Feature/3176 json

[mitzimorris]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

Add callback writer to output JSON data.

Intended Effect

Allow structured outputs in JSON format, per design-docs spec https://github.com/stan-dev/design-docs/blob/master/designs/0032-stan-output-formats.md

How to Verify

Unit tests

Side Effects

None

Documentation

N/A

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Coumbia University

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[mitzimorris]

OK, ready for re-review

[mitzimorris]

looking at write_vector code - if a vector is empty, we should output the empty vector, no? current implementation doesn't.

[mitzimorris]

ready for re-re-review

[WardBrian]

Three small things but otherwise I think this is ready. New tests all look great.

[bob-carpenter]

The efficient way to do this if you know the instantiations at compile time would be with the CRTP. But I wouldn't do that if the virtual function calls aren't a bottleneck.

[SteveBronder]

Actually @mitzimorris and @WardBrian one last thing, how should NA and infinite values get handled here? I'm unfamiliar with how json should handle these.

[WardBrian]

We print them such that the JSON parser we use can read them back in. This involves using some non-standard atoms, since the JSON spec provides no proper format for these:

Value	""JSON""
Not-a-number	NaN
Infinity	Inf
Negative infinity	-Inf

[mitzimorris]

OK to merge?

[WardBrian]

I think so, @SteveBronder?

[SteveBronder]

Yep good to merge!