module YAML::Serializable

Overview

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

Example

require "yaml"

class Location
  include YAML::Serializable

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

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

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

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"

Usage

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?
end

YAML::Field properties:

• ignore: if true skip this field in serialization and deserialization (by default false)
• ignore_serialize: If truthy, skip this field in serialization (default: false). The value can be any Crystal expression and is evaluated at runtime.
• ignore_deserialize: if true skip this field in 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::ParseContext, YAML::Nodes::Node) and to_yaml(value, YAML::Nodes::Builder). Examples of converters are a Time::Format instance 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
end

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

NOTE YAML::Serializable defines an internal constructor on any including type, which means the default constructor (def initialize; end) is absent unless explicitly defined by the user, even when all instance variables have a default initializer.

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
end

a = A.from_yaml("---\na: 1\nb: 2\n") # => A(@yaml_unmapped={"b" => 2}, @a=1)
a.yaml_unmapped["b"].raw.class       # => Int64
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?
end

Discriminator field

A very common YAML serialization strategy for handling different objects under a same hierarchy is to use a discriminator field. For example in GeoJSON each object has a "type" field, and the rest of the fields, and their meaning, depend on its value.

You can use YAML::Serializable.use_yaml_discriminator for this use case.

after_initialize method

#after_initialize is a method that runs after an instance is deserialized from YAML. It can be used as a hook to post-process the initialized object.

Example:

require "yaml"

class Person
  include YAML::Serializable
  getter name : String

  def after_initialize
    @name = @name.upcase
  end
end

person = Person.from_yaml "---\nname: Jane\n"
person.name # => "JANE"

Defined in:

Macro Summary

• use_yaml_discriminator(field, mapping)

  Tells this class to decode YAML by using a field as a discriminator.

Instance Method Summary

• #to_yaml(yaml : YAML::Nodes::Builder)

Macro Detail

require "yaml"

abstract class Shape
  include YAML::Serializable

  use_yaml_discriminator "type", {point: Point, circle: Circle}

  property type : String
end

class Point < Shape
  property x : Int32
  property y : Int32
end

class Circle < Shape
  property x : Int32
  property y : Int32
  property radius : Int32
end

Shape.from_yaml(%(
  type: point
  x: 1
  y: 2
)) # => #<Point:0x10373ae20 @type="point", @x=1, @y=2>

Shape.from_yaml(%(
  type: circle
  x: 1
  y: 2
  radius: 3
)) # => #<Circle:0x106a4cea0 @type="circle", @x=1, @y=2, @radius=3>

Instance Method Detail