1.10 : Terraform Expressions — Conditional, Dynamic & Splat

🎯 Goal: Replace repetitive or verbose logic with 3 expressive patterns — no functions needed.

✅ You’ll master:

✅ Conditional (condition ? true : false) → Environment-aware configs

✅ Dynamic blocks → Generate nested configs (security groups, IAM policies)

✅ Splat ([*]) → Extract lists from count resources in one line

🧠 Why Expressions > Copy-Paste

❌ Without expressions:

10x ingress blocks for 10 security rules

if env == "prod" logic duplicated across 5 resources

Manual aws_instance.example[0].id, [1].id, [2].id

✅ With expressions:

→ DRY, readable, and scalable configs — before you learn functions.

📦 Expression Cheat Sheet

Expression	Syntax	Use Case	When to Use
Conditional	`cond ? true_val : false_val`	Toggle values by env/flag	One-off decisions (instance size, tags)
Dynamic Block	`dynamic "block" { for_each = ... }`	Generate nested blocks (e.g., `ingress`)	Variable-length nested configs
Splat	`resource[*].attr`	Extract lists from `count` resources	Feed IDs/IPs to other resources

💡 Golden Rule:

“If you’re repeating a block 3+ times — dynamic is calling.”

✏️ Hands-On: Expressions in Action

🔹 Prerequisites (`variables.tf`)

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

variable "instance_count" {
  type    = number
  default = 2
}

# 🔹 For Dynamic Blocks: list of security rules
variable "ingress_rules" {
  type = list(object({
    from_port   = number
    to_port     = number
    protocol    = string
    cidr_blocks = list(string)
    description = string
  }))
  default = [
    {
      from_port   = 80
      to_port     = 80
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
      description = "HTTP"
    },
    {
      from_port   = 443
      to_port     = 443
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
      description = "HTTPS"
    }
  ]
}

1️⃣ Conditional Expression — Environment-Aware EC2

`main.tf`

resource "aws_instance" "app" {
  count = var.instance_count

  ami           = "ami-0c7217cdde317cfec"  # Amazon Linux 2
  # ✅ Conditional: dev = t2.micro, prod = t3.medium
  instance_type = var.env == "dev" ? "t2.micro" : "t3.medium"

  tags = {
    Name        = "app-${count.index}"
    Environment = var.env
  }
}

✅ How it works: