Type Constraints
Base Syntax
Use :type key in a param or option declarations to add type coercer.
require 'dry-initializer'
class User
extend Dry::Initializer
param :name, type: proc(&:to_s)
end
user = User.new :Andrew
user.name # => "Andrew"
Any object that responds to #call with 1 argument can be used as a type. Common examples are proc(&:to_s) for strings, method(:Array) (for arrays) or Array.method(:wrap) in Rails, ->(v) { !!v } (for booleans), etc.
Dry Types as coercers
Another important example is the usage of dry-types as type constraints:
require 'dry-initializer'
require 'dry-types'
class User
extend Dry::Initializer
param :name, type: Dry::Types['strict.string']
end
user = User.new :Andrew # => #<TypeError ...>
Positional Argument
Instead of :type option you can send a constraint/coercer as the second argument:
require 'dry-initializer'
require 'dry-types'
class User
extend Dry::Initializer
param :name, Dry::Types['coercible.string']
param :email, proc(&:to_s)
end
Array Types
As mentioned above, the :type option takes a callable object... with one important exception.
You can use arrays for values that should be wrapped to array:
class User
extend Dry::Initializer
option :name, proc(&:to_s)
option :emails, [proc(&:to_s)]
end
user = User.new name: "joe", emails: :"joe@example.com"
user.emails # => ["joe@example.com"]
user = User.new name: "jane", emails: [:"jane@example.com", :"jane@example.org"]
user.emails # => ["jane@example.com", "jane@example.org"]
You can wrap the coercer into several arrays as well:
class User
extend Dry::Initializer
option :emails, [[proc(&:to_s)]]
end
user = User.new name: "joe", emails: "joe@example.com"
user.emails # => [["joe@example.com"]]
Eventually, you can use an empty array as a coercer. In that case we just wrap the source value(s) into array, not modifying the items:
class Article
extend Dry::Initializer
option :tags, []
end
article = Article.new(tags: 1)
article.tags # => [1]
Nested Options
Sometimes you need to describe a structure with nested options. In this case you can use a block with options inside.
class User
extend Dry::Initializer
option :name, proc(&:to_s)
option :emails, [] do
option :address, proc(&:to_s)
option :description, proc(&:to_s)
end
end
user = User.new name: "joe",
emails: { address: "joe@example.com", description: "Job email" }
user.emails.class # => Array
user.emails.first.class # => User::Emails
user.emails.first.address # => "joe@example.com"
user.emails.to_h # => [{ address: "joe@example.com", description: "Job email" }]
Notice how we mixed array wrapper with a nested type.
The only syntax restriction here is that you cannot use a positional param inside the block.
Back References
Sometimes you need to refer back to the initialized instance. In this case use a second argument to explicitly give the instance to a coercer:
class Location < String
attr_reader :parameter # refers back to its parameter
def initialize(name, parameter)
super(name)
@parameter = parameter
end
end
class Parameter
extend Dry::Initializer
param :name
param :location, ->(value, param) { Location.new(value, param) }
end
offset = Parameter.new "offset", location: "query"
offset.name # => "offset"
offset.location # => "query"
offset.location.parameter == offset # true