Pkl Has a Free API: Apple's Configuration Language That Catches Errors Before Deployment

#pkl #configuration #apple #devops

YAML doesn't validate. JSON doesn't have comments. TOML doesn't do schemas. Apple built Pkl to fix all three.

What Is Pkl?

Pkl (pronounced "pickle") is a configuration language from Apple. It's like YAML but with types, validation, and IDE support.

// server.pkl
port: UInt16 = 8080
host: String = "localhost"
maxConnections: UInt = 100

database {
  host: String = "db.example.com"
  port: UInt16 = 5432
  name: String = "myapp"
  maxPoolSize: UInt(isBetween(1, 100)) = 20
}

If you set port = -1 → compile error. If you set maxPoolSize = 200 → compile error. Before your app even starts.

Type Safety

// Define a schema
class ServerConfig {
  host: String
  port: UInt16
  ssl: Boolean = false
  workers: UInt(isPositive) = 4
  allowedOrigins: Listing<String>
  timeouts {
    read: Duration = 30.s
    write: Duration = 30.s
    idle: Duration = 120.s
  }
}

// Use it — any mistake is caught at evaluation time
production: ServerConfig = new {
  host = "api.example.com"
  port = 443
  ssl = true
  workers = 8
  allowedOrigins {
    "https://example.com"
    "https://app.example.com"
  }
}

Generate Any Format

# Generate JSON
pkl eval server.pkl -f json

# Generate YAML
pkl eval server.pkl -f yaml

# Generate .properties
pkl eval server.pkl -f properties

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