Sjsonnet

A JVM implementation of the Jsonnet configuration language.

Usage

Sjsonnet can be used from Java:

<dependency>
    <groupId>com.lihaoyi</groupId>
    <artifactId>sjsonnet_2.13</artifactId>
    <version>0.2.6</version>
</dependency>
sjsonnet.SjsonnetMain.main0(
    new String[]{"foo.jsonnet"},
    sjsonnet.SjsonnetMain.createParseCache(),
    System.in,
    System.out,
    System.err,
    os.package$.MODULE$.pwd(),
    scala.None$.empty()
);

From Scala:

"com.lihaoyi" %% "sjsonnet" % "0.2.6" // SBT
ivy"com.lihaoyi::sjsonnet:0.2.6" // Mill
sjsonnet.SjsonnetMain.main0(
    Array("foo.jsonnet"),
    sjsonnet.SjsonnetMain.createParseCache(),
    System.in,
    System.out,
    System.err,
    os.pwd, // working directory
    None
);

As a standalone executable assembly:

$ curl -L https://github.com/lihaoyi/sjsonnet/releases/download/0.2.6/sjsonnet.jar > sjsonnet.jar

$ chmod +x sjsonnet.jar

$ ./sjsonnet.jar
error: Need to pass in a jsonnet file to evaluate
usage: sjsonnet [sjsonnet-options] script-file

  -i, --interactive  Run Mill in interactive mode, suitable for opening REPLs and taking user input
  -n, --indent       How much to indent your output JSON
  -J, --jpath        Specify an additional library search dir (right-most wins)
  -o, --output-file  Write to the output file rather than stdout
  ...

$ ./sjsonnet.jar foo.jsonnet

Or from Javascript:

$ curl -L https://github.com/databricks/sjsonnet/releases/download/0.2.6/sjsonnet.js > sjsonnet.js

$ node

> require("./sjsonnet.js")

> SjsonnetMain.interpret("local f = function(x) x * x; f(11)", {}, {}, "", (wd, imported) => null)
121

> SjsonnetMain.interpret(
    "local f = import 'foo'; f + 'bar'", // code
    {}, // extVars
    {}, // tlaVars
    "", // initial working directory

    // import callback: receives a base directory and the imported path string,
    // returns a 2-element array of the resolved file path and file contents
    (wd, imported) => [wd + "/" + imported, "local bar = 123; bar + bar"])
'246bar'

Note that since Javascript does not necessarily have access to the filesystem, you have to provide an explicit import callback that you can use to resolve imports yourself (whether through Node's fs module, or by emulating a filesystem in-memory)

Running deeply recursive Jsonnet programs

The depth of recursion is limited by JVM stack size. You can run Sjsonnet with increased stack size as follows:

java -Xss100m -cp sjsonnet.jar sjsonnet.SjsonnetMain foo.jsonnet

The -Xss option above is responsible for JVM stack size. Please try this if you ever run into sjsonnet.Error: Internal Error ... Caused by: java.lang.StackOverflowError ....

There is no analog of --max-stack/-s option of google/jsonnet. The only stack size limit is the one of the JVM.

Architecture

Sjsonnet is implementated as a straightforward AST interpreter. There are roughly 3 phases:

  • sjsonnet.Parser: parses an input String into a sjsonnet.Expr, which is a Syntax Tree representing the Jsonnet document syntax, using the Fastparse parsing library

  • sjsonnet.Evaluator: recurses over the sjsonnet.Expr and converts it into a sjsonnet.Val, a data structure representing the Jsonnet runtime values (basically lazy JSON which can contain function values).

  • sjsonnet.Materializer: recurses over the sjsonnet.Val and converts it into an output ujson.Expr: a non-lazy JSON structure without any remaining un-evaluated function values. This can be serialized to a string formatted in a variety of ways

These three phases are encapsulated in the sjsonnet.Interpreter object.

Some notes on the values used in parts of the pipeline:

  • sjsonnet.Expr: this represents {...} object literal nodes, a + b binary operation nodes, function(a) {...} definitions and f(a) invocations, etc.. Also keeps track of source-offset information so failures can be correlated with line numbers.

  • sjsonnet.Val: essentially the JSON structure (objects, arrays, primitives) but with two modifications. The first is that functions like function(a){...} can still be present in the structure: in Jsonnet you can pass around functions as values and call then later on. The second is that and object values & array entries are lazy: e.g. [error 123, 456][1] does not raise an error because the first (erroneous) entry of the array is un-used and thus not evaluated.

Performance

Due to pervasive caching, sjsonnet is much faster than google/jsonnet. See this blog post for more details:

Here's the latest set of benchmarks I've run comparing Sjsonnet against google/jsonnet and google/go-jsonnet, measuring the time taken to
evaluate the test_suite/ folder (smaller is better):

Sjsonnet 0.1.5 Sjsonnet 0.1.6
Scala 2.13.0 14.26ms ± 0.22 6.59ms ± 0.27
Scala 2.12.8 18.07ms ± 0.30 9.29ms ± 0.26
google/jsonnet google/go-jsonnet
~1277ms ~274ms

google/jsonnet was built from source on commit f59758d1904bccda99598990f582dd2e1e9ad263, while google/go-jsonnet was go geted at version v0.13.0. You can see the source code of the benchmark in
SjsonnetTestMain.scala

Laziness

The Jsonnet language is lazy: expressions don't get evaluated unless their value is needed, and thus even erroneous expressions do not cause a failure if un-used. This is represented in the Sjsonnet codebase by sjsonnet.Val.Lazy: a wrapper type that encapsulates an arbitrary computation that returns a sjsonnet.Val.

sjsonnet.Val.Lazy is used in several places, representing where laziness is present in the language:

  • Inside sjsonnet.Scope, representing local variable name bindings

  • Inside sjsonnet.Val.Arr, representing the contents of array cells

  • Inside sjsonnet.Val.Obj, representing the contents of object values

Unlike google/jsonnet, Sjsonnet caches the results of lazy computations the first time they are evaluated, avoiding wasteful re-computation when a value is used more than once.

Standard Library

Different from google/jsonnet, Sjsonnet does not implement the Jsonnet standard library std in Jsonnet code. Rather, those functions are implemented as intrinsics directly in the host language (in Std.scala). This allows both better error messages when the input types are wrong, as well as better performance for the more computationally-intense builtin functions.

Client-Server

Sjsonnet comes with a built in thin-client and background server, to help mitigate the unfortunate JVM warmup overhead that adds ~1s to every invocation down to 0.2-0.3s. For the simple non-client-server executable, you can use

./mill show sjsonnet[2.13.0].assembly

To create the executable. For the client-server executable, you can use

./mill show sjsonnet[2.13.0].server.assembly

By default, the Sjsonnet background server lives in ~/.sjsonnet, and lasts 5 minutes before shutting itself when inactive.

Since the Sjsonnet client still has 0.2-0.3s of overhead, if using Sjsonnet heavily it is still better to include it in your JVM classpath and invoke it programmatically via new Interpreter(...).interpret(...).

Publishing

To publish, run the following commands:

./mill -i mill.scalalib.PublishModule/publishAll lihaoyi:$SONATYPE_PASSWORD $GPG_PASSWORD --publishArtifacts __.publishArtifacts --release true

./mill -i show sjsonnet[2.13.0].js.fullOpt
./mill -i show sjsonnet[2.13.0].jvm.assembly

Changelog

0.2.6

  • Add std.flatMap, std.repeat, std.clamp, std.member, std.stripChars, std.rstripChars, std.lstripChars

0.2.4

  • Add support for syntactical key ordering #53
  • Bump dependency versions

0.2.2

  • Bump verion of Scalatags, uPickle

0.1.9

  • Bump version of FastParse

0.1.8

  • Bump versions of OS-Lib, uJson, Scalatags

0.1.7

  • Support std lib methods that take a key lambda #40
  • Handle hex in unicode escaoes #41
  • Add encodeUTF8, decodeUTF8 std lib methdos #42
  • Properly fail on non-boolean conditionals #44
  • Support YAML-steam output #45

0.1.6

  • ~2x performance increase

0.1.5

  • Javascript support, allowing Sjsonnet to be used in the browser or on Node.js
  • Performance improvements

0.1.4

  • Scala 2.13 support
  • Performance improvements

0.1.3

  • Add std.mod, std.min and std.max
  • Performance improvements

0.1.2

  • Improvements to error reporting when types do not match

0.1.1

  • Performance improvements to the parser via upgrading to Fastparse 2.x

0.1.0

  • First release