https://docs.ropensci.org/jsonld,
https://github.com/ropensci/jsonld#readme (devel)
https://www.w3.org/TR/json-ld (spec)

Convert R objects to JSON and vice versa
This package is a fork of the RJSONIO package by Duncan Temple
Lang. It builds on the parser from RJSONIO, but implements a different
mapping between R objects and JSON strings. The C code in this package is
mostly from Temple Lang, the R code has been rewritten from scratch. In
addition to drop-in replacements for fromJSON and toJSON, the package has
functions to serialize objects. Furthermore, the package contains a lot of
unit tests to make sure that all edge cases are encoded and decoded
consistently for use with dynamic data in systems and applications.

Jeroen Ooms, Duncan Temple Lang, Jonathan Wallace
This package is a fork of the RJSONIO package by Duncan Temple
Lang. It builds on the same libjson c++ parser, but implements a smarter
mapping between JSON data and R classes. This is particularly useful when
working with JSON data from pipelines and web APIs. The vignettes describe
the behavior in great detail. In addition to drop-in replacements for
toJSON and fromJSON, the package contains functions to validate, prettify
and minify JSON, and many unit tests to verify that all edge cases are
encoded and decoded consistently for use with dynamic data in systems
and applications.
testthat, knitr, reshape2, httr (>= 0.3), RCurl, rjson, plyr

testthat, knitr, reshape2, httr (>= 0.4), rjson, plyr

A Robust, High Performance JSON Parser and Generator for R
Jeroen Ooms, Duncan Temple Lang, Lloyd Hilaiel
A fast JSON parser and generator optimized for statistical data and
the web. Started out as a fork of RJSONIO, but has been completely rewritten
in recent versions. The package offers flexible, robust, high performance
tools for working with JSON in R and is particularly powerful for building
pipelines and interacting with web APIs. The parser implementation is based
on the mapping described in the vignette of the package (Ooms, 2014). In
addition to drop-in replacements for toJSON and fromJSON, jsonlite contains
functions to stream, validate, and prettify JSON data, and many unit
tests to verify that all edge cases are encoded and decoded consistently
for use with dynamic data in systems and applications.