Deep Merge strategy exception for some of the values

[alandreasyans]

Conditional Deep Merge Control in Terragrunt Includes

Background

Terragrunt's deep merge strategy is a powerful feature for managing configuration across multiple HCL files. However, current implementations lack granular control over variable merging.

Current Limitations

Developers currently face challenges when:

• Wanting to selectively merge some variables deeply
• Needing to completely replace specific variables
• Implementing conditional merge strategies within the same configuration

Detailed Problem Statement

Existing Merge Behavior

When using include with merge_strategy = "deep", Terragrunt performs a comprehensive deep merge of all input variables. This approach doesn't provide flexibility for:

• Partial variable overrides
• Selective merge control
• Complex configuration scenarios

Proposed Solution

Introduce a mechanism to provide per-variable merge control, such as:

• An ignore_deep_merge flag
• Explicit merge strategy configuration
• Conditional merge annotations

Parent source.hcl:

terraform {
  source = "${get_repo_root()}/modules//my_module"
}

inputs = {
  my_list_var1 = ["1","2","3","4"]
  my_list_var2 = ["5","6","7","8"]
}

Child child.hcl:

include "root" {
  path = find_in_parent_folders("root.hcl")
}

include "source" {
  path = "${dirname(find_in_parent_folders("root.hcl"))}/source.hcl"
  merge_strategy = "deep"
}

inputs = {
  # Proposed: Granular merge control
  my_list_var1 = ["x","y"]        # Deep merge (keeps original + new)
  my_list_var2 = ["a","b"]         # Shallow merge (replaces original)
}

Expected Behavior

my_list_var1: ["1","2","3","4", "x", "y"]
my_list_var2: ["a", "b"]

Proposed Syntax Options

Option 1: Inline Annotation

inputs = {
  my_list_var2 = ["a", "b"] # ignore_deep_merge
}

Option 2: Explicit Configuration

inputs = {
  my_list_var2 = {
    value = ["a", "b"]
    ignore_deep_merge = true
  }
}

Option 3: Merge Strategy Decorator

inputs = {
  my_list_var2 = merge(["a", "b"], {
    _merge_strategy = "shallow"
  })
}