For / If Expressions

Carina supports for and if expressions to dynamically generate resources. This guide shows you how to iterate over collections and conditionally create resources.

For expressions

A for expression creates multiple resources by iterating over a list or map.

Iterating over a list

The simplest form iterates over a list of values:

provider awscc {
  region = awscc.Region.ap_northeast_1
}

let vpcs = for env in ['dev', 'stg'] {
  awscc.ec2.Vpc {
    cidr_block = '10.0.0.0/16'

    tags = {
      Name        = "vpc-${env}"
      Environment = env
    }
  }
}

This creates two VPCs, one for each environment.

Indexed iteration

Use the (index, value) form to access the iteration index:

let vpcs = for (i, env) in ['dev', 'stg'] {
  awscc.ec2.Vpc {
    cidr_block = cidr_subnet('10.0.0.0/8', 8, i)

    tags = {
      Name        = "vpc-${env}"
      Environment = env
    }
  }
}

The index i starts at 0. Here it is used with cidr_subnet to assign different CIDR blocks to each VPC.

Iterating over a map

Use key, value binding to iterate over map entries:

let cidrs = {
  dev = '10.0.0.0/16'
  stg = '10.1.0.0/16'
}

let vpcs = for name, cidr in cidrs {
  awscc.ec2.Vpc {
    cidr_block = cidr

    tags = {
      Name        = "vpc-${name}"
      Environment = name
    }
  }
}

Accessing for expression results

The result of a for expression is a list. You can access individual elements with index syntax:

let vpcs = for env in ['dev', 'stg'] {
  awscc.ec2.Vpc {
    cidr_block = '10.0.0.0/16'

    tags = {
      Name = "vpc-${env}"
    }
  }
}

# Reference the first VPC's attributes
awscc.ec2.Subnet {
  vpc_id            = vpcs[0].vpc_id
  cidr_block        = '10.0.1.0/24'
  availability_zone = 'ap-northeast-1a'

  tags = {
    Name = 'dev-subnet'
  }
}

Local variables in for body

You can define local let bindings inside a for body:

let vpc = awscc.ec2.Vpc {
  cidr_block = '10.0.0.0/16'
}

let subnets = for (i, az) in ['ap-northeast-1a', 'ap-northeast-1c'] {
  let cidr = cidr_subnet('10.0.0.0/16', 8, i)

  awscc.ec2.Subnet {
    vpc_id            = vpc.vpc_id
    cidr_block        = cidr
    availability_zone = az

    tags = {
      Name = "subnet-${az}"
    }
  }
}

For with modules

You can call modules inside for expressions to create multiple instances of a module. See the Using Modules guide for a full example.

If expressions

An if expression conditionally creates a resource or selects a value.

Conditional resources

Create a resource only when a condition is true:

let enabled = true

let vpc = if enabled {
  awscc.ec2.Vpc {
    cidr_block = '10.0.0.0/16'

    tags = {
      Name = 'conditional-vpc'
    }
  }
}

When enabled is false, no VPC is created and the let binding is empty.

Conditional values with if/else

Use if/else as a value expression to choose between two values:

let is_production = true

awscc.ec2.Vpc {
  cidr_block = if is_production { '10.0.0.0/16' } else { '172.16.0.0/16' }

  tags = {
    Name = if is_production { 'prod-vpc' } else { 'dev-vpc' }
  }
}

This creates a single VPC but uses different CIDR blocks and names depending on the condition.

Combining for and if

You can use if expressions inside for bodies and vice versa:

let environments = {
  dev = '10.0.0.0/16'
  stg = '10.1.0.0/16'
  prd = '10.2.0.0/16'
}

let vpcs = for name, cidr in environments {
  awscc.ec2.Vpc {
    cidr_block           = cidr
    enable_dns_hostnames = if name == 'prd' { true } else { false }

    tags = {
      Name        = "vpc-${name}"
      Environment = name
    }
  }
}