Intersection
Intersection types are specified using the & operator. It combines two
compatible types into a single one with properties from each.
One example is a Hash that allows any keys, but requires one of them to be named id:
Id = Types::Hash.schema(id: Types::Integer)
HashWithId = Id & Types::Hash
Id[{id: 1}] # => {:id=>1}
Id[{id: 1, message: 'foo'}] # => {:id=>1}
Id[{message: 'foo'}] # => Dry::Types::MissingKeyError: :id is missing in Hash input
HashWithId[{ message: 'hello' }] # => Dry::Types::MissingKeyError: :id is missing in Hash input
HashWithId[{ id: 1, message: 'hello' }] # => {:id=>1, :message=>"hello"}