JSON::Jsonnet

Perl XS bindings to libjsonnet (Google Jsonnet C/C++ API).

This module provides a thin, low-level interface to the official Jsonnet VM. You can evaluate Jsonnet snippets/files, use multi/stream outputs, and register custom import and native callbacks from Perl.

Requirements

This distribution requires libjsonnet (development headers and shared library).

You need:

• libjsonnet.h
• libjsonnet.so (or equivalent)
• C++ standard library for linking (libstdc++)

Jsonnet version 0.21.x or newer is recommended.

Debian / Ubuntu

sudo apt install libjsonnet-dev jsonnet

Fedora

sudo dnf install jsonnet jsonnet-devel

Arch

sudo pacman -S jsonnet

macOS (Homebrew)

brew install jsonnet

From source (if packages are unavailable)

git clone https://github.com/google/jsonnet.git
cd jsonnet
make libjsonnet.so
sudo cp include/libjsonnet.h /usr/local/include/
sudo cp libjsonnet.so /usr/local/lib/
sudo ldconfig   # on Linux

Installation

Standard CPAN install

cpanm JSON::Jsonnet

If libjsonnet is in a non-standard prefix

Set JSONNET_PREFIX so the build can find headers and library:

JSONNET_PREFIX=/opt/jsonnet cpanm JSON::Jsonnet

or when building from the repo:

JSONNET_PREFIX=/opt/jsonnet perl Makefile.PL
make
make test
make install

The build will also try pkg-config libjsonnet automatically when available.

Usage

Basic evaluation

use JSON::Jsonnet;

my $vm = JSON::Jsonnet->new(
    ext_vars => { foo => "bar" },
);

my $json = $vm->evaluate_snippet("snippet", '{ x: std.extVar("foo") }');
print $json;

Evaluate a file

my $json = $vm->evaluate_file("main.jsonnet");
print $json;

Multi output

Multi mode returns a hashref where keys are output filenames:

my $out = $vm->evaluate_file_multi("multi.jsonnet");

for my $name (sort keys %$out) {
    print "== $name ==\n";
    print $out->{$name};
}

Stream output

Stream mode returns an arrayref of JSON texts:

my $items = $vm->evaluate_snippet_stream("s", '[{a:1},{b:2}]');

for my $j (@$items) {
    print $j;
}

Import callback

Register a custom importer. The callback must return:

• ($found_here, $content) on success
• (undef, $error_text) on failure

$vm->import_callback(sub {
    my ($base, $rel) = @_;

    # Example: virtual import
    return ("virtual.jsonnet", '{ z: 42 }')
        if $rel eq "virtual.jsonnet";

    return (undef, "no such import: $rel");
});

my $json = $vm->evaluate_snippet("x", q'
local v = import "virtual.jsonnet";
{ z: v.z }
');
print $json;

Note: once you set import_callback, it replaces the default importer. If you want filesystem imports too, implement them in your callback.

Native callbacks (std.native)

$vm->native_callback(
    "add",
    sub { my ($a, $b) = @_; $a + $b },
    [qw(a b)],
);

print $vm->evaluate_snippet("n", 'std.native("add")(2, 3)');

Native callbacks accept primitive Jsonnet values (string/number/bool/null). Return values can be scalars, arrayrefs, hashrefs, or undef.

VM Options

You can set tuning parameters either via constructor or later:

my $vm = JSON::Jsonnet->new(
    max_stack         => 1000,
    gc_min_objects    => 10,
    gc_growth_trigger => 2.5,
    max_trace         => 20,
    string_output     => 0,
);

$vm->jpath_add("./libsonnet");
$vm->ext_var("env", "prod");
$vm->tla_var("cfg", "value");

string_output => 1 makes top-level Jsonnet strings return without JSON quotes.

Troubleshooting

undefined symbol: jsonnet_make when loading the module

Your Jsonnet.so was built without linking to libjsonnet.

Fix by installing libjsonnet-dev or rebuilding with a correct prefix:

JSONNET_PREFIX=/path/to/jsonnet perl Makefile.PL
make clean
make
make test

Build cannot find libjsonnet.h

Install the development package (libjsonnet-dev, jsonnet-devel) or provide JSONNET_PREFIX.

Import errors after setting import_callback

Setting an import callback replaces default imports. Handle filesystem imports yourself if needed.

License

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Author

Sergey Kovalev info@neolite.ru