First release of Nickel

11 March 2022 — by Yann Hamdaoui

I am excited to announce the first release of Nickel! In the original introductory blog post, I’ve written about why we, at Tweag, are developing yet another configuration language. Our goal is to empower people to write correct, programmable and maintainable configurations. Nickel targets Nix, Infrastructure-as-Code, build systems, and more. We think such a tool would greatly help taming the growing complexity of configuring systems.

Since the first blog post, we have made progress. We backed up and changed our mind sometimes. We had a lot of discussions, and we also experimented quite a bit. There is still a lot to do and there are many imperfect aspects of the language to improve.

Still, I think we have reached a point where Nickel is consistent enough for people to try it more seriously. That way we can get our target users involved and not lose sight of their actual issues and wishes.

Beware: this is a 0.1 release. We are making no backward-compatibility guarantees at this time, because we don’t want to tie our hands too early and get stuck with retrospectively unfortunate decisions. Of course, we’ll try to minimize breaking changes as much as possible. You shouldn’t use this version in production yet, however, we encourage you to try Nickel wherever you think makes sense: personal projects, experimenting outside of the production environment, etc.

What’s here

To try the examples from this section, you can use the online playground.


Apart from a few syntax differences, basic Nickel is close to JSON. Here is an example of a service configuration:

  kind = "Service",
  apiVersion = "v1",
  route = "/api/v1", = "phabricator",

  service = { = "phabricator",
    ports = [
      {port = 80, protocol = "http"},
      {port = 443, protocol = "https"}

Nickel has the same primitive data types as JSON: numbers, string, lists and records (objects in JSON).

Unlike JSON, Nickel is programmable. You can use variables to have a single source of truth for your data. You can use functions to transform and describe dependencies between data. The following example has some repetition, and while it is fabricated, such repetition is actually quite frequent in real-life configurations, for example in Kubernetes. Repetition can lead to inconsistencies and make modifying data a burden. Let’s see how we can get rid of repetition in Nickel:

let app_name = "phabricator" in

let protocol_from_port = fun port =>
  if port == 80 then "http"
  else if port == 443 then "https"
  else "" in

  kind = "Service",
  apiVersion = "v1",
  route = "%{app_name}/api/%{apiVersion}", = app_name,

  service = { = app_name,
    ports =
      (fun p => {port = p, protocol = protocol_from_port p})
      [80, 443],

let app_name = "phabricator" in ... defines an immutable variable app_name with value "phabricator". fun port => ... defines a function of one argument named port. The string interpolation %{app_name} substitutes app_name by its value inside the string.

In this new version,

  • The name phabricator is now centralized in app_name. It ensures that all the occurrences are the same, and that changing the name only amounts to updating app_name.
  • The array of records ports is now generated, and each protocol field is deduced automatically from the corresponding port number using the function protocol_from_port. Generating ports ensures each port number and its associated protocol are consistent, and make modification easy (just modify directly the port list).
  • The route field refers to another field of the same record, apiVersion. In Nickel, records are recursive by default, meaning that fields can freely refer to each others. This makes it simple to express inter-dependencies of fields. Here, indeed, the route depends on the apiVersion. If we change the api version, the route will automatically stay in sync.


The previous code is dynamically typed, which is the default in Nickel. This makes it easy to write a configuration. However, dynamic type errors can quickly become hard to debug, especially when using functions.

To help in writing bug free code, Nickel features a gradual type system. In practice, this means you can leverage static typing by simply annotating a particular block, typically a function, with a type. The typechecker will then rigorously verify this block. Here is a tweak of our previous protocol_from_port with a type annotation (I replaced the "" with null on the last line):

# In file wrong.ncl
let protocol_from_port : Num -> Str = fun port =>
  if port == 80 then "http"
  else if port == 443 then "https"
  else null in

If we try to evaluate this program, the typechecker will rightfully reject it. Indeed, null is not a valid value for a string:

$ nickel -f wrong.ncl
error: incompatible types
  ┌─ repl-input-7:4:8
4 │   else null in
  │        ^^^^ this expression
  = The type of the expression was expected to be `Str`
  = The type of the expression was inferred to be `Dyn`

Contracts (schemas)

The end result of the evaluation of a Nickel program is typically a JSON file (or YAML, or TOML). This file is then fed to a system (e.g. Kubernetes). This file must certainly follow the requirements imposed by this system. Such requirements can usually be expressed as data schemas, specifying which fields are allowed, which are mandatory, and what kind of data can go in each field.

Some such requirements can be enforced by using types. However, a large number of them are out of reach of static typing. For example, take a random field from the kubernetes openAPI specification:

"serverAddress": {
  "default": "",
  "description": "[..] This can be a hostname, hostname:port, IP or IP:port.",
  "type": "string"

The last part of the description is a good example: being a string containing a hostname, hostname:port, IP or IP:port. Not only this is out of reach of most static type systems, but openAPI is not able to express this property either! The type field just indicates string, which is much less precise.

Nickel can express the full actual specification of serverAddress using contracts. Contracts are a validation system that can enforce arbitrary properties (you can provide your own validation functions). Contracts are designed to be written in an intuitive way, like schemas:

# We assume the boolean functions is_hostname and is_ip are defined before
let Address = contract.from_predicate (fun value =>
  builtin.is_string value &&
  (value == "" ||
   is_hostname value ||
   is_ip value) in

# Defines a simple schema for our configuration
let Schema = {
      | Address
      | doc "[..] This can be a hostname, hostname:port, IP or IP:port."
      | default = "",
    # ... rest of the contract
} in

  serverAddress = "",
  # ... rest of the configuration
} | Schema

If we try to export this program, we get a contract violation, because our IP is not valid (there is one extra number):

error: contract broken by a value
   ┌─ :1:1
 1 │ Address
   │ ------- expected type
   ┌─ repl-input-2:11:19
11 │   serverAddress = "",
   │                   ^^^^^^^^^^^^^^^ applied to this expression

  ┌─ repl-input-2:3:9
3 │       | Address
  │         ^^^^^^^ bound here

This was a simple example, but I could have used custom messages to make the error even more helpful. Thanks to contracts, invalid configurations can be caught early, instead of downstream in the pipeline when trying to deploy our services. Beyond just making our deployment work, we can even imagine using contracts for enforcing additional properties, such as security-related rules.


One last important aspect of Nickel is the merge operation, written &. Merging combines records recursively and provides a way of writing modular configurations:

# file service.ncl
  name = "phabricator",
  kind = `Service,
  firewall.openPorts = [80, 443],

# file security.ncl
  useFirewall = true,
  firewall = {
    use = "iptables",
    openPorts | default = [],
    allowedProtocols = protocol_from_port openPorts,

# file main.ncl
let service = import "service.ncl" in
let security = import "security.ncl" in
service & security

Here we split security options from the service definition. The merge operator recursively combines the two resulting in the following JSON:

  "firewall": {
    "use": "iptables",
    "openPorts": [
    "allowedProtocols": [
  "kind": "Service",
  "name": "phabricator",
  "useFirewall": true

firewall has been specified by pieces in the two different files and combined by merging. The service needs to open some ports: the firewall.ncl closes them all by default (by defining openPorts = [], but makes this option overridable thanks to the default annotation. What’s more, merging plays well with recursive records, as it automatically updated the value of allowedProtocols, which depends on openPorts.

Going further

You can look at the main README for a general description of the project. The first blog post series explains the inception of Nickel, and the following posts focus on specific aspects of the language. For a (very) condensed version of this blog post, see the release notes. Finally, the most complete source remains the user manual.

What’s not here (yet)

We’ve been mostly focusing on designing and implementing the core language.

Tooling and documentation

Tooling and documentation are available, but not yet comprehensive. Still, check out the vim plugin or the VSCode plugin, and the editor-agnostic LSP server. For documentation, you can look at the website, and in particular the user manual.


Not much effort has been put into making the interpreter fast or memory savvy. This is a subject we plan to work on in the future.

Nickel and Nix

Although Nickel is at heart a generalist configuration language, Nix has been a target application from day one, and was one of the original motivations for Nickel. We think Nickel could make an impact there and really improve the user experience.

There are several possible approaches to integrate Nickel with Nix, with varying power, ergonomics and required effort. Some approaches would require modifying either Nix itself, Nickel, or both.

For the time being, we haven’t yet worked out one robust, practical and powerful solution to use Nickel as a front-end for Nix development. However, we have been actively thinking about it. And now, Nix integration is the very next step on the roadmap, in parallel with collecting feedback and usage reports to help build the future of Nickel.

In the meantime, we open-sourced the result of our experiments of writing a simple Nix shell in Nickel. The idea has been to find the more direct route, involving only pure Nix and Nickel, and without augmenting either the Nix or the Nickel interpreter. The current code is experimental at best, but may serve as a basis for more Nickel in Nix, for the more adventurous among you: nickel-nix.


We are happy to announce the first release of the Nickel configuration language. You can use it wherever you would normally use JSON, YAML or TOML, but feel limited by using static text or ad-hoc templating languages.

This release is both usable and not yet ready for production. Your feedback, ideas and opinions are invaluable: please use Nickel, break it, do cool things we haven’t even imagined, and most importantly, please let us know about it!

About the author

Yann Hamdaoui

Yann is working at Tweag on the Nickel programming language, a next-generation configuration language to manage the growing complexity of Infrastructure-as-Code and a candidate successor for the language Nix.

If you enjoyed this article, you might be interested in joining the Tweag team.

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


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