Handle unmarshalling errors better

[markphelps]

It would be great if we could handle unmarshalling errors more gracefully during evaluation.

Example

Ideas

1. Catch unmarshalling errors and return a nicer error that informs the user that values must be strings
2. Infer "true" from true and 'autobox' the type into a string

[markphelps]

This is handled by grpc-gateway, so we should implement our own error handler per: https://mycodesmells.com/post/grpc-gateway-error-handler

https://grpc-ecosystem.github.io/grpc-gateway/docs/customizingyourgateway.html

[aaronraff]

I can take a a look into this one. I'm leaning towards starting with idea 1, if that sounds like a good plan to you.

[aaronraff]

I started taking a look into this over the weekend. I took a look at the blog post that you linked in this issue. After implementing that, this is the error that is made available to us through the custom handler: rpc error: code = InvalidArgument desc = json: cannot unmarshal bool into Go value of type string.

It seems like we would probably need to add some custom logic when decoding the json (https://golang.org/pkg/encoding/json/#Decoder). Is this desired, or did you have a simpler solution in mind for this? Maybe I am missing something here.

[markphelps]

Thanks for taking a look @aaronraff!

My initial thoughts were to be able to return something like context values must be strings or something similar if we failed to unmarshal those values, however after thinking about it some more, it may not be worth the trouble at the moment to implement a custom decoder just to do that.

What do you think?

[aaronraff]

I don't mind trying it out, but if there are other issues that you feel are more important, I can jump on one of those.

Sounds like a cool idea for a library though!

[markphelps]

I think im gonna close this since it's not really critical. There's another issue that I could use your help on re the automatic 'slugifying' of flag/segment keys from the name that was added in #155. I'll create an issue for that