The data templating language
Jsonnet

Language Design

Objectives

JSON has emerged as the defacto standard for communication of structured data, both between machines and at the human / machine boundary. However, in large quantities JSON can be unwieldy for humans, especially when duplication needs to be kept in sync between different parts of the data structure. Many people address this by writing scripts that generate JSON. Typically these are written in general purpose programming languages like Python. However, maintaining these scripts can be non-trivial, especially for persons unfamiliar with the generation code.

Jsonnet attempts to solve this problem in a specialized and principled way. Its design was guided by the following criteria:

Before Jsonnet, no existing configuration languages or DSLs satisfied these criteria. In fact, the vast majority only satisfied one or two of them.

Rationale for extending JSON

Extending JSON means building a language that fits within the existing constraints of the JSON specification. This means that all JSON data must behave the same in Jsonnet (i.e. be emitted unchanged), with additional behaviors being expressible using constructs that would be syntax errors in standard JSON. This choice did constrain us substantially, but we believe it was the correct one for the following reasons:

A common case of Jsonnet program is a program that is mostly JSON data, but has a few occurences of Jsonnet constructs. In this case, someone who knows JSON can maintain the file with no additional knowledge.

Systems that accept JSON can be transparently modified to accept Jsonnet instead by inserting a step to convert the Jsonnet to JSON. If you want to be extra paranoid, you can invoke Jsonnet only if the JSON parsing failed, but this is not actually necessary.

Rationale for Reflection

In JSON, there is an object construct, that is used as a dictionary, and like an object. Jsonnet is the same, the object construct is often used as a dictionary as well as a conventional object. The importance of this is that it is necessary to iterate over dictionaries, test for the existence of a key, and build them with computed key names. These features when applied to objects give us reflection. So naturally, Jsonnet has reflection because objects and dictionaries are the same thing.

Rationale for Turing-Completeness

Jsonnet is Turing-complete as it is possible to write non-terminating programs. Configurations should always terminate, so ideally we would consider non-termination as an error. However, doing so in general is impossible and even when constrained to practical use cases, it remains impractical. Typical approaches for enforcing termination either restrict the language (e.g. to primitive recursion), which makes some programs impossible / difficult to write, or, alternatively, require the programmer to provide evidence that the program terminates, via some sort of annotation / energy function. Both of these make the programmer's life more difficult.

Furthermore, non-Turing complete languages can take arbitrary CPU or RAM by running intensive algorithms on large input. So enforcing termination is firstly not practical, and secondly does not actually solve the more practical problem of bounding resource consumption during execution.

For Jsonnet we decided restricting termination would create more problems than it would solve.

Rationale for Dynamic Typing

Modern type systems typically either require annotations, use type inference, or are dynamic. The former two approaches have two advantages: Firstly, some errors can be detected before execution (some correct programs are also rejected). Secondly, they can be implemented more efficiently since the additional knowledge about the program means special memory representations can be used, and also some instructions can be elided. However annotations are additional bureaucracy and type inference produces unification errors that the programmer has to understand and fix.

Dynamic typing means checking types at run-time, and raising errors via the language's existing error reporting mechanism. This is conceptually much simpler for the programmer. In a configuration language, runtime performance is not as important as it is in most other contexts, and additionally programs tend to execute quickly so testing is much easier. On the other hand, simplicity is of the utmost importance.

For Jsonnet, i.e. in the context of configuration languages, we decided that simplicity was far more important than the minimal benefits of runtime efficiency and static error detection.

Rationale for Object-Oriented Semantics

Object-oriented semantics (specifically, late binding), are ideal for deriving variants from existing data. Most modern object-oriented languages distinguish between classes and objects. Classes can be extended, or instantiated into objects, and objects cannot be extended. However some are prototype-based, where the concept of class and object are essentially fused. In Jsonnet, we decided to use the latter form because JSON does not have classes, and it's simpler to only have one concept instead of two.

Furthermore, we decided to use mixin semantics for inheritance, because they are the most powerful and flexible form of inheritance that has been widely studied, yet remain remarkably simple to define and use.

One novelty is our notion of a first class super construct, which can be used outside the context of field lookup, i.e. super.f. The behavior of super by itself is given formally in the specification. We decided not to restrict super in order to keep the language simple (fewer rules).

