module YAML

Overview

The YAML module provides serialization and deserialization of YAML to/from native Crystal data structures.

Parsing with #parse and #parse_all

YAML#parse will return an Any, which is a convenient wrapper around all possible YAML 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"

Parsing with YAML#mapping

YAML#mapping defines how an object is mapped to YAML. Mapped data is accessible through generated properties like Foo#bar. It is more type-safe and efficient.

Generating with YAML.build

Use YAML.build, which uses YAML::Builder, to generate YAML by emitting scalars, sequences and mappings:

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"

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.

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

Defined in:

yaml/any.cr
yaml/builder.cr
yaml/mapping.cr
yaml.cr

Class Method Summary

Macro Summary

Class Method Detail

def self.build(io : IO, &block) #

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


[View source]
def self.build(&block) #

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]
def self.dump(object, io : IO) #

Serializes an object to YAML, writing it to io.


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def self.dump(object) : String #

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


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def self.parse(data : String | IO) : Any #

Deserializes a YAML document.

# ./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"
# => }

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def self.parse_all(data : String) : Array(Any) #

Deserializes multiple YAML documents.

# ./foo.yml
foo: bar
---
hello: world
require "yaml"

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

[View source]

Macro Detail

macro mapping(properties, strict = false) #

The YAML.mapping macro defines how an object is mapped to YAML.

It takes named arguments, a named tuple literal or a hash literal as argument, in which attributes and types are defined. Once defined, Object#from_yaml populates properties of the class from the YAML document.

require "yaml"

class Employee
  YAML.mapping(
    title: String,
    name: String,
  )
end

employee = Employee.from_yaml("title: Manager\nname: John")
employee.title # => "Manager"
employee.name  # => "John"

employee.name = "Jenny"
employee.name # => "Jenny"

Attributes not mapped with YAML.mapping are not defined as properties. Also, missing attributes raise a ParseException.

employee = Employee.from_yaml("title: Manager\nname: John\nage: 30")
employee.age # undefined method 'age'. (compile error)

Employee.from_yaml("title: Manager") # raises YAML::ParseException

You can also define attributes for each property.

class Employer
  YAML.mapping(
    title: String,
    name: {
      type:    String,
      nilable: true,
      key:     "firstname",
    },
  )
end

Available attributes:

  • type (required) defines its type. In the example above, title: String is a shortcut to title: {type: String}.
  • nilable defines if a property can be a Nil. Passing T? as a type has the same effect.
  • default: value to use if the property is missing in the YAML document, or if it's null and nilable was not set to true. If the default value creates a new instance of an object (for example [1, 2, 3] or SomeObject.new), a different instance will be used each time a YAML document is parsed.
  • key defines which key to read from a YAML document. It defaults to the name of the property.
  • converter takes an alternate type for parsing. It requires a #from_yaml method in that class, and returns an instance of the given type. Examples of converters are Time::Format and Time::EpochConverter for Time.
  • setter: if true, will generate a setter for the variable, true by default
  • getter: if true, will generate a getter for the variable, true by default
  • presence: if true, a {{key}}_present? method will be generated when the key was present (even if it has a null value), false by default

This macro by default defines getters and setters for each variable (this can be overrided with setter and getter). The mapping doesn't define a constructor accepting these variables as arguments, but you can provide an overload.

The macro basically defines a constructor accepting a YAML::PullParser that reads from it and initializes this type's instance variables.

This macro also declares instance variables of the types given in the mapping.


[View source]
macro mapping #

This is a convenience method to allow invoking YAML.mapping with named arguments instead of with a hash/named-tuple literal.


[View source]