Watt TF

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Deep Merge ​

By default, when a transformation target points to an existing map or object, watt-tf performs a deep merge instead of completely overwriting the structure. This allows you to inject or update specific nested keys while preserving the rest of the existing data structure.

How It Works ​

When a transform block is executed, the engine checks the data type of the resolved value:

  1. Scalar Values / Arrays: Overwrite the target path.
  2. Maps (map[string]any): The engine recursively traverses the keys. If a key exists in both the original data and the transformation value, it drills deeper. If it encounters a conflict between a map and a scalar, the transformation value takes precedence.

Concrete Example ​

Imagine you have a Terraform configuration with existing tags, and you want to inject a globally managed environment tag, plus some team-specific infrastructure details.

1. Input Data ​

yaml
environment: "Production"
ami: "ami-0c55b159cbfafe1f0"
instance_type: "t2.micro"
tags:
  Name: "web-server"
  Project: "Phoenix"

2. Configuration (blueprint.yaml) ​

yaml
transform:
  - target: "resource.aws_instance.web"
    value:
      ami: ${input.ami}
      instance_type: ${input.instance_type}
      tags:
        Name: ${input.tags.Name}
        Project: ${input.tags.Project}
  
  - target: "resource.aws_instance.web.tags"
    if: input.environment == "Production"
    value:
      Environment: "Production"
      `[company.com/managed-by](https://company.com/managed-by)`: "Watt TF"
      Project: "Phoenix-Prod" # Overwrites the existing Project key

3. Resulting Output ​

The existing keys (Name) are preserved, the conflicting key (Project) is updated, and new keys (Environment, company.com/managed-by) are seamlessly injected:

json
{
  "resource": {
    "aws_instance": {
      "web": {
        "ami": "ami-0c55b159cbfafe1f0",
        "instance_type": "t2.micro",
        "tags": {
          "Name": "web-server",
          "Project": "Phoenix-Prod",
          "Environment": "Production",
          "[company.com/managed-by](https://company.com/managed-by)": "Watt TF"
        }
      }
    }
  }
}

Nested Deep Merging

Deep merging works across multiple nested layers. If your value block mirrors the structure of the input target, Watt TF will safely navigate down the tree.