "$.package.publish.license"
}

It's strange to me that the path in the outermost error has the same path of the innermost error, I'd expect them to look like this:

 {
  "causes": [
    {
      "causes": [
        {
          "causes": [],
          "message": "No value found",
          "path": "$.package.publish.license"
        }
      ],
      "message": "Could not decode PublishConfig",
      "path": "$.package.publish"
    }
  ],
  "message": "Could not decode PackageConfig",
  "path": "$.package"
}

I couldn't find a way to change withPath to do this though, presumably because this is all top-down.
In any case, this is no big deal in a world where decoding a record yields at most one error - though once you have different keys produce different errors, then lifting the path of the innermost error all the way up doesn't make sense anymore.