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module YAML

The YAML module provides serialization and deserialization of YAML version 1.1 to/from native Crystal data structures, with the additional independent types specified in http://yaml.org/type/

Parsing with #parse and #parse_all

YAML.parse will return an Any, which is a convenient wrapper around all possible YAML core types, making it easy to traverse a complex YAML structure but requires some casts from time to time, mostly via some method invocations.

require "yaml"

data = YAML.parse <<-END
         ---
         foo:
           bar:
             baz:
               - qux
               - fox
         END
data["foo"]["bar"]["baz"][1].as_s # => "fox"

YAML.parse can read from an IO directly (such as a file) which saves allocating a string:

require "yaml"

yaml = File.open("path/to/file.yml") do |file|
  YAML.parse(file)
end

Parsing with from_yaml

A type T can be deserialized from YAML by invoking T.from_yaml(string_or_io). For this to work, T must implement new(ctx : YAML::PullParser, node : YAML::Nodes::Node) and decode a value from the given node, using ctx to store and retrieve anchored values (see YAML::PullParser for an explanation of this).

Crystal primitive types, Time, Bytes and Union implement this method. YAML::Serializable can be used to implement this method for user types.

Dumping with YAML.dump or #to_yaml

YAML.dump generates the YAML representation for an object. An IO can be passed and it will be written there, otherwise it will be returned as a string. Similarly, #to_yaml (with or without an IO) on any object does the same.

For this to work, the type given to YAML.dump must implement to_yaml(builder : YAML::Nodes::Builder).

Crystal primitive types, Time and Bytes implement this method. YAML::Serializable can be used to implement this method for user types.

yaml = YAML.dump({hello: "world"})                               # => "---\nhello: world\n"
File.open("foo.yml", "w") { |f| YAML.dump({hello: "world"}, f) } # writes it to the file
# or:
yaml = {hello: "world"}.to_yaml                               # => "---\nhello: world\n"
File.open("foo.yml", "w") { |f| {hello: "world"}.to_yaml(f) } # writes it to the file

Class methods

.build(io : IO, &) : Nil

Writes YAML into the given IO. A YAML::Builder is yielded to the block.

View source

.build

Returns the resulting String of writing YAML to the yielded YAML::Builder.

require "yaml"

string = YAML.build do |yaml|
  yaml.mapping do
    yaml.scalar "foo"
    yaml.sequence do
      yaml.scalar 1
      yaml.scalar 2
    end
  end
end
string # => "---\nfoo:\n- 1\n- 2\n"
View source

.dump(object, io : IO)

Serializes an object to YAML, writing it to io.

View source

.dump(object) : String

Serializes an object to YAML, returning it as a String.

View source

.libyaml_version : SemanticVersion

Returns the used version of libyaml.

View source

.parse(data : String | IO) : Any

Deserializes a YAML document according to the core schema.

# ./foo.yml
data:
  string: "foobar"
  array:
    - John
    - Sarah
  hash: {key: value}
  paragraph: |
    foo
    bar

require "yaml"

YAML.parse(File.read("./foo.yml"))
# => {
# => "data" => {
# => "string" => "foobar",
# => "array" => ["John", "Sarah"],
# => "hash" => {"key" => "value"},
# => "paragraph" => "foo\nbar\n"
# => }
View source

.parse_all(data : String) : Array(Any)

Deserializes multiple YAML documents according to the core schema.

# ./foo.yml
foo: bar
---
hello: world

require "yaml"

YAML.parse_all(File.read("./foo.yml"))
# => [{"foo" => "bar"}, {"hello" => "world"}]
View source