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Function distinct()

📝 Description

The distinct function takes a list and returns a new list with all duplicate elements removed, keeping only the first occurrence of each element.

Use Case: Cleaning up lists of CIDR blocks, removing redundant tags, or ensuring that a load balancer doesn't try to register the same target ID twice.

💻 Syntax

distinct(list)

💡 Examples

1. Basic De-duplication:

# Result: ["a", "b", "c"]
output "unique_list" {
  value = distinct(["a", "b", "a", "c", "b"])
}

2. Cleaning up IP Whitelists:

Imagine you merge two lists of IPs and want to remove overlaps.

locals {
  office_ips = ["1.1.1.1", "2.2.2.2"]
  vpn_ips    = ["2.2.2.2", "3.3.3.3"]

  # Result: ["1.1.1.1", "2.2.2.2", "3.3.3.3"]
  all_unique_ips = distinct(concat(local.office_ips, local.vpn_ips))
}

3. Practical Scenario: Security Groups

If several modules output the same Security Group ID, distinct ensures your instance resource only receives a clean, unique list.

resource "aws_instance" "app" {
  # ... 
  vpc_security_group_ids = distinct(var.sg_ids)
}

⚠️ Key Details

  • Order Preservation: distinct keeps the elements in the order they first appeared.

  • Types Matter: Just like contains(), it treats the number 1 and the string "1" as different elements.

  • Empty Lists: If the input list is empty, it returns an empty list without error.

  • [[concat]] — Combine multiple lists before calling distinct.

  • [[flatten]] — Use this before distinct if you are dealing with nested lists.

  • Official HashiCorp Docs