Vagrantfile ruby syntax explanation

Question

I'm confused with the ruby syntax used to configure Vagrant. Especially with this construct. Is this an assignment, a method call, or something else? Is it pure ruby or vagrant specific dialect?

config.vm.network "forwarded_port", guest: 3000, host: 3000

And this one. Is the "ansible" an assignment or an argument, and where |ansible| comes from?

config.vm.provision "ansible" do |ansible|
  ansible.playbook = "provisioners/docker.yml"
end

Where I can find more information about those specific expressions?

Answer 1

Those are DSLs, Ruby is a very good language for writing DSL, take a look a this other question

Although those are DSLs, you can throw vanilla Ruby code outside those blocks, and probably inside as well as long as it gets evaluated.

Answer 2

The Vagrantfile is written in standard Ruby syntax.

Here is an example Vagrantfile

Vagrant.configure("2") do |config|
  # this is an evaluation statement
  # .box is an string attribute
  config.vm.box = "debian/stretch64"

  # this is a method call
  # .synced_folder is a method that takes two positional arguments ('synced_folder', '/vagrant'), 
  # followed by some keyword arguments (disabled: true)
  config.vm.synced_folder 'synced_folder', '/vagrant', disabled: true

  # this is a method call, followed by a "do ... end" block
  # .provider is a method that takes one positional argument (:libvirt)
  config.vm.provider :libvirt do |node|
    # these are two evaluation statements
    node.cpus = 4
    node.memory = 4096
  end
end

From the official documents, you can see the variable "string" in "config.vm.box (string)", but not following the methods config-vm-provider and config-vm-synced_folder.

I was also confused about the Vagrantfile syntax, even after reading the offical documents. I think it's because Ruby looks very different to me compared to other languages I used before.