Rationale for Pure Functional Semantics

There are ideological and practical reasons for designing Jsonnet as a pure functional language. On the practical side, pure functional languages have no side-effects, which (together with determinism) gives us the property of hermiticity. This allows code to be treated as data, because the code will always evaluate to the same thing, and it is not allowed to make changes to its environment (e.g. by writing to files). By analogy, compressed data can be decompressed on demand because it will always produce the same data and the decompression process doesn't have any external side-effects.

Furthermore, the property of referential transparency essentially extends the hermiticity property into submodules of the Jsonnet code itself, which means that different parts of the code cannot interfere with each other and everything composes in a very predictable way. In a language whose purpose is simply to define data, the property of referential transparency is very natural. Ideologically speaking, functional programming language advocates have always claimed that functional languages are excellent at defining, translating, and refining structured data.

Rationale for Lazy Semantics

The late binding from the object oriented semantics already embodies some of the features of a lazy language. Errors do not occur unless a field is actually dereferenced, and cyclic structures can be created. For example the following is valid even in an eager version of the language:

local x = {a: "apple", b: y.b},
      y = {a: x.a, b: "banana"};
x

It would therefore be confusing if the following was not also valid, which leads us to lazy semantics for arrays.

local x = ["apple", y[1]],
      y = [x[0], "banana"];
x

Therefore, for consistency, the whole language is lazy. It does not harm the language to be lazy: Performance is not significantly affected, stack traces are still possible, and it doesn't interfere with I/O (because there is no I/O). There is also a precedent for laziness, e.g. in Makefiles and the Nix expression language.

Arguably, laziness brings real benefits in terms of abstraction and modularity. It is possible to build infinite data-structures, and there is unrestricted beta expansion. For example, the following 2 snippets of code are only equivalent in a lazy language.

if x == 0 then 0 else if x > 0 then 1 / x else -1/x
local r = 1 / x;
if x == 0 then 0 else if x > 0 then r else -r

Modularity and Encapsulation

In Jsonnet, a module is typically a Jsonnet file that defines an object whose fields contain useful values, such as functions or objects that can specialized for a particular purpose via extension. Using an object at the top level of the module allows adding other fields later on, without having to alter user code. When writing such a module, it is advisable to expose only the interface to the module, and not its implementation. This is called encapsulation, and it allows changing the implementation later, despite the module being imported by many other Jsonnet files.

Jsonnet's primary feature for encapsulation is the local keyword. This makes it possible to define variables that are visible only to the module, and impossible to access from outside. The following is a simple example. Other code can import util.jsonnet but will not be able to see the internal object, and therefore not the function square.

// util.jsonnet
local internal = {
    square(x):: x*x,
};
{
    euclidianDistance(x1, y1, x2, y2)::
        std.sqrt(internal.square(x2-x1) + internal.square(y2-y1)),
}

It is also possible to store square in a field, which exposes it to those importing the module:

// util2.jsonnet
{
    square(x):: x*x,
    euclidianDistance(x1, y1, x2, y2)::
        std.sqrt(self.square(x2-x1) + self.square(y2-y1)),
}

This allows users to redefine the square function, as shown in the very strange code below. In some cases, this is actually what you want and is very useful. But it does make it harder to maintain backwards compatibility. For example, if you later change the implementation of euclidianDistance to inline the square call, then user code will behave differently.

// myfile.jsonnet
local util2 = import "util2.jsonnet" { square(x):: x*x*x };
{
    crazy: util2.euclidianDistance(1,2,3,4)
}

In conclusion, Jsonnet allows you to either expose these details or hide them. So choose wisely, and know that everything you expose will potentially be used in ways that you didn't expect. Keep your interface small if possible.

A common belief is that languages should make local the default state, with an explicit construct to allow outside access. This ensures that things are not accidentally (or apathetically) exposed. In the case of Jsonnet, backwards compatibility with JSON prohibits that design since in JSON everything is a visible field.

Miscellaneous Design Choices

The language is designed to be implementable via desugaring, i.e. there is a simple core language and the other constructs are translated down to this core language before interpretation. This technique allows a language to have considerable expressive power, while remaining easy to implement.

The string formating operator % is defined to behave identically to the Python equivalent, and therefore very similarly to the C printf micro-language. We decided it would be better to be compatible with an existing popular language wherever possible, and Python is often the language of choice for operations people.

In contrast, we decided that the semantics of Python's and and or operators, which have meaning beyond boolean logic, might be confusing for those without prior knowledge of such languages. We therefore implemented the simpler && and || operators, which only operate on booleans. More complex cases can be implemented explicitly with the if construct.