Pkl Has a Free Configuration Language — Type-Safe Configs That Generate JSON, YAML, and More

Pkl (by Apple) is a configuration language — write type-safe configs and generate JSON, YAML, XML, or any format.

What You Get for Free

• Type safety — catch config errors at build time
• Validation — constraints on values (ranges, patterns)
• Modules — reusable config templates
• Multi-format output — generate JSON, YAML, plist, properties
• IDE support — IntelliJ and VS Code extensions
• Packages — share config schemas as packages
• Templating — inheritance, composition, overrides
• CLI — evaluate, test, format Pkl files

Quick Start

brew install pkl

// server.pkl
host: String = "localhost"
port: Int(isBetween(1, 65535)) = 8080
database {
  url: String = "postgres://localhost/mydb"
  maxConnections: Int(isPositive) = 10
}

pkl eval server.pkl -f json
# {"host":"localhost","port":8080,"database":{"url":"postgres://localhost/mydb","maxConnections":10}}

pkl eval server.pkl -f yaml
# host: localhost
# port: 8080
# database:
#   url: postgres://localhost/mydb
#   maxConnections: 10

Why Developers Switch from Raw YAML

YAML has no types, no validation, and indentation bugs:

• Type-safe — port: Int catches port: "8080" at build time
• Validation — isBetween(1, 65535) rejects port 70000
• Multi-format — one source, generate JSON, YAML, XML
• IDE support — autocomplete, error highlighting

A team deployed to production with maxConnections: "10" (string, not int) in YAML — caused a 3-hour outage. After Pkl: type system would have caught it at build time.

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