Using expr

Use the expr expression language in bricks.json for dynamic configuration, conditional logic, string operations, and runtime property references.

Bluebricks leverages the expr expression language to enable dynamic configurations within your bricks.json files. This allows for powerful manipulations, conditional logic, and more.

What is expr?

expr is a lightweight, high-performance expression language for Go, designed for dynamic evaluation of expressions. In Bluebricks, it lets you:

• Perform arithmetic and string operations

• Use conditional logic (if/else, ternary)

• Access built-in functions for manipulating strings, lists, maps, etc.

• Reference properties, data, and secrets dynamically

Common Use Cases

• Basic arithmetic and string operations: Concatenation, addition, subtraction, etc.

• Conditional expressions: Ternary operators or if/else logic.

• Functions: Built-in functions for manipulating strings, lists, maps, etc.

Examples

1. Concatenating strings and using current time

{
  "packages": [
    {
      "name": "my_s3_bucket",
      "version": "1.0.0",
      "props": {
        "bucket_name": {
            "value": "now().Format('02012006030405')+'_sample_bucket'+(Props.vpc_id ?? Data.vpc1.vpc_id)"
        }
      }
    }
  ]
}

• now().Format('02012006030405') generates a timestamp string.

• '_sample_bucket' is a static string.

• (Props.vpc_id ?? Data.vpc1.vpc_id) uses the null coalescing operator: it will use Props.vpc_id if defined, otherwise Data.vpc1.vpc_id.

2. Conditional Logic (Ternary Operator)

{
  "props": {
    "environment": {
      "type": "string",
      "default": "dev"
    }
  },
  "packages": [
    {
      "name": "my_instance",
      "version": "1.0.0",
      "props": {
        "instance_type": {
            "value": "Props.environment == 'prod' ? 'm5.xlarge' : 't3.medium'"
      }
    }
  ]
}

This sets instance_type to m5.xlarge if Props.environment is prod, otherwise t3.medium.

3. Accessing nested properties and list elements

{
  "props": {
    "network_config": {
      "type": "object",
      "default": {
        "subnets": ["subnet-abc", "subnet-def"],
        "security_groups": ["sg-123"]
      }
    }
  },
  "packages": [
    {
      "name": "my_service",
      "version": "1.0.0",
      "props": {
        "subnet_id": { "value": "Props.network_config.subnets[0]" },
        "security_group_id": { "value": "Props.network_config.security_groups[0]" }
      }
    }
  ]
}

Further Reading

• expr language documentation

• expr playground

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For more advanced usage, see the official expr documentation or try out expressions in the playground above.