type_to_form returns non-JSON string in case of error

[agoose77]

Version of Awkward Array

main

Description and code to reproduce

When using the RDataFrame integration, users attempting to work with unsupported column types see a cryptic error message. This occurs because type_to_form returns the string "unsupported type" for unsupported types, which then triggers an exception when Awkward attempts to call json.loads on the generated form.

Could we use std::variant here to catch the error case?

[agoose77]

@jpivarski is this next-release? It's a new feature, with a hard-to-decode error message for unsupported use, so I'm 50-50 on it

[jpivarski]

No, this issue is not about changing the API of the RDataFrame bridge, it's about fixing a bug.

The touchstone is, "Could someone's workflow start to depend on this, and then they'd have to adjust when we fix it?" I think it's pretty clearly "no" in this case.

[agoose77]

🥳 less work! I'm all about the principle of least action.

For clarity; I was trying to gauge our pain threshold for the first release.