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A simple and extendible type system for Ruby with support for kernel coercions, form coercions, sum types, constrained types and default-value types.

Used by:


dry-data vs virtus

Virtus has been a successful library, unfortunately it is "only" a by-product of an ActiveRecord ORM which carries many issues typical to ActiveRecord-like features that we all know from Rails, especially when it comes to very complicated coercion logic, mixing unrelated concerns, polluting application layer with concerns that should be handled at the bounderies etc.

dry-data has been created to become a better tool that solves similar (but not identical!) problems related to type-safety and coercions. It is a superior solution because:


Add this line to your application's Gemfile:

gem 'dry-data'

And then execute:

$ bundle

Or install it yourself as:

$ gem install dry-data


You can use dry-data for defining various data types in your application, like domain entities and value objects or hashes with coercible values used to handle params.

Built-in types are grouped under 5 categories:

  • default: pass-through without any checks
  • strict - doesn't coerce and checks the input type against the primitive class
  • coercible - tries to coerce and raises type-error if it failed
  • form - non-strict coercion types suitable for form params
  • maybe - accepts either a nil or something else

Configuring Types Module

In dry-data a type is an object with a constructor that knows how to handle input. On top of that there are high-level types like a sum-type, constrained type, optional type or default value type.

To acccess all the built-in type objects you can configure dry-data with a namespace module:

module Types

Dry::Data.configure do |config|
  config.namespace = Types

# after defining your custom types (if you've got any) you can finalize setup

# this defines all types under your namespace, in example:
# => #<Dry::Data::Type:0x007feffb104aa8 @constructor=#<Method: Kernel.String>, @primitive=String>

With types accessible as constants you can easily compose more complex types, like sum-types or constrained types, in hash schemas or structs:

Dry::Data.configure do |config|
  config.namespace = Types


module Types
  Email = String.constrained(format: /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z]+)*\.[a-z]+\z/i)
  Age = Int.constrained(gt: 18)

class User < Dry::Data::Struct
  attribute :name, Types::String
  attribute :email, Types::Email
  attribute :age, Types::Age

Built-in Type Categories

Assuming you configured types under Types module namespace:


  • Types::Nil
  • Types::Symbol
  • Types::Class
  • Types::True
  • Types::False
  • Types::Date
  • Types::DateTime
  • Types::Time

Coercible types using kernel coercion methods:

  • Types::Coercible::String
  • Types::Coercible::Int
  • Types::Coercible::Float
  • Types::Coercible::Decimal
  • Types::Coercible::Array
  • Types::Coercible::Hash

Optional strict types:

  • Types::Maybe::Strict::String
  • Types::Maybe::Strict::Int
  • Types::Maybe::Strict::Float
  • Types::Maybe::Strict::Decimal
  • Types::Maybe::Strict::Array
  • Types::Maybe::Strict::Hash

Optional coercible types:

  • Types::Maybe::Coercible::String
  • Types::Maybe::Coercible::Int
  • Types::Maybe::Coercible::Float
  • Types::Maybe::Coercible::Decimal
  • Types::Maybe::Coercible::Array
  • Types::Maybe::Coercible::Hash

Coercible types suitable for form param processing:

  • Types::Form::Nil
  • Types::Form::Date
  • Types::Form::DateTime
  • Types::Form::Time
  • Types::Form::True
  • Types::Form::False
  • Types::Form::Bool
  • Types::Form::Int
  • Types::Form::Float
  • Types::Form::Decimal

Strict vs Coercible Types

Types::Strict::Int[1] # => 1
Types::Strict::Int['1'] # => raises Dry::Data::ConstraintError

# coercible type-check group
Types::Coercible::String[:foo] # => 'foo'
Types::Coercible::Array[:foo] # => [:foo]

# form group
Types::Form::Date['2015-11-29'] # => #<Date: 2015-11-29 ((2457356j,0s,0n),+0s,2299161j)>

Optional Types

All built-in types have their optional versions too, you can access them under "Types::Maybe::Strict" and "Maybe::Coercible" categories:

Types::Maybe::Int[nil] # None
Types::Maybe::Int[123] # Some(123)

Types::Maybe::Coercible::Float[nil] # None
Types::Maybe::Coercible::Float['12.3'] # Some(12.3)

You can define your own optional types too:

maybe_string = Types::Strict::String.optional

# => None

# => None

# => Some('something')

# => Some('SOMETHING')



A type with a default value will return the configured value when the input is nil:

PostStatus = Types::Strict::String.default('draft')

PostStatus[nil] # "draft"
PostStatus["published"] # "published"
PostStatus[true] # raises ConstraintError


You can specify sum types using | operator, it is an explicit way of defining what are the valid types of a value.

