Nickel is the cheap configuration language.
Its purpose is to automate the generation of static configuration files - think JSON, YAML, XML, or your favorite data representation language - that are then fed to another system. It is designed to have a simple, well-understood core: it is in essence JSON with functions.
Nickel's salient traits are:
- Lightweight: Nickel is easy to embed. An interpreter should be simple to implement. The reference interpreter can be called from many programming languages.
- Composable code: the basic building blocks for computing are functions. They are first-class citizens, which can be passed around, called and composed.
- Composable data: the basic building blocks for data are records (called objects in JSON). In Nickel, records can be merged at will, including associated metadata (documentation, default values, type contracts, etc).
- Typed, but only when it helps: static types improve code quality, serve as documentation and eliminate bugs early. But application-specific self-contained code will always evaluate to the same value, so type errors will show up at runtime anyway. Some JSON is hard to type. There, types are only a burden. Whereas reusable code - that is, functions - is evaluated on potentially infinitely many different inputs, and is impossible to test exhaustively. There, types are precious. Nickel has types, but you get to choose when you want it or not, and it handles safely the interaction between the typed and the untyped world.
- Design by contract: complementary to the type system, contracts are a principled approach to checking assertions. The interpreter automatically inserts assertions at the boundary between typed and untyped code. Nickel lets users add arbitrary assertions of their own and easily understand why when assertions fail.
The motto guiding Nickel's design is:
Great defaults, design for extensibility
There should be a standard, clear path for common things. There should be no arbitrary restrictions that limit what you can do you the one day you need to go beyond.
Nickel is a good fit in any situation where you need to generate a complex configuration, be it for a single app, a machine, whole infrastructure, or a build system.
The motivating use cases are in particular:
- The Nix package manager: Nix is a declarative package manager using its own language for specifying packages. Nickel is an evolution of the Nix language, while trying to overcome some of its limitations.
- Infrastructure as code: infrastructure is becoming increasingly complex, requiring a rigorous approach to deployment, modification and configuration. This is where a declarative approach also shines, as adopted by Terraform, NixOps or Kubernetes, all requiring potentially complex generation of configuration.
- Build systems: build systems (like Bazel) need a specification of the dependency graph.
Most aforementioned projects have their own bespoke configuration language. See Comparison. In general, application-specific languages might suffer from feature creep, lack of abstractions or just feel ad hoc. Nickel buys you more for less.
Please follow the getting started guide for Nickel users on the nickel-lang
website. The instructions below are
either reproduced for this document to be self-contained or because
they are aimed toward hacking on the Nickel interpreter itself (e.g. building
the nickel-lang
crate documentation).
-
Get a Nickel binary:
- With flake-enabled Nix, run
Nickel directly with
nix run github:tweag/nickel
. You can use our binary cache to prevent rebuilding a lot of packages. Pass arguments to Nickel with an extra--
as innix run github:tweag/nickel -- repl
, - Again with flake-enabled Nix, you can install Nickel in your profile with
nix profile add github:tweag/nickel
. Thenickel
command is then in your$PATH
and is available anywhere. - Without Nix, you can use
cargo run
after building, passing arguments with an extra--
as incargo run -- -f program.ncl
.
- With flake-enabled Nix, run
Nickel directly with
-
Run your first program:
$ nickel <<< 'let x = 2 in x + x' 4
Or load it from a file:
$ echo 'let s = "world" in "Hello, " ++ s' > program.ncl $ nickel -f program.ncl "Hello, world"
-
Start a REPL:
$ nickel repl nickel> let x = 2 in x + x 4 nickel>
Use
:help
for a list of available commands. -
Export your configuration to JSON, YAML or TOML:
$ nickel export --format json <<< '{foo = "Hello, world!"}'
{
"foo": "Hello, world!"
}
Use nickel help
for a list of subcommands, and nickel help <subcommand>
for help about a specific subcommand.
Nickel has syntax highlighting plugins for Vim/Neovim, and VSCode. In-editor diagnostics, type hints, and auto-completion are provided by the Nickel Language Server. Please follow the LSP guide to set up syntax highlighting and NLS.
You can format Nickel source code using Topiary:
topiary -i -f my-config.ncl
Please follow the Formatting Capabilities section of the LSP documentation to know how to hook up the Nickel LSP and topiary in order to enable formatting inside your code editor.
-
Download build dependencies:
-
With Nix: If you have Nix installed:
nix-shell nix develop # if you use Nix Flakes
You will be dropped in a shell, ready to build. You can use our binary cache to prevent rebuilding a lot of packages.
-
Without Nix: otherwise, follow this guide to install Rust and Cargo first.
-
-
Build Nickel:
cargo build --release
And voilà! Generated files are placed in
target/release
.
Run tests with
cargo test
The user manual is available on the nickel-lang.org
website, and in this
repository as a collection of Markdown files in doc/manual
.
To get the documentation of the nickel-lang
codebase itself:
-
Build the doc:
cargo doc --no-deps
-
Open the file
target/doc/nickel/index.html
in your browser.
You can find examples in the ./examples
directory.
Nickel is currently released in version 1.0
. We expect the core design of the
language to be stable and the language to be useful for real-world applications.
The next steps we plan to work on are:
- Nix integration: being able to seamlessly use Nickel to write packages and shells (nickel-nix)
- Custom merge functions (second part of the overriding proposal)
- Incremental evaluation: design an incremental evaluation model and a caching mechanism in order to perform fast re-evaluation upon small changes to a configuration.
- Performance improvements
- CUE is a configuration language with a focus on data validation. It introduces a new constraint system backed by a solid theory which ensures strong guarantees about your code. It allows for very elegant schema specifications. In return, the cost to pay is to abandon functions and Turing-completeness. Nickel's merge system is inspired by the one of CUE, even if since Nickel does have general functions and is Turing-complete, they are necessarily different.
- Nix: The Nix language, or Nix expressions, is one of the main inspirations for Nickel. It is a very simple yet powerful lazy functional language. We strive to retain this simplicity, while adding typing capabilities, modularity, and detaching the language from the Nix package manager.
- Dhall is a statically typed configuration language. It is also inspired by Nix, to which it adds a powerful static type system. However, this forces the programmer to annotate all of their code with types.
- Jsonnet is another language which could be dubbed as "JSON with functions" (and others things as well). It is a lazy functional language with object-oriented features, among which inheritance is similar to Nickel's merge system. One big difference with Nickel is the absence of typing.
- Pulumi is not a language in itself, but a cloud tool (like Terraform) where you can use your preferred language for describing your infrastructure. This is a different approach to the problem, with different trade-offs.
- Starlark is the language of Bazel, which is a dialect of Python. It does not have types and recursion is forbidden, making it not Turing-complete.
See RATIONALE.md for the design rationale and a more detailed comparison with these languages.
Language | Typing | Recursion | Evaluation | Side-effects |
---|---|---|---|---|
Nickel | Gradual (dynamic + static) | Yes | Lazy | Yes (constrained, planned) |
Starlark | Dynamic | No | Strict | No |
Nix | Dynamic | Yes | Lazy | Predefined and specialized to package management |
Dhall | Static (requires annotations) | Restricted | Lazy | No |
CUE | Static (everything is a type) | No | Lazy | No, but allowed in the separated scripting layer |
Jsonnet | Dynamic | Yes | Lazy | No |
JSON | None | No | Strict | No |
YAML | None | No | N/A | No |
TOML | None | No | N/A | No |