Strict JSON Encoding and Decoding via the 'Jansson' C Library

An R-safe profile of RFC 8259 JSON: parsing and generation backed by the 'Jansson' C library, linked as a system library where one is available and compiled from the bundled sources otherwise. The parser rejects, with classed conditions carrying line, column, and byte position: malformed or truncated input, trailing content, duplicate object keys at any depth, invalid UTF-8, escapes encoding a null character, reals overflowing double, and integer literals whose magnitude exceeds 2^53, the range within which a double represents every integer exactly. Number literals with a fraction or exponent convert by ordinary correctly rounded IEEE 754 double conversion. Objects decode to named lists in key order, arrays to unnamed lists, and scalars to length-one vectors. The encoder maps named lists to objects in insertion order, unnamed lists to arrays, guarantees that every finite double, signed zero included, round-trips to the exact same value (whole-number doubles are written as integers), and refuses values with no faithful JSON representation (NA, NaN, infinities, named atomic vectors, classed objects) instead of guessing. No R package dependencies.

janssonr

Strict JSON for R, backed by the system Jansson C library. Two functions, zero R package dependencies.

janssonr implements an R-safe profile of RFC 8259: RFC-conformant parsing plus deliberate extra restrictions, so that what parses maps predictably into R (integer literals are exact or refused; fraction and exponent literals convert by correctly rounded double conversion) and everything that encodes is faithfully representable in JSON. Where common JSON packages guess (collapse duplicate keys, truncate at NUL, round big integers, stringify NA), janssonr refuses with a classed condition.

API

from_json('{"a": 1, "b": [true, null]}')
#> $a
#> [1] 1
#> $b
#> $b[[1]]
#> [1] TRUE
#> $b[[2]]
#> NULL

to_json(list(a = 1L, b = list(TRUE, NULL)))
#> [1] "{\"a\":1,\"b\":[true,null]}"

Decoding (from_json)

Rejected, with a classed error: malformed or truncated input, trailing content, duplicate object keys at any depth (including inside arrays of objects), invalid UTF-8, lone surrogates, NUL escapes, reals overflowing double (1e999), and integer literals whose magnitude exceeds 2^53 (no silent precision loss). The exactness guarantee is for integer literals; number literals with a fraction or exponent (1.5, 9007199254740993.0, 1e-999) use ordinary correctly rounded double conversion, so writing .0 after a big integer opts into rounding. Nesting is capped at 1024 containers by janssonr's own guard, independent of the Jansson version underneath.

Errors carry line, column, position (byte offset) and a stable code string ("duplicate_key", "invalid_utf8", ...). Every integer-form refusal is "integer_precision": detected post-parse (beyond 2^53) it carries an RFC 6901 JSON Pointer in path; detected by the lexer (beyond the parser's integer range) it carries source coordinates instead. "numeric_overflow" is real-form overflow (1e999).

Encoding (to_json)

Output is compact UTF-8, one line, no trailing newline. Refused: NA of any type, NaN, infinities, named atomic vectors, attributes other than list names, classed objects, duplicate or NA keys, raw, complex, functions, environments.

The double round-trip guarantee is value-level: from_json(to_json(x)) returns bit-identical doubles, but the lexical spelling of non-integral reals may differ across Jansson versions.

Conditions

janssonr_input_error   janssonr_parse_error   janssonr_encode_error
                \             |                   /
                 \____ janssonr_error ___________/

Installation

janssonr needs R >= 4.4. On Unix it links against the system Jansson library when a usable one is found, version 2.11 or newer (Ubuntu 20.04+, Debian bullseye+, RHEL 8+, current Homebrew all qualify):

# Debian/Ubuntu          # Fedora/RHEL              # macOS
apt install libjansson-dev   dnf install jansson-devel   brew install jansson

Without a system jansson the package compiles its bundled Jansson 2.15.1 sources (src/jansson/) instead, so installation works on machines with no jansson at all — CRAN's macOS builders among them. Install the development package first if you want the system-linked build. On Windows the bundled copy is always used: Rtools carries no jansson.

remotes::install_github("cornball-ai/janssonr")

From the apt repository (Ubuntu, binary)

Prebuilt r-cornball-janssonr packages are published to https://cornball-ai.github.io/janssonr/, built per suite so a client is never offered a build compiled against a different release. The repository is unsigned, so clients opt in with Trusted: yes. Set Suites to your release codename, noble for 24.04:

sudo tee /etc/apt/sources.list.d/janssonr.sources > /dev/null <<'EOF'
Types: deb
URIs: https://cornball-ai.github.io/janssonr
Suites: noble
Components: main
Trusted: yes
Enabled: yes
EOF

sudo apt update
sudo apt install r-cornball-janssonr

The binary needs only the Jansson runtime (libjansson4), never a compiler or libjansson-dev. Debs are built by tools/build-deb.sh and indexed by tools/mkrepo.sh (rapt's repository pattern).

Design notes

• All Jansson interaction sits behind one internal C seam (src/jr_jansson.h); Unix links the system library when one is present, the bundled copy compiles in otherwise (and always on Windows), and the package code cannot tell the difference. The real-number contract stays value-level partly because the two linkages may carry different Jansson versions.
• No memory is leaked when R errors mid-conversion: the Jansson tree is owned by an external pointer finalizer from the moment it exists.
• janssonr never touches Jansson's process-global state (json_object_seed(), json_set_alloc_funcs()), which is shared with any other in-process user of libjansson.