In example dry-data defines bool type which is a sum-type consisting of true and false types which is expressed as Types::True | Types::False (and it has its strict version, too).

Another common case is defining that something can be either nil or something else:

nil_or_string = Types::Nil | Types::Strict::String

nil_or_string[nil] # => nil
nil_or_string["hello"] # => "hello"

Constrained Types

You can create constrained types that will use validation rules to check if the input is not violating any of the configured contraints. You can treat it as a lower level guarantee that you're not instantiating objects that are broken.

All types support constraints API, but not all constraints are suitable for a particular primitive, it's up to you to set up constraints that make sense.

Under the hood it uses dry-logic and all of its predicates are supported.

string = Types::Strict::String.constrained(min_size: 3)

# => "foo"

# => Dry::Data::ConstraintError: "fo" violates constraints

email = Types::Strict::String.constrained(
  format: /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z]+)*\.[a-z]+\z/i

email["[email protected]"]
# => "[email protected]"

# => Dry::Data::ConstraintError: "fo" violates constraints


In many cases you may want to define an enum. For example in a blog application a post may have a finite list of statuses. Apart from accessing the current status value it is useful to have all possible values accessible too. Furthermore an enum is a int => value map, so you can store integers somewhere and have them mapped to enum values conveniently.

# assuming we have types loaded into `Types` namespace
# we can easily define an enum for our post struct
class Post < Dry::Data::Struct
  Statuses = Types::Strict::String.enum('draft', 'published', 'archived')

  attribute :title, Types::Strict::String
  attribute :body, Types::Strict::String
  attribute :status, Statuses

# enum values are frozen, let's be paranoid, doesn't hurt and have potential to
# eliminate silly bugs
Post::Statuses.values.frozen? # => true
Post::Statuses.values.all?(&:frozen?) # => true

# you can access values using indices or actual values
Post::Statuses[0] # => "draft"
Post::Statuses['draft'] # => "draft"

# it'll raise if something silly was passed in
Post::Statuses['something silly']
# => Dry::Data::ConstraintError: "something silly" violates constraints

# nil is considered as something silly too
# => Dry::Data::ConstraintError: nil violates constraints


The built-in hash type has constructors that you can use to define hashes with explicit schemas and coercible values using the built-in types.

Hash Schema

# using simple kernel coercions
hash = Types::Hash.schema(name: Types::String, age: Types::Coercible::Int)

hash[name: 'Jane', age: '21']
# => { :name => "Jane", :age => 21 }

# using form param coercions
hash = Types::Hash.schema(name: Types::String, birthdate: Form::Date)

hash[name: 'Jane', birthdate: '1994-11-11']
# => { :name => "Jane", :birthdate => #<Date: 1994-11-11 ((2449668j,0s,0n),+0s,2299161j)> }

Strict Schema

Strict hash will raise errors when keys are missing or value types are incorrect.

hash = Types::Hash.strict(name: 'string', age: 'coercible.int')

hash[email: '[email protected]', name: 'Jane', age: 21]
# => Dry::Data::SchemaKeyError: :email is missing in Hash input

Symbolized Schema

Symbolized hash will turn string key names into symbols

hash = Types::Hash.symbolized(name: Types::String, age: Types::Coercible::Int)

hash['name' => 'Jane', 'age' => '21']
# => { :name => "Jane", :age => 21 }


The built-in array type supports defining member type:

PostStatuses = Types::Strict::Array.member(Types::Coercible::String)

PostStatuses[[:foo, :bar]] # ["foo", "bar"]


You can define struct objects which will have readers for specified attributes using a simple dsl:

class User < Dry::Data::Struct
  attribute :name, Types::Maybe::Coercible::String
  attribute :age, Types::Coercible::Int

user = User.new(name: nil, age: '21')

user.name # None
user.age # 21

user = User(name: 'Jane', age: '21')

user.name # => Some("Jane")
user.age # => 21


You can define value objects which will behave like structs and have equality methods too:

class Location < Dry::Data::Value
  attribute :lat, Types::Strict::Float
  attribute :lat, Types::Strict::Float

loc1 = Location.new(lat: 1.23, lng: 4.56)
loc2 = Location.new(lat: 1.23, lng: 4.56)

loc1 == loc2
# true

Status and Roadmap

This library is in an early stage of development but you are encouraged to try it out and provide feedback.

For planned features check out the issues.


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.


Bug reports and pull requests are welcome on GitHub at https://github.com/dryrb/dry-data.