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HCL syntax

Chapter objectives​

  • Understand the HCL syntax
  • Master the data types
  • Use expressions and functions
  • Write clean HCL code

1 - Introduction to HCL​

What is HCL?​

HCL (HashiCorp Configuration Language) is a declarative language designed to be:

  • Human-readable
  • Easily editable
  • JSON-compatible

Basic structure​

# Commentaire sur une ligne

/* Commentaire
   sur plusieurs
   lignes */

# Bloc avec arguments
type "label" "name" {
  argument1 = "valeur"
  argument2 = 123
  
  # Bloc imbriqué
  nested_block {
    nested_arg = true
  }
}

2 - Types of blocks​

Resource​

# Crée une ressource dans le cloud
resource "aws_instance" "web" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
  
  tags = {
    Name = "WebServer"
  }
}

Variable​

# Déclare une variable d'entrée
variable "instance_type" {
  description = "Type d'instance EC2"
  type        = string
  default     = "t2.micro"
}

Output​

# Exporte une valeur
output "instance_ip" {
  description = "IP publique de l'instance"
  value       = aws_instance.web.public_ip
}

Provider​

# Configure un provider
provider "aws" {
  region = "eu-west-1"
}

Locals​

# Définit des valeurs locales
locals {
  environment = "production"
  common_tags = {
    Environment = local.environment
    Project     = "MyProject"
  }
}

3 - Data types​

Primitive types​

# String
name = "hello"
message = "Hello, ${var.name}!"

# Number
count = 42
price = 19.99

# Bool
enabled = true
disabled = false

Complex types​

# List (ordered collection)
availability_zones = ["eu-west-1a", "eu-west-1b", "eu-west-1c"]

# Map (key-value pairs)
tags = {
  Name        = "WebServer"
  Environment = "Production"
}

# Set (unique values, no order)
unique_ports = toset([80, 443, 8080])

# Object (structured type)
server_config = {
  name   = "web-01"
  cpu    = 4
  memory = 8
}

# Tuple (fixed-length list with specific types)
mixed = ["hello", 42, true]

Type declaration​

variable "string_var" {
  type = string
}

variable "number_var" {
  type = number
}

variable "bool_var" {
  type = bool
}

variable "list_var" {
  type = list(string)
}

variable "map_var" {
  type = map(string)
}

variable "object_var" {
  type = object({
    name    = string
    age     = number
    enabled = bool
  })
}

variable "any_var" {
  type = any
}

4 - References and expressions​

Reference resources​

# Référence simple
resource "aws_instance" "web" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
  subnet_id     = aws_subnet.main.id  # Référence
}

resource "aws_subnet" "main" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
}

Types of references​

# Ressource
aws_instance.web.id
aws_instance.web.public_ip

# Variable
var.instance_type
var.environment

# Local
local.common_tags
local.environment

# Data source
data.aws_ami.ubuntu.id

# Module output
module.vpc.vpc_id

# Provider
provider.aws

Interpolation​

# Interpolation de variables
name = "server-${var.environment}-${count.index}"

# Interpolation conditionnelle
instance_type = var.environment == "production" ? "t2.large" : "t2.micro"

# Interpolation avec fonctions
bucket_name = lower("MyBucket-${random_id.bucket.hex}")

5 - Operators​

Arithmetic​

# Addition, soustraction, multiplication, division
total = 10 + 5      # 15
diff  = 10 - 5      # 5
prod  = 10 * 5      # 50
quot  = 10 / 5      # 2
mod   = 10 % 3      # 1 (modulo)
neg   = -10         # -10

Comparison​

# Égalité et inégalité
equal     = var.env == "prod"
not_equal = var.env != "dev"

# Comparaison numérique
greater = 10 > 5
less    = 5 < 10
gte     = 10 >= 10
lte     = 5 <= 10

Logical​

# AND, OR, NOT
both   = var.enabled && var.ready
either = var.primary || var.secondary
negate = !var.disabled

6 - Conditional expressions​

Ternary​

# condition ? true_value : false_value
instance_type = var.environment == "production" ? "t2.large" : "t2.micro"

# Imbriqué
size = var.env == "prod" ? "large" : (var.env == "staging" ? "medium" : "small")

With count​

resource "aws_instance" "web" {
  count = var.create_instance ? 1 : 0
  
  ami           = "ami-12345678"
  instance_type = "t2.micro"
}

With for_each​

resource "aws_instance" "web" {
  for_each = var.create_instances ? var.instances : {}
  
  ami           = each.value.ami
  instance_type = each.value.type
}

7 - Loops and iterations​

for expression​

# Transformer une liste
upper_names = [for name in var.names : upper(name)]
# ["alice", "bob"] -> ["ALICE", "BOB"]

# Filtrer une liste
adults = [for p in var.people : p.name if p.age >= 18]

# Créer une map depuis une liste
name_map = {for s in var.servers : s.id => s.name}
# [{id="1", name="web"}] -> {"1" = "web"}

