Pkl Has a Free Configuration API from Apple That Replaces YAML and JSON

#pkl #configuration #apple #devops

Pkl is a configuration language from Apple that catches errors before deployment. Type-safe, programmable configs that generate YAML, JSON, or any format.

Basic Syntax

// config.pkl
name = "my-app"
port = 8080
debug = false

database {
  host = "localhost"
  port = 5432
  name = "mydb"
  maxConnections = 20
}

features {
  enableNewUI = true
  maxUploadSizeMB = 50
}

Type Safety

// AppConfig.pkl — define your schema
class DatabaseConfig {
  host: String
  port: UInt16  // Must be 0-65535
  name: String
  maxConnections: Int(isBetween(1, 100))  // Validated!
}

class AppConfig {
  name: String
  port: UInt16
  debug: Boolean
  database: DatabaseConfig
}

// If someone sets maxConnections = 200, Pkl catches it at generation time

Generate YAML/JSON

# Generate YAML
pkl eval config.pkl -f yaml
# Output:
# name: my-app
# port: 8080
# database:
#   host: localhost
#   port: 5432

# Generate JSON
pkl eval config.pkl -f json

# Generate for Kubernetes
pkl eval k8s-deployment.pkl -f yaml > deployment.yaml

Programmable Configs

// environments.pkl
local baseConfig: AppConfig = new {
  name = "my-app"
  database {
    maxConnections = 20
  }
}

dev: AppConfig = (baseConfig) {
  port = 3000
  debug = true
  database {
    host = "localhost"
  }
}

prod: AppConfig = (baseConfig) {
  port = 8080
  debug = false
  database {
    host = "db.production.internal"
    maxConnections = 100
  }
}

Kubernetes Example

// k8s.pkl
import "package://pkg.pkl-lang.org/pkl-k8s/k8s@1.0.0#/api/apps/v1/Deployment.pkl"

output {
  renderer = new YamlRenderer {}
  value = new Deployment {
    metadata {
      name = "my-app"
      namespace = "production"
    }
    spec {
      replicas = 3
      template {
        spec {
          containers {
            new {
              name = "app"
              image = "my-app:latest"
              ports { new { containerPort = 8080 } }
            }
          }
        }
      }
    }
  }
}

Why This Matters

Type-safe: Catch config errors before deployment
IDE support: Autocomplete, hover docs, error highlighting
Multi-format: Generate YAML, JSON, Property Lists
From Apple: Battle-tested in Apple's internal infrastructure

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