HCL2 Syntax: Blocks, Arguments, Types & Identifiers

The Big Idea

HashiCorp Configuration Language (HCL2) is neither a general-purpose programming language like Python or Go, nor a static data serialization format like JSON or YAML. It was purposely engineered to balance human readability with powerful expressions, interpolation, and validation.

Every Terraform configuration is built from blocks and arguments. A block has a type, zero or more labels, and a body delimited by curly braces. Inside the body, arguments assign values to identifiers using the equals operator (=).

Understanding how to reference attributes across blocks is the cornerstone of writing modular Terraform. When you declare aws_subnet.public.vpc_id = aws_vpc.main.id, you are not just passing a value: you are telling the Terraform compiler that the subnet cannot be created until the VPC exists.

Core Block Anatomy & Structural Types

# 1. Terraform settings block: defines engine & provider requirements
terraform {
  required_version = ">= 1.9.0"
}

# 2. Local values: compute reusable values and composite objects
locals {
  common_tags = {
    Owner       = "platform-team"
    Environment = "production"
    CostCenter  = "infrastructure"
  }
  
  # Structural object type
  network_config = {
    vpc_cidr     = "10.100.0.0/16"
    subnet_count = 3
    is_public    = true
  }
}

# 3. Resource block with two labels: type and local identifier
resource "aws_vpc" "core" {
  cidr_block           = local.network_config.vpc_cidr
  enable_dns_hostnames = true

  tags = merge(
    local.common_tags,
    { Name = "core-enterprise-vpc" }
  )
}

# 4. Referencing exported attributes
resource "aws_subnet" "public_a" {
  vpc_id            = aws_vpc.core.id # Implicit dependency reference
  cidr_block        = "10.100.1.0/24"
  availability_zone = "us-east-1a"

  tags = merge(local.common_tags, { Name = "public-subnet-1a" })
}