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]" }
      }
    }
  ]
}

4. Null coalescing for fallbacks

Use the ?? operator to provide fallback values:

{
  "packages": [
    {
      "name": "@bluebricks/terraform_aws_subnet",
      "id": "web_subnet",
      "props": {
        "vpc_id": {
          "value": "Props.vpc_id ?? Data.shared_vpc.vpc_id"
        },
        "availability_zone": {
          "value": "Props.az ?? 'us-west-2a'"
        }
      }
    }
  ]
}

Props.vpc_id ?? Data.shared_vpc.vpc_id uses Props.vpc_id if defined, otherwise falls back to Data.shared_vpc.vpc_id.

5. String concatenation

Build complex strings from multiple sources:

{
  "packages": [
    {
      "name": "@bluebricks/terraform_aws_route53",
      "id": "dns_record",
      "props": {
        "domain_name": {
          "value": "Props.service_name + '.' + Props.environment + '.' + Props.base_domain"
        }
      }
    }
  ]
}

6. Mathematical operations

Perform calculations for resource sizing:

{
  "packages": [
    {
      "name": "@bluebricks/terraform_aws_instance",
      "id": "web_servers",
      "props": {
        "count": {
          "value": "Props.environment == 'prod' ? Props.base_instance_count * 2 : Props.base_instance_count"
        }
      }
    }
  ]
}

7. Complex conditional logic

Combine multiple conditions:

{
  "packages": [
    {
      "name": "@bluebricks/terraform_aws_alb",
      "id": "load_balancer",
      "props": {
        "ssl_certificate_arn": {
          "value": "Props.enable_ssl && Props.environment == 'prod' ? Secrets.prod_ssl_cert : (Props.enable_ssl ? Secrets.dev_ssl_cert : '')"
        }
      }
    }
  ]
}

8. Output aggregation with expressions

Combine outputs from multiple packages:

{
  "outs": {
    "all_endpoints": {
      "value": "Data.web_server.endpoint + ',' + Data.api_server.endpoint + ',' + Data.admin_server.endpoint",
      "type": "string",
      "description": "All service endpoints"
    },
    "monitoring_dashboard": {
      "value": "Props.enable_monitoring ? Data.monitoring.dashboard_url : 'Monitoring disabled'",
      "type": "string",
      "description": "Monitoring dashboard URL or status"
    }
  }
}

Best practices

• Use expressions for dynamic values, not static ones

• Keep expressions readable; avoid deeply nested ternaries

• Use null coalescing (??) for safe fallbacks

• Test complex expressions with different input combinations

Further reading

• expr language documentation

• expr playground

• Blueprint Composition Patterns: architectural patterns that use expressions in context