22 October 2020 — by Yann Hamdaoui
Nickel: better configuration for less

We are making the Nickel repository public. Nickel is an experimental configuration language developed at Tweag. While this is not the time for the first release yet, it is an occasion to talk about this project. The goal of this post is to give a high-level overview of the project. If your curiosity is tickled but you are left wanting to learn more, fear not, as we will publish more blog posts on specific aspects of the language in the future. But for now, let’s have a tour!

[Disclaimer: the actual syntax of Nickel being still worked on, I’m freely using as-of-yet non-existing syntax for illustrative purposes. The underlying features are however already supported.]

The inception

We, at Tweag, are avid users of the Nix package manager. As it happens, the configuration language for Nix (also called Nix) is a pretty good configuration language, and would be applicable to many more things than just package management.

All in all, the Nix language is a lazy JSON with functions. It is simple yet powerful. It is used to generate Nix’s package descriptions but would be well suited to write any kind of configuration (Terraform, Kubernetes, etc…).

The rub is that the interpreter for Nix-the-language is tightly coupled with Nix-the-package manager. So, as it stands, using the Nix language for anything else than package management is a rather painful exercise.

Nickel is our attempt at answering the question: what would Nix-the-language look like if it was split from the package manager? While taking the opportunity to improve the language a little, building on the experience of the Nix community over the years.

What’s Nickel, exactly ?

Nickel is a lightweight generic configuration language. In that it can replace YAML as your application’s configuration language. Unlike YAML, though, it anticipates large configurations by being programmable. Another way to use Nickel is to generate static configuration files — e.g. in JSON, YAML — that are then fed to another system. Like Nix, it is designed to have a simple, well-understood core: at its heart, it is JSON with functions.

But past experience with Nix also brings some insights on which aspects of the language could be improved. Whatever the initial scope of a language is, it will almost surely be used in a way that deviates from the original plan: you create a configuration language to describe software packages, and next thing you know, somebody needs to implement a topological sort.

Nickel strives to retain the simplicity of Nix, while extending it according to this feedback. Though, you can do perfectly fine without the new features and just write Nix-like code.

Yet another configuration language

At this point you’re probably wondering if this hasn’t already been done elsewhere. It seems that more and more languages are born every day, and surely there already exist configuration languages with a similar purpose to Nickel: Starlark, Jsonnet, Dhall or CUE, to name a few. So why Nickel?


Perhaps the most important difference with other configuration languages is Nickel’s approach to typing.

Some languages, such as Jsonnet or Starlark, are not statically typed. Indeed, static types can be seen as superflous in a configuration language: if your program is only run once on fixed inputs, any type error will be reported at run-time anyway. Why bother with a static type system?

On the other hand, more and more systems rely on complex configurations, such as cloud infrastructure (Terraform, Kubernetes or NixOps), leading the corresponding programs to become increasingly complex, to the point where static types are beneficial. For reusable code — that is, library functions — static types add structure, serve as documentation, and eliminate bugs early.

Although less common, some configuration languages are statically typed, including Dhall and CUE.

Dhall features a powerful type system that is able to type a wide range of idioms. But it is complex, requiring some experience to become fluent in.

CUE is closer to what we are striving for. It has an optional and well-behaved type system with strong guarantees. In exchange for which, one can’t write nor type higher-order functions in general, even if some simple functions are possible to encode.

Gradual typing

Nickel, features a gradual type system. Gradual types are unobtrusive: they make it possible to statically type reusable parts of your programs, but you are still free to write configurations without any types. The interpreter safely handles the interaction between the typed and untyped worlds.

Concretely, typed library code like this:

// file: mylib.ncl
  numToStr : Num -> Str = fun n => ...;
  makeURL : Str -> Str -> Num -> Str = fun proto host port =>
    "${proto}://${host}:${numToStr port}/";

can coexist with untyped configuration code like this:

// file: server.ncl
let mylib = import "mylib.ncl" in
let host = "myproject.com" in
  host = host;
  port = 1;
  urls = [
    mylib.makeURL "myproto" host port,
    {protocol = "proto2"; server = "sndserver.net"; port = 4242}

In the first snippet, the body of numToStr and makeURL are statically checked: wrongfully calling numToStr proto inside makeURL would raise an error even if makeURL is never used. On the other hand, the second snippet is not annotated, and thus not statically checked. In particular, we mix an URL represented as a string together with one represented as a record in the same list. The interpreter rather inserts run-time checks, or contracts, such that if makeURL is misused then the program fails with an appropriate error.

Gradual types also lets us keep the type system simple: even in statically typed code if you want to write a component that the type checker doesn’t know how to verify, you don’t have to type-check that part.


Complementary to the static type system, Nickel offers contracts. Contracts offer precise and accurate dynamic type error reporting, even in the presence of function types. Contracts are used internally by Nickel’s interpreter to insert guards at the boundary between typed and untyped chunks. Contracts are available to the programmer as well, to give them the ability to enforce type assertions at run-time in a simple way.

One pleasant consequence of this design is that the exposure of the user to the type system can be progressive:

  • Users writing configurations can just write Nix-like code while ignoring (almost) everything about typing, since you can seamlessly call a typed function from untyped code.
  • Users writing consumers or verifiers of these configurations would use contracts to model data schemas.
  • Users writing libraries would instead use the static type system.

An example of contract is given in the next section.


While the basic computational blocks are functions, the basic data blocks in Nickel are records (or objects in JSON). Nickel supports writing self-documenting record schemas, such as:

  host | type: Str
       | description: "The host name of the server."
       | default: "fallback.myserver.net"

  port | type: Num
       | description: "The port of the connection."
       | default: 4242

  url | type: Url
      | description: "The host name of the server."

Each field can contain metadata, such as a description or default value. These aim at being displayed in documentation, or queried by tools.

The schema can then be used as a contract. Imagine that a function has swapped two values in its output and returns:

  host = "myproject.com",
  port = "myproto://myproject.com:1/",
  url = 1

Without types, this is hard to catch. Surely, an error will eventually pop up downstream in the pipeline, but how and when? Using the schema above will make sure that, whenever the fields are actually evaluated, the function will be blamed in the type error.

Schemas are actually part of a bigger story involving merging records together, which, in particular, lets the schema instantiate missing fields with their default values. It is very much inspired by the NixOs module system and the CUE language, but it is a story for another time.


I hope that I gave you a sense of what Nickel is trying to achieve. I only presented its most salient aspects: its gradual type system with contracts, and built-in record schemas. But there is more to explore! The language is not ready to be used in real world applications yet, but a good share of the design presented here is implemented. If you are curious about it, check it out!

This article is licensed under a Creative Commons Attribution 4.0 International license.
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