Pkl Has a Free Configuration Language by Apple — Type-Safe Config That Generates JSON, YAML, and Plist

The Config Problem

YAML: indentation errors that break prod. JSON: no comments, no variables. TOML: limited nesting. Jsonnet: nobody knows it.

Pkl (Pickle) by Apple is a typed configuration language. Write config once, generate JSON, YAML, XML, or Plist. With validation.

What Pkl Gives You

Typed Configuration

class ServerConfig {
  host: String
  port: Int(isBetween(1, 65535))
  debug: Boolean = false
  workers: Int(isPositive) = 4
}

production: ServerConfig = new {
  host = "api.example.com"
  port = 443
  workers = 16
}

staging: ServerConfig = (production) {
  host = "staging.example.com"
  debug = true
  workers = 2
}

Staging inherits from production. Override only what's different.

Validation Built In

class DatabaseConfig {
  host: String(!isEmpty)
  port: Int(isBetween(1, 65535))
  maxConnections: Int(isBetween(1, 1000))
  sslMode: "disable"|"require"|"verify-full"
}

Invalid config? Compile-time error. Not a runtime crash.

Generate Any Format

pkl eval config.pkl -f json    # → config.json
pkl eval config.pkl -f yaml    # → config.yaml
pkl eval config.pkl -f plist   # → config.plist
pkl eval config.pkl -f xml     # → config.xml

Modules and Imports

import "base-config.pkl"

server = (base.server) {
  host = "custom.example.com"
}

Share config modules across teams. Publish to a registry.

Dynamic Values

local replicas = 3

services: Listing<Service> = new {
  for (i in IntSeq(1, replicas)) {
    new {
      name = "worker-\(i)"
      port = 8080 + i
    }
  }
}

IDE Support

VS Code extension with autocomplete, type checking, and inline errors.

Quick Start

brew install pkl
pkl eval config.pkl

SDKs available for: Swift, Kotlin, Java, Go.

Why This Matters

Configuration is code — it should have types, validation, and IDE support. Pkl brings software engineering practices to the one place that still uses untyped text files.

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Managing configuration for data pipelines? Check out my web scraping actors on Apify Store — configurable data extraction. For custom solutions, email [e-mail].