Amongst other types, there is separate kind of container types, with the most recognizable examples of Enumerable, Array and Hash. It is not clear how to define types, because one scenario requires Hash values to be symbol, another one requires them to be integers. To handle this, concept of parametrized types (or generics), widely known in statically typed languages, is introduced.

Type may define one or more parameters during it’s definition using .parameter() call. Parameters can be used to substitute types, consider following example:

# input:
pagination:
  number: 3
  total: 271
content:
  - item #1
  - item #2
  - ...

class Pagination
  attribute :number, Integer
  attribute :total, Integer
end

class Page
  attribute :pagination, Pagination
  attribute :content, [Enumerable, T: parameter(:E)]
end

What has happened there? Page :content attribute has been declared with type Enumerable, and it’s parameter T had been substituted with Page parameter E. To finalize this up, resolve page parameters in mapping definition:

AMA::Entity::Mapper.map(input, [Page, E: Person])

In fact, parameters are resolved with type collections rather than singular types, so this is perfectly valid (and, in fact, the only way to allow nils in array):

AMA::Entity::Mapper.map(input, [Enumerable, T: [Float, Integer, NilClass]])