Optional Values
Optional values
Use the .optional method to get a type that has all the same features but also accepts nil:
Types::Strict::String[nil]
# => raises Dry::Types::ConstraintError
optional_string = Types::Strict::String.optional
optional_string[nil]
# => nil
optional_string['something']
# => "something"
optional_string[123]
# raises Dry::Types::ConstraintError
Under the hood this creates a sum type. Types::String.optional is just syntactic sugar for Types::Strict::Nil | Types::Strict::String.
Maybe values
For more advanced usage, use types under the Maybe namespace to get optional types that return an instance of Dry::Monads::Maybe::Some from dry-monads.
This functionality is not available by default - it must be loaded using Dry::Types.load_extensions(:maybe) and you need to add dry-monads to your Gemfile:
require 'dry-types'
Dry::Types.load_extensions(:maybe)
module Types
include Dry::Types.module
end
Types::Maybe::Strict::Integer[nil] # None
Types::Maybe::Strict::Integer[123] # Some(123)
Types::Maybe::Coercible::Float[nil] # None
Types::Maybe::Coercible::Float['12.3'] # Some(12.3)
# 'Maybe' types can also accessed by calling '.maybe' on a regular type:
Types::Strict::Integer.maybe # equivalent to Types::Maybe::Strict::Integer
You can define your own optional types:
maybe_string = Types::Strict::String.maybe
maybe_string[nil]
# => None
maybe_string[nil].fmap(&:upcase)
# => None
maybe_string['something']
# => Some('something')
maybe_string['something'].fmap(&:upcase)
# => Some('SOMETHING')
maybe_string['something'].fmap(&:upcase).value
# => "SOMETHING"