Crystal by example: YAML

Like JSON, Crystal has built-in support for the YAML format. You can serialize, decode, and more. This is possible with the yaml module.

YAML strings are created using heredocs. When parsing the YAML string with YAML.parse, it will return a hash map with the contents.

YAML.dump allows creating YAML from Crystal data structures, and this method returns a string with the YAML content. You can use to_yaml to create the exact same behavior.

The generated --- is called frontmatter and is generally used for adding metadata.

require "yaml"

yaml_string = <<-YAML
---
development:
    servers:
        - us-east-2
        - sa-east-1
YAML

parsed = YAML.parse yaml_string
puts parsed, parsed["development"]["servers"][1]

puts YAML.dump({math: true, constants: [3.14, 2.72, 6.28]})

$ crystal run yaml.cr

{"development" => {"servers" => ["us-east-2", "sa-east-1"]}}
sa-east-1
---
math: true
constants:
- 3.14
- 2.72
- 6.28

Like the JSON module (this module is heavily inspired by the JSON module), you can build YAML with YAML.build. The build method takes a block and offers methods for creating arrays or keys.

my_yaml = YAML.build do |yaml|
  yaml.mapping do
    yaml.scalar "production"
    yaml.mapping do
      yaml.scalar "servers"
      yaml.sequence do
        yaml.scalar "us-east-1"
        yaml.scalar "sa-east-1"
      end
    end
  end
end

puts my_yaml

$ crystal run yaml_build.cr

---
production:
  servers:
  - us-east-1
  - sa-east-1