Splitting cfndsl templates into multiple files

template.rb

require_relative "subtemplate.rb"

CloudFormation {
  instance_type = external_parameters.fetch(:instance_type, "t2.micro")

  my_instance(instance_type)
}

subtemplate.rb

def my_instance(instance_type)
  EC2_Instance(:MyInstance) {
    InstanceType instance_type
  }
end

params.yml

instance_type: m4.xlarge

run.sh

$ cfndsl -p -y params.yml template.rb
{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Resources": {
    "MyInstance": {
      "Properties": {
        "InstanceType": "m4.xlarge"
      },
      "Type": "AWS::EC2::Instance"
    }
  }
}