# Transformer une map
upper_tags = {for k, v in var.tags : k => upper(v)}

count​

resource "aws_instance" "web" {
  count = 3
  
  ami           = "ami-12345678"
  instance_type = "t2.micro"
  
  tags = {
    Name = "web-${count.index}"  # web-0, web-1, web-2
  }
}

# Accéder aux instances
output "instance_ids" {
  value = aws_instance.web[*].id
}

for_each​

# Avec un set
resource "aws_iam_user" "users" {
  for_each = toset(["alice", "bob", "charlie"])
  
  name = each.value
}

# Avec une map
resource "aws_instance" "servers" {
  for_each = {
    web = { type = "t2.micro", ami = "ami-web" }
    api = { type = "t2.small", ami = "ami-api" }
    db  = { type = "t2.medium", ami = "ami-db" }
  }
  
  ami           = each.value.ami
  instance_type = each.value.type
  
  tags = {
    Name = each.key
  }
}

8 - Built-in functions​

String functions​

# Manipulation de strings
lower("HELLO")                    # "hello"
upper("hello")                    # "HELLO"
title("hello world")              # "Hello World"
trim("  hello  ")                 # "hello"
trimprefix("helloworld", "hello") # "world"
trimsuffix("helloworld", "world") # "hello"
replace("hello", "l", "x")        # "hexxo"
split(",", "a,b,c")               # ["a", "b", "c"]
join("-", ["a", "b", "c"])        # "a-b-c"
format("Hello, %s!", "World")     # "Hello, World!"
substr("hello", 0, 3)             # "hel"

Numeric functions​

abs(-5)           # 5
ceil(4.2)         # 5
floor(4.8)        # 4
max(1, 5, 3)      # 5
min(1, 5, 3)      # 1
pow(2, 3)         # 8
signum(-5)        # -1

Collection functions​

# Listes
length([1, 2, 3])                 # 3
element(["a", "b", "c"], 1)       # "b"
index(["a", "b", "c"], "b")       # 1
contains(["a", "b"], "a")         # true
concat([1, 2], [3, 4])            # [1, 2, 3, 4]
flatten([[1, 2], [3, 4]])         # [1, 2, 3, 4]
distinct([1, 1, 2, 2, 3])         # [1, 2, 3]
reverse([1, 2, 3])                # [3, 2, 1]
sort(["c", "a", "b"])             # ["a", "b", "c"]
slice([1, 2, 3, 4], 1, 3)         # [2, 3]
range(1, 5)                       # [1, 2, 3, 4]

# Maps
keys({a = 1, b = 2})              # ["a", "b"]
values({a = 1, b = 2})            # [1, 2]
lookup({a = 1}, "a", 0)           # 1
lookup({a = 1}, "b", 0)           # 0
merge({a = 1}, {b = 2})           # {a = 1, b = 2}

File functions​

file("script.sh")                 # Contenu du fichier
fileexists("config.yaml")         # true/false
filebase64("image.png")           # Base64 du fichier
templatefile("template.tpl", {
  name = "World"
})                                # Fichier rendu

Type functions​

tostring(123)                     # "123"
tonumber("123")                   # 123
tobool("true")                    # true
tolist(toset(["a", "b"]))        # ["a", "b"]
tomap({a = 1})                    # {a = 1}
toset([1, 1, 2])                  # toset([1, 2])
try(var.value, "default")         # var.value ou "default"
can(var.value.nested)             # true si accessible

9 - Dynamic blocks​

Syntax​

resource "aws_security_group" "example" {
  name = "example"

  dynamic "ingress" {
    for_each = var.ingress_rules
    content {
      from_port   = ingress.value.from_port
      to_port     = ingress.value.to_port
      protocol    = ingress.value.protocol
      cidr_blocks = ingress.value.cidr_blocks
    }
  }
}

Associated variable​

variable "ingress_rules" {
  type = list(object({
    from_port   = number
    to_port     = number
    protocol    = string
    cidr_blocks = list(string)
  }))
  
  default = [
    {
      from_port   = 80
      to_port     = 80
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    },
    {
      from_port   = 443
      to_port     = 443
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    }
  ]
}

10 - HCL best practices​

Formatting​

# Formater automatiquement
terraform fmt

# Vérifier le formatage
terraform fmt -check

Naming​

# ✅ Bon - snake_case
resource "aws_instance" "web_server" {}
variable "instance_count" {}

# ❌ Mauvais
resource "aws_instance" "WebServer" {}
variable "instanceCount" {}

Organization​

# ✅ Un fichier par type
# variables.tf - toutes les variables
# outputs.tf - tous les outputs
# main.tf - ressources principales

# ✅ Grouper les ressources liées
# networking.tf - VPC, subnets, routes
# compute.tf - EC2, ASG
# database.tf - RDS, ElastiCache

Summary​

Key points
  • HCL is declarative and readable
  • Use the appropriate types for validation
  • for expressions enable powerful transformations
  • count and for_each to create multiple resources
  • Always format with terraform fmt

Practical exercises​

  1. Create variables of different types
  2. Use a for expression to transform a list
  3. Create resources with for_each
  4. Explore the string functions

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