nickel

Nickel Configuration Language

Nickel is a configuration language designed to automate generation of static configuration files (JSON, YAML, TOML, XML). It combines gradual typing with runtime contracts to provide both static checking for complex logic and flexible validation for configuration data.

Key Concepts

Gradual Typing

Nickel supports both static and dynamic typing. Mix typed and untyped code within the same configuration:

• Typed regions: Functions with static type checking for complex logic
• Dynamic regions: Flexible configuration values validated at runtime with contracts
• Types are optional—choose when to use them

Contracts

Contracts are runtime assertions that validate values satisfy specific properties. They act as schemas:

• Built-in contracts: Number, String, Bool, Dyn, Array, record contracts
• Custom contracts: Functions that check predicates and return values or errors
• Composition: Combine multiple contracts with boolean logic (std.contract.one_of, std.contract.all_of)
• Metadata: Contracts attach to fields using the | operator

Merge System

The & operator merges records with symmetric semantics:

• Symmetric merging: Both sides contribute to the result
• Recursive merging: Nested records merge recursively
• Metadata composition: Documentation, defaults, and contracts merge together
• Priorities and defaults: Control override behavior with metadata

Installation

Using mise

Add to mise.toml:

[tools]
nickel = "latest"

[tasks.validate-config]
script = "nickel eval config.ncl"

[tasks.export-config]
script = "nickel export --format json config.ncl"

Using Cargo

cargo install nickel-lang-cli

CLI Usage

Evaluate Nickel Files

nickel eval config.ncl
nickel eval config.ncl --output result.json

Export to Different Formats

# Export to JSON
nickel export config.ncl --format json

# Export to YAML
nickel export config.ncl --format yaml

# Export to TOML
nickel export config.ncl --format toml

# Merge and export
nickel export base.json overrides.ncl --format yaml