dry-struct is a gem built on top of
dry-types which provides virtus-like DSL for defining typed struct classes.
You can define struct objects which will have readers for specified attributes using a simple dsl:
require 'dry-struct' module Types include Dry.Types() end class User < Dry::Struct attribute :name, Types::String.optional attribute :age, Types::Coercible::Integer end user = User.new(name: nil, age: '21') user.name # nil user.age # 21 user = User.new(name: 'Jane', age: '21') user.name # => "Jane" user.age # => 21
optional type means that the value can be nil, not the key in the hash can be skipped.
Dry::Struct::Value is deprecated in 1.2.0. Structs are already meant to be immutable, freezing them doesn't add any value (no pun intended) beyond a bad example of defensive programming.
You can define value objects which will behave like structs but will be deeply frozen:
class Location < Dry::Struct::Value attribute :lat, Types::Float attribute :lng, Types::Float end loc1 = Location.new(lat: 1.23, lng: 4.56) loc2 = Location.new(lat: 1.23, lng: 4.56) loc1.frozen? # true loc2.frozen? # true loc1 == loc2 # true
Dry::Struct out of the box uses hash schemas from
dry-types for processing input hashes.
with_key_transform are exposed as
class User < Dry::Struct transform_keys(&:to_sym) attribute :name, Types::String.optional attribute :age, Types::Coercible::Integer end User.new('name' => 'Jane', 'age' => '21') # => #<User name="Jane" age=21>
This plays nicely with inheritance, you can define a base struct for symbolizing input and then reuse it:
class SymbolizeStruct < Dry::Struct transform_keys(&:to_sym) end class User < SymbolizeStruct attribute :name, Types::String.optional attribute :age, Types::Coercible::Integer end
Validating data with dry-struct
Please don't. Structs are meant to work with valid input, it cannot generate error messages good enough for displaying them for a user etc. Use
dry-validation for validating incoming data and then pass its output to structs.
Differences between dry-struct and virtus
dry-struct look somewhat similar to Virtus but there are few significant differences:
- Structs don't provide attribute writers and are meant to be used as "data objects" exclusively
- Handling of attribute values is provided by standalone type objects from
dry-types, which gives you way more powerful features
- Handling of attribute hashes is provided by standalone hash schemas from
dry-types, which means there are different types of constructors in
- Structs are not designed as swiss-army knives, specific constructor types are used depending on the use case
- Struct classes quack like
dry-types, which means you can use them in hash schemas, as array members or sum them