Sjsonnet
A JVM implementation of the Jsonnet configuration language.
Usage
Sjsonnet can be used from Java:
<dependency>
<groupId>com.lihaoyi</groupId>
<artifactId>sjsonnet_2.12</artifactId>
<version>0.1.3</version>
</dependency>
sjsonnet.SjsonnetMain.main0(
new String[]{"foo.jsonnet"},
sjsonnet.SjsonnetMain.createParseCache(),
System.in,
System.out,
System.err,
os.package$.MODULE$.pwd()
);
From Scala:
"com.lihaoyi" %% "sjsonnet" % "0.1.3" // SBT
ivy"com.lihaoyi::sjsonnet:0.1.3" // Mill
sjsonnet.SjsonnetMain.main0(
Array("foo.jsonnet"),
sjsonnet.SjsonnetMain.createParseCache(),
System.in,
System.out,
System.err,
os.pwd // working directory
);
Or as a standalone executable assembly:
$ curl -L https://github.com/lihaoyi/sjsonnet/releases/download/0.1.3/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
Architecture
Sjsonnet is implementated as a straightforward AST interpreter. There are roughly 3 phases:
-
sjsonnet.Parser
: parses an inputString
into asjsonnet.Expr
, which is a Syntax Tree representing the Jsonnet document syntax, using the Fastparse parsing library -
sjsonnet.Evaluator
: recurses over thesjsonnet.Expr
and converts it into asjsonnet.Val
, a data structure representing the Jsonnet runtime values (basically lazy JSON which can contain function values). -
sjsonnet.Materializer
: recurses over thesjsonnet.Val
and converts it into an outputujson.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 andf(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 likefunction(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
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.Lazy
: a
wrapper type that encapsulates an arbitrary computation that returns a
sjsonnet.Val
.
sjsonnet.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 build 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.assembly
To create the executable. For the client-server executable, you can use
mill show sjsonnet.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(...)
.