require 'rom'

Batman = Struct.new(:secret_name)

Transproc.register(:to_batman, proc { |name| Batman.new(name) })

class GothamCityMapper < ROM::Mapper
  attribute :superhero, type: :batman
end

m = GothamCityMapper.build

puts m.call [{ superhero: 'Bruce Wayne' }]
# => [{:superhero=>#<struct Batman secret_name="Bruce Wayne">}]