Function merge()

📝 Description

The merge function takes an arbitrary number of maps and combines them into a single map. If a key exists in multiple maps, the value from the last map in the argument list wins (it overwrites the previous ones).

Use Case: Combining global tags (like Owner, Environment) with resource-specific tags. This is the industry standard for tagging in AWS, Azure, and GCP.

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💻 Syntax

merge(map1, map2, map3...)

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💡 Examples

1. Basic Merging:

# Result: { "a" = 1, "b" = 2, "c" = 3 }
output "combined" {
  value = merge({a=1, b=2}, {c=3})
}

2. Overwriting Values (The "Last Wins" Rule):

If the same key appears twice, the rightmost map takes priority.

locals {
  default_tags = {
    Environment = "Dev"
    ManagedBy   = "Terraform"
  }

  extra_tags = {
    Environment = "Prod" # This will overwrite "Dev"
    Project     = "Wiki"
  }
}

# Result: { "Environment" = "Prod", "ManagedBy" = "Terraform", "Project" = "Wiki" }
output "final_tags" {
  value = merge(local.default_tags, local.extra_tags)
}

3. Practical Scenario: Complex Tagging

resource "aws_instance" "web" {
  # ... other config ...

  tags = merge(
    var.common_tags,
    {
      Name = "web-server-01"
      Role = "frontend"
    }
  )
}

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⚠️ Key Details

• All arguments must be maps.

• If you pass an empty map {}, it simply gets ignored and doesn't affect the result.

• Order matters! Always put your "overrides" or more specific maps at the end of the function call.

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🔗 Related Functions

• [[lookup]] — Find a specific value in a map.

• [[keys]] — List all keys from the resulting merged map.

• Official HashiCorp Docs