🎯 Goal: Move from expressions → functions — reusable, built-in logic for safe, DRY configs.

✅ You’ll master:

• ✅ String: lower(), replace(), substr(), trim()
• ✅ Numeric: max(), min(), abs()
• ✅ Collection: length(), concat(), merge()
• ✅ Type Conversion: toset(), tonumber(), tostring()
• ✅ Lookup: lookup() for env-specific configs
• ✅ for expressions (not for_each!) for list/map transforms

🧠 Why Functions Matter

❌ Without functions:

• Manual S3 bucket name cleanup → bucket = "Project Alpha!!!" → apply fails
• Copy-paste tags = { Env = "dev", Cost = "eng" } across 20 resources
• Hardcoded instance_type = "t3.medium" in prod

✅ With functions:

→ Infrastructure that self-corrects invalid inputs

→ Truly DRY configs (no repeated logic)

→ Environment-aware resources (dev/staging/prod)

💡 Key Insight:

Terraform has 100+ built-in functions — but no custom functions.

→ Use them like LEGO blocks — small, composable, powerful.

📦 Function Cheat Sheet (Part 1)

🎯 Golden Rule:

“Chain functions like shell pipes: lower(replace(...)) → clean, readable, safe.”

✏️ Hands-On: Functions in Action

🔹 Prerequisites (variables.tf)

variable "project_name" {
  type    = string
  default = "Project Alpha Resource!!"
}

variable "bucket_name_raw" {
  type    = string
  default = "Project Alpha Storage!! 123"
}

variable "allowed_ports_str" {
  type    = string
  default = "80,443,8080,3306"
}

variable "instance_sizes" {
  type = map(string)
  default = {
    dev      = "t2.micro"
    staging  = "t3.small"
    prod     = "t3.large"
  }
}

variable "default_tags" {
  type = map(string)
  default = {
    ManagedBy = "Terraform"
    Company   = "TechCorp"
  }
}

variable "env_tags" {
  type = map(string)
  default = {
    Environment = "dev"
    CostCenter  = "engineering"
  }
}

1️⃣ String Functions — S3 Bucket Name Sanitization