🔗 Repo: github.com/Push/terraform-aws-labs/day07

🎯 Goal: Master all 7 type constraints — write configs that fail fast on invalid inputs.

✅ You’ll master:

🧠 Why Type Constraints Matter

❌ var.region = 123 → crashes late (during apply)

✅ type = string → fails early (during plan/validate)

Type constraints = self-documenting, self-validating configs.

📦 Type Constraints Cheat Sheet

Category Type Format Use Case Duplicates? Ordered? Index Access?
Primitive string "hello" Names, IDs, tags — — —
number 42, 3.14 Ports, counts, sizes — — —
bool true, false Flags (e.g., monitoring) — — —
Complex list(<TYPE>) ["a", "b"] Ordered lists (AZs, ports) ✅ ✅ ✅ ([0])
set(<TYPE>) ["a", "b"] Unique values (regions, tags) ❌ ❌ ❌ (→ tolist())
map(<TYPE>) {k="v"} Key-value (tags, env vars) Keys ❌ ❌ ❌ (→ ["key"])
tuple([T1, T2]) [42, "tcp"] Fixed-position mixed types ✅ ✅ ✅ ([0])
object({k=T}) {port=443} Structured data (config blocks) Keys ❌ ❌ ❌ (["key"])
Special null null Optional/unset values — — —
any 42, "hi" Legacy/fallback (avoid in prod) — — —

💡 Golden Rule:

“Use the most specific type possible — not any.”

✏️ Hands-On: Type Constraints in Action

1️⃣ Primitive Types — The Basics

variables.tf

# 🔹 string (names, regions)
variable "region" {
  type        = string
  default     = "us-east-1"
  description = "AWS region"
}

# 🔹 number (ports, counts)
variable "instance_count" {
  type        = number
  default     = 1
  description = "Number of EC2 instances"
}

# 🔹 bool (flags)
variable "enable_monitoring" {
  type        = bool
  default     = true
  description = "Enable CloudWatch detailed monitoring"
}

main.tf

resource "aws_instance" "app" {
  count         = var.instance_count          # ← number
  ami           = "ami-0c7217cdde317cfec"
  instance_type = "t3.micro"
  monitoring    = var.enable_monitoring      # ← bool

  tags = {
    Region = var.region                      # ← string
  }
}

✅ Validation:

terraform plan -var="region=123" → ❌ Invalid value for "region": string required.