add 2nd XForm validator option

[MartijnR]

I'm in the process of building an Enketo-form-engine-based nodeJS XForm validator. I hope you will agree this will nicely enrich the ODK ecosystem (assuming it's done fairly well). I think the command line interface will be identical to ODK Validate.

The main reason for this project is that some adopters use customized Enketo engines with custom XPath functions (and they'll be using a custom version of the new validator).

This issue is to discuss adding a 2nd validator option to pyxform with a configuration item to swap from ODK Validate to the new validator. The default would remain ODK Validate.

Please let me know if you any ideas, comments, concerns!

[lognaturel]

Could warning rather than throwing an error on missing function address the problem at least in the short term? getodk/validate#35 It seems this could be a simpler approach though it does have the downside of pushing the validation of actual function names to the client.

[MartijnR]

Thanks for bringing that up. That development would help some people with being able to at least use custom functions.

However, one difficulty is with training users that some 'function not supported' warnings are okay and others are not. They also would miss out on parameter checks for those function calls.

I think, generally it is better to output an error for the use of unsupported XPath functions (if the validator kept up-to-date with the spec).

[MartijnR]

Here's a web interface around the new validator (and also ODK validate): https://validate.enketo.org/. Comments welcome here: https://github.com/enketo/enketo-validate/issues