Great Nickel configurations from little merges grow

2 November 2023 — by Yann Hamdaoui

This blog post is part of the series exploring the foundations of the Nickel configuration language.

  1. Presenting Nickel: better configuration for less
  2. Programming with contracts in Nickel
  3. Types à la carte in Nickel
  4. Great configurations from little merges grow

We previously looked at the core language, then contracts, and finally typing. The last important remaining piece to explore is the merge system. In this post, we’ll see how to use the Nickel merge system to write reusable configuration modules, and why the merge approach seems to be more adapted for modular configurations than plain functions, despite Nickel being a functional language.

Programmable configuration

One of the starting points of Nickel has been the limits of static data formats such as JSON, TOML or YAML when tackling large configurations. In particular, those formats have no way of reusing values of your configuration. Have a single value used in several places (say, an IP)? Have this virtual machine description block used several times, almost identically but for a few parameters? In JSON, the best you can do is to copy and paste, praying for the different copies to not drift away with time.

In Nickel, you can just use variables and functions:

let master_ip = "" in
let machine = fun {ip, open_ports} => {
  # machine definition depending on ip and open_ports
} in

  ip = master_ip,
  cmd = "ssh -p 8070 user@%{master_ip}",
  client1 = machine {ip = "", open_ports = [22]},
  client2 = machine {ip = "", open_ports = [80]},

In the zoology of programming abstractions, functions are the simplest and most ubiquitous concept of all. For the machine example, a (pure) function is exactly what we look for: an expression with unknown parameters which can be selected at usage time. Virtually every programming language has functions in some form, even if they’re not called that way (for example, in Terraform’s configuration language, modules are more or less functions1).

You might even want not to hardcode some parameters directly in the configuration but rather fetch them from the command line or an environment variable for each deployment, turning your whole configuration to a function:

fun {param1, param2, ..} =>
  # your config

This pattern probably looks familiar to Nix developers: this is exactly how a Nix package is defined, where arguments are other packages (programs, tools and libraries) this package depends on.

Functions are simple yet powerful and flexible. They can be composed, passed around, returned… so, aren’t functions sufficient?

Issues with functions

Functions might actually not be the best solution for reusable configuration parts, for the following reasons.

Functions are opaque

Functions are opaque values that need to be fed arguments before producing any data. If your whole configuration is now a function, you can’t manipulate or explore it anymore. You need to provide arguments first.

A basic example is the nickel query subcommand, which extracts information and documentation from a configuration (the example comes from the Nickel repository):

$ nickel query -f examples/config-gcc/config-gcc.ncl
Available fields
• flags
• optimization_level
• path_libc

$ nickel query -f examples/config-gcc/config-gcc.ncl path_libc
• contract: Path,SharedObjectFile
• default: "/lib/x86_64-linux-gnu/"
• documentation: Path to libc.

But query doesn’t work on functions!

$ nickel repl
nickel> let example1 =
  let value = 1 in {left = "a", right = 1}
nickel> :query example1
Available fields
• left
• right

nickel> let example2 =
  fun param => let value = 1 in {left = param, right = 1}
nickel> :query example2


query can perform some evaluation and sees through the let-binding in the first example, but it can’t evaluate inside the body of a function before it’s applied.

This is frustrating, because most of the time, 90% of the structure and the content of the configuration is known while just a few values depend on the parameters. This is a well-known issue in Nix2.

Functions aren’t always nice to combine

I previously said that functions were composable, and I stand by this statement in general. Functions lend themselves particularly well to sequential composition, that is piping several operations one after the other. Applying array operations in Nickel is indeed particularly pleasant3:

let do_stuff : Array Number -> Number
  = fun array =>
    # keep odd numbers
    |> std.array.filter (fun x => x % 2 == 1)
    # add one
    |> ((+) 1)
    # multiply them together
    |> std.array.reduce_left (*)
do_stuff [1, 2, 3, 4, 5] # 48

However, when writing configurations, it’s not that common to “pipe” many configuration parts together this way (we do pipe operations to generate individual values as in the example above - that’s why Nickel has functions, they are great for that). Instead, we want to paste many configuration parts. Pasting can be done using Nickel’s merge operator &. Merge works well on its own, but using functions for abstraction is still awkward. Consider for example:

# network.ncl
fun {ip, firewall_enabled, ipv6_enabled} => { .. }
# services.ncl
fun {httpd_enabled, openssh_version} => { .. }
# volumes.ncl
fun {volume_max_size, lvm_enabled} => { .. }

If we want to combine those parts, we have to glue everything together:

let network = import "network.ncl" in
let services = import "services.ncl" in
let volumes = import "volumes.ncl" in

fun {
} =>
  network { ip, firewall_enabled, ipv6_enabled }
  & services { httpd_enabled, openssh_version }
  & volumes { volume_max_size, lvm_enabled }

The result is mostly boilerplate repeating information we already know. If you need different combinations somewhere else, such as network and services only, you need to write all the corresponding wrapper functions.

Functions are nice to compose sequentially but not as nice to compose in parallel, which is what we do here.

Functions are hard to override

Overriding is the act of taking an existing configuration (in our case a function that may have already been applied) but tweaking just one parameter. Isn’t overriding the essence of configuration, after all? Toggle a flag, replace a value, and run again.

Overriding turns out to be a hard problem when configuration is built from composing functions. If you control the source code, you can change the value provided to the configuration-as-a-function at the call-site. But if this function calls other functions internally, the parameters to the inner call aren’t necessarily accessible. You need to bubble them up to the root function’s argument.

It also happens that one doesn’t control the call-site or the function’s code, because it’s in a library. That is, overriding is opt-in for the author of the function, but the one who needs it is the consumer. It’s hard for the author to know in advance everything that could be useful to expose for overriding.

Overriding is typically an issue in Nix: when pulling package descriptions from the central repository (Nixpkgs), the package is “already applied”. For example, we build Nickel with Nix, and we need to make sure the version of wasm-bindgen we pull from Nixpkgs is the same as the one set in Cargo.toml. A naive way would be to do something like (in Nix, // overrides a value in a record):

wasm_bindgen_cli =
  pkgs.wasm-bindgen-cli // {version = wasm_bindgen_fixed_version;}

Sadly, this doesn’t work. This code overrides the final version field correctly, but not the other values which depend on version (such as src). What we want to override is the original version used when calling the package-as-a-function, not the one propagated in the end result. But once an arbitrary function has been applied, there’s no coming back.

The actual Nix code to build Nickel calls to a mysterious override function, which is a bespoke mechanism of Nixpkgs to simulate this kind of overriding. Spoiler: there are several competing such overriding mechanisms in Nix, they are all hard to wrap your head around, and you can’t always override everything you would like to.

Merging system

In Nickel, we advocate for a different model to write and assemble configuration parts. Instead of writing functions that return records and pipe them together, we write records directly where all fields might not have a definition yet. We refer to configurations written in this style as partial configurations.

Nickel’s records are recursive, meaning that a field definition may depend on other fields of the same record:

# machine.ncl
  cmd = "ssh -p 8070 user@%{ip}",

Here, ip doesn’t have a definition. This configuration is partial and can’t be exported yet:

$ nickel export -f machine.ncl
error: missing definition for `ip`
  ┌─ machine.ncl:2:3
1 │ ╭ {
2 │ │   ip,
  │ │   ^^ required here
3 │ │   cmd = "ssh -p 8070 user@%{ip}",
4 │ │ }
  │ ╰─' in this record

However, merging this snippet with a record defining ip does the trick:

$ echo '(import "machine.ncl") & {ip = ""}' > config.ncl
$ nickel export -f config.ncl
  "ip": ""
  "cmd": "ssh -p 8070 [email protected]",

Deep down, recursive records could be seen as just a fancy representation for functions (in the same way as objects are poor man’s closures). But we’ll see how this representation makes many configuration-related operations much easier.

Records are transparent

As opposed to functions, partial records don’t need to hang on their missing field. We can extract documentation from them, get completion in the LSP, use nickel query, and so on. The structure of the final configuration is already apparent: there are just some holes to be filled.

Records remember

As opposed to functions, recursive records remember the relation between their different fields even if they have all been given a value at this point. Overriding a field will automatically recompute the new value of its reverse dependencies. Remember the wasm-bindgen example, where we had to use Nixpkgs’ override. In Nickel, you don’t need any of that. The following is a similar example, albeit simplified, where the cmd field depends on the the ip field (similar to version in the wasm-bindgen example). In the partial configuration model, you can naively override ip:

# machines.ncl
let init_machine = {
  ip = "",
  cmd = "ssh -p 8070 user@%{ip}",
} in

  old = init_machine,
  new = old & { ip | force = "" },

ip is properly updated in new:

$ nickel export -f packages.ncl
  "old": {
    "ip": ""
    "cmd": "ssh -p 8070 [email protected]",
  "new": {
    "ip": ""
    "cmd": "ssh -p 8070 [email protected]",

Records are nice to combine

Finally, recall the example from Functions aren’t always nice to combine with a network part, a services part and a volumes part.

We had to redeclare the full list of parameters and correctly forward them to subfunctions.

With records, you can just merge them. In some sense, merge is like parallel composition: it pastes records together, the fields without definition in the result (the arguments) being the union of the fields without definition from each part.

  all = network & services & volumes,
  partial = network & services,

Toward modules

Nickel is centered around records as the primary unit of configuration. Metadata can be attached to record fields, giving them more expressive power and the ability to better describe an interface for a partial configuration. Combined with field metadata, records can come close to the notion of modules in other languages. For example:

  inputs | not_exported = {
        | String
        | doc "Doc of foo",
        | Number
        | doc "Doc of bar",
        | Bool
        | optional,
  local | not_exported = {
      computed = std.string.to_lower,

  some_config_option = + 1,
  other_option = std.string.join ["Hello", local.computed],
  last_option = "values are %{local.computed} and %{std.to_string}",

We used the not_exported metadata to signal that the inputs and local fields are high-level values that are used to generate the configuration, but they shouldn’t be part of the final export. inputs is supposed to be set by consumers of this module. local contains intermediate values that are computed from the inputs and possibly reused many times in the outputs.

Finally, the outputs lie at the root of the record. We could also have put them under an output field, requiring to extract the output field in the end (but, in return, we can then remove the not_exported).

All of those values are readily and automatically overridable (using the force metadata for fields that already have a non-default value), and changes are propagated to reverse dependencies accordingly. The proposed layout is rather arbitrary: Nickel has no notion of inputs, local and outputs, but rather of fields with and without definition as well as priorities.

The metadata associated to inputs define a clear typed module interface which can be leveraged by the Nickel tooling.

Nickel might get a first-class notion of modules one day, if only to enforce a standard structure across projects. But, as far as functionality is concerned, the current merge system can already get you pretty far in an intuitive way.


In this post, we explored the merge system of Nickel and how to leverage it to write natural configuration parts that are easy to query, to combine and to tweak. We supported our claim that partial configuration is a better suited model than functions (or other abstractions, for that matter) for configuration management, and showed that the combination of merging and field metadata is an expressive combo.

  1. According to Terraform’s own manual
  2. Nix language changes, Eelco Dolstra
  3. the pipe operator |> can appear magical, but it’s just syntactic sugar for reverse application, that is data |> f |> g is g (f data)

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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