/Crystal 0.31

module YAML::Serializable


The YAML::Serializable module automatically generates methods for YAML serialization when included.


require "yaml"

class Location
  include YAML::Serializable

  @[YAML::Field(key: "lat")]
  property latitude : Float64

  @[YAML::Field(key: "lng")]
  property longitude : Float64

class House
  include YAML::Serializable
  property address : String
  property location : Location?

house = House.from_yaml(%({"address": "Crystal Road 1234", "location": {"lat": 12.3, "lng": 34.5}}))
house.address  # => "Crystal Road 1234"
house.location # => #<Location:0x10cd93d80 @latitude=12.3, @longitude=34.5>
house.to_yaml  # => "---\naddress: Crystal Road 1234\nlocation:\n  lat: 12.3\n  lng: 34.5\n"

houses = Array(House).from_yaml("---\n- address: Crystal Road 1234\n  location:\n    lat: 12.3\n    lng: 34.5\n")
houses.size    # => 1
houses.to_yaml # => "---\n- address: Crystal Road 1234\n  location:\n    lat: 12.3\n    lng: 34.5\n"


Including YAML::Serializable will create #to_yaml and self.from_yaml methods on the current class, and a constructor which takes a YAML::PullParser. By default, these methods serialize into a yaml object containing the value of every instance variable, the keys being the instance variable name. Most primitives and collections supported as instance variable values (string, integer, array, hash, etc.), along with objects which define to_yaml and a constructor taking a YAML::PullParser. Union types are also supported, including unions with nil. If multiple types in a union parse correctly, it is undefined which one will be chosen.

To change how individual instance variables are parsed and serialized, the annotation YAML::Field can be placed on the instance variable. Annotating property, getter and setter macros is also allowed.

require "yaml"

class A
  include YAML::Serializable

  @[YAML::Field(key: "my_key", emit_null: true)]
  getter a : Int32?

YAML::Field properties:

  • ignore: if true skip this field in seriazation and deserialization (by default false)
  • key: the value of the key in the yaml object (by default the name of the instance variable)
  • converter: specify an alternate type for parsing and generation. The converter must define from_yaml(YAML::PullParser) and to_yaml(value, YAML::Builder) as class methods. Examples of converters are Time::Format and Time::EpochConverter for Time.
  • presence: if true, a @{{key}}_present instance variable will be generated when the key was present (even if it has a null value), false by default
  • emit_null: if true, emits a null value for nilable property (by default nulls are not emitted)

Deserialization also respects default values of variables:

require "yaml"

struct A
  include YAML::Serializable
  @a : Int32
  @b : Float64 = 1.0

A.from_yaml("---\na: 1\n") # => A(@a=1, @b=1.0)

Extensions: YAML::Serializable::Strict and YAML::Serializable::Unmapped.

If the YAML::Serializable::Strict module is included, unknown properties in the YAML document will raise a parse exception. By default the unknown properties are silently ignored. If the YAML::Serializable::Unmapped module is included, unknown properties in the YAML document will be stored in a Hash(String, YAML::Any). On serialization, any keys inside yaml_unmapped will be serialized appended to the current yaml object.

require "yaml"

struct A
  include YAML::Serializable
  include YAML::Serializable::Unmapped
  @a : Int32

a = A.from_yaml("---\na: 1\nb: 2\n") # => A(@yaml_unmapped={"b" => 2_i64}, @a=1)
a.to_yaml                            # => "---\na: 1\nb: 2\n"

Class annotation YAML::Serializable::Options

supported properties:

  • emit_nulls: if true, emits a null value for all nilable properties (by default nulls are not emitted)
require "yaml"

@[YAML::Serializable::Options(emit_nulls: true)]
class A
  include YAML::Serializable
  @a : Int32?

Defined in:



Instance Method Summary

Constructor Detail

def self.new(*, __context_for_yaml_serializable ctx : YAML::ParseContext, __node_for_yaml_serializable node : YAML::Nodes::Node)Source

Instance Method Detail

def to_yaml(yaml : YAML::Nodes::Builder)Source

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Licensed under the Apache License, Version 2.0.