Pkl Has a Free API — Heres How Apple Reinvented Configuration Files

Pkl (pronounced 'pickle') is Apple's new configuration language — type-safe, composable, and generates JSON, YAML, or any format. Replace your brittle YAML with something that actually catches errors.

Why Pkl?

• Type-safe: Catch config errors before deployment
• Composable: Inherit, extend, amend configurations
• Multi-output: Generate JSON, YAML, XML, properties
• IDE support: Autocomplete, go-to-definition
• Validation: Built-in constraints
• Apple-backed: Open source from Apple

Install

# macOS
brew install pkl

# Linux
curl -L https://github.com/apple/pkl/releases/latest/download/pkl-linux-amd64 -o pkl
chmod +x pkl

Basic Config

// config.pkl
name = "my-app"
version = "1.2.3"

server {
  host = "0.0.0.0"
  port = 8080
  maxConnections = 100
}

database {
  url = "postgres://localhost:5432/mydb"
  pool {
    min = 5
    max = 20
  }
}

Generate JSON:

pkl eval config.pkl -f json

Type Safety

class ServerConfig {
  host: String
  port: Int(isBetween(1, 65535))
  maxConnections: Int(isPositive)
  tls: Boolean = false
}

class DatabaseConfig {
  url: String
  pool: PoolConfig
}

class PoolConfig {
  min: Int(isPositive)
  max: Int(this >= outer.min)  // max must be >= min
}

server: ServerConfig = new {
  host = "0.0.0.0"
  port = 8080
  maxConnections = 100
}

database: DatabaseConfig = new {
  url = "postgres://localhost:5432/mydb"
  pool = new {
    min = 5
    max = 20
  }
}

Environments (Amend)

// production.pkl
amends "config.pkl"

server {
  tls = true
  maxConnections = 1000
}

database {
  url = "postgres://prod-db:5432/mydb"
  pool {
    min = 20
    max = 100
  }
}

Production inherits all defaults, only overrides what changes.

Generate Kubernetes YAML

output {
  renderer = new YamlRenderer {}
}

apiVersion = "apps/v1"
kind = "Deployment"
metadata {
  name = "my-app"
  labels {
    ["app"] = "my-app"
  }
}
spec {
  replicas = 3
  template {
    spec {
      containers {
        new {
          name = "my-app"
          image = "my-app:1.2.3"
          ports {
            new { containerPort = 8080 }
          }
        }
      }
    }
  }
}

Validation

port: Int(isBetween(1, 65535))
email: String(matches(Regex("^[^@]+@[^@]+\\.[^@]+$")))
replicas: Int(this >= 1 && this <= 100)
memory: DataSize(this >= 256.mb && this <= 16.gb)

Real-World Use Case

A team had 50 YAML files for Kubernetes deployments. A typo in replicas: 30 (should have been 3) caused a $2,000 cloud bill. After switching to Pkl with replicas: Int(this <= 10), such errors are caught at generate time, not in production.

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