Sjsonnet

A JVM implementation of the Jsonnet configuration language.

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.

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(...).