Terraform Locals, Functions & Expressions: Writing Clean, DRY Infrastructure Code

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What Are Locals in Terraform?

Local values let you define a named value once, and use it multiple times throughout your configuration.

Instead of repeating:

resource "aws_s3_bucket" "logs" {
  bucket = "${var.project_name}-${var.env}-logs"
}

You can define it cleanly:

locals {
  bucket_name = "${var.project_name}-${var.env}-logs"
}

resource "aws_s3_bucket" "logs" {
  bucket = local.bucket_name
}

Now you have a central place to update naming logic or calculated values.

Common Use Cases for Locals

• Naming conventions (e.g., app-environment-region)

• Shared tags

• Common resource names

• Complex expressions simplified for reuse

• Nested conditionals or long functions extracted for clarity

Terraform Functions Overview

Terraform has a rich set of built-in functions to transform data. They're grouped into:

🧪 Example 1: Naming Standard

locals {
  resource_name = format("%s-%s-%s", var.project, var.env, var.region)
}

Output: myapp-prod-us-east-1

🧪 Example 2: Shared Tags

locals {
  common_tags = {
    project   = var.project
    env       = var.env
    managedby = "terraform"
  }
}

resource "aws_s3_bucket" "logs" {
  bucket = "my-bucket"
  tags   = local.common_tags
}

Expressions in Terraform

Expressions are combinations of values and operators that compute a value.

You’ve seen these already in action:

• Interpolation: "${var.region}-logs"

• Conditional: var.enabled ? "on" : "off"

• For loops: for x in var.tags : upper(x)

• Merging maps or lists

🔁 For Expressions

locals {
  upper_tags = [for tag in var.tags : upper(tag)]
}

❓ Conditional Expression

locals {
  instance_type = var.env == "prod" ? "t3.large" : "t3.micro"
}

This replaces multi-block logic with elegant, readable one-liners.

Hands-On Example: Smarter Infra Config with Locals + Functions

variables.tf

variable "env" {
  type    = string
  default = "dev"
}

variable "project" {
  type    = string
  default = "mlplatform"
}

variable "region" {
  type    = string
  default = "us-east-1"
}

locals.tf

locals {
  standard_name = format("%s-%s-%s", var.project, var.env, var.region)

  common_tags = {
    project = var.project
    env     = var.env
    owner   = "ml-team"
  }

  instance_type = var.env == "prod" ? "t3.large" : "t3.micro"
}

main.tf

resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = local.instance_type
  tags          = local.common_tags
}

Your config is now clean, DRY, and easily reusable across environments.

Common Pitfalls / Pro Tips

✅ Name your locals logically, as you would variables
 ✅ Use format() instead of string concatenation
 ✅ Don’t overuse locals — they’re for logic, not input values
 ✅ Split locals into locals.tf file for clarity
 ✅ Use can() and try() for defensive logic in expressions

💡 Tip of the Day:

📚 Resources & References

1️⃣ Terraform Locals Documentation
🔗 Docs
How to define reusable values inside your configuration.

2️⃣ Terraform Functions List
🔗 Docs
Full list of built-in string, numeric, list, and utility functions.

3️⃣ Expressions in Terraform
🔗 Docs
Guide on using operators, loops, conditionals, and interpolations.

4️⃣ Best Practices for Reusability
🔗 HashiCorp Learn
Clean, modular infra code with locals and functions.

5️⃣ Using can() and try() in Terraform
🔗 Defensive Programming
How to write safe, fault-tolerant expressions